Observational Techniques in X-Ray Astronomy

OBSERVATIONAL TECHNIQUES iN X-RAY ASTRONOMYI

By R. GIACCONI, H. GURSKY, AND L. P. VAN SPEYBROECK A merican Science a.nd Engineering, Cambridge, M assa.chusetts

INTRODUCTION

X-ray astronomy deals with measurements of electromagnetic radiation in the energy range of about 0.2 to 100 keV originating outside the solar system. As with any other form of electromagnetic radiation, the relevant quantities to be determined are the intensity, the spectral content, and the polarization of the radiation. Since these are astronomical observations it is

necessary to localize the sources of the radiation, study their structure, establish time variations, and determine the effects of the in terstel lar medium on the radiation. Except for measurements of polarization that are still in the planning stage, all of these measurements have been made on one or more of the X-ray sources. The experimental and theoretical results in X-ray astronomy have been the subject of recent review articles by Gould (1), Morrison (2), and Friedman (3). Earlier reviews by Oda (4), Giacconi & Gursky (5), Hayakawa, Matsuoka & Sugimoto (6), Hayakawa & Matsuoka (7), and Giacconi et al. (8) also contain useful material.

This paper reviews the various techniques that have evolved for use in X-ray astronomy. The rocket payload used by Giacconi, Gursky, Paolini & Rossi (9) in the first discovery of a stellar X-ray source in 1962 is shown in Figure 1 together with a more modern rocket payload. The field has progressed rapidly since 1962 and many of the instruments that were utilized in important experiments during the last few years are only of historic significance now. \IYe shall attempt to present the general requirements for performing su ccessful experiments without necessarily presenting detailed analysis of particular instruments, and to discuss the particular quantities that are apparently important and the intrinsic limitations that are imposed by nature, both in the instruments and in the phenomena.

It is useful at the outset to point out the fundamental differences in the manner in which X-ray observations are performed, compared to other astronomical observations. First, since the Earth's atmosphere absorbs these X rays very strongly (see Figure 2), measurements must be made from sounding rockets or satellites, or at least the instruments must be carried to balloon altitudes from which a limited class of experiments can be performed. In addition to the limitations in weight and volume imposed by the experiment carrier, one no longer has the very rigid inertial platform provided by the Earth. The experimenter must provide the means for stabilizing the carrier and determining its orientation with respect to the celestial sphere.

1 The survey of literature for this review was concluded in January 1968.

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374 GTACCONI, GURSKY & VAN SPEYB ROECK

FIG.!. The sounding rocket payload on the left was used in the first discovery of a stellar X-ray source by Giacconi, Gursky, Paolini & Rossi in 1962 (9). The rocket payload on the right was used in a recent scan of the Cygnus region (:lO, :l4, 38).

Also, the very low fluxes of radiation make the use of efficient quantum de

tectors mandatory; the ultimate limit in precision and sensitivity is invari

ably the statistical fluctuations in the number of recorded photons. Finally,

X-ray focusing optics with its potential for high-resolution imagery and the

various techniques for high-dispersion spectroscopy have not yet been

utilized in celestial studies. Angular resolution has been achieved with

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 375

ffi iE rn o ��-+--+--4--�--+-�---r--+--��-+--+-�--�--+-�---r--+-� ��'�-+--+--4--�--+-� __ �--+--� ��-+--+--4--�r-+-��h-+-

FIG. 2. Attenuation of electromagnetic radiation in the atmosphere. Solid curves indicate altitude (and corresponding pressure expressed as a fraction of 1 atm) at which a given attenuation occurs for radiation of a given wavelength.

mechanical collimators, and spectra have been measured by nondispersive techniques.

THE PRESENT STATUS OF X-RAY ASTRONOMY

The outstanding feature of X-ray astronomy is the existence of discrete sources of X rays. As seen in Figure 3, their distribution over the sky makes it clear that the majority of the known sources lie within our Galaxy; however, several objects are reported at high galactic latitudes and may be as-

90

o+-++-I-++-

-90 @ IDENTIFIED OPTICALLY

FIG. 3. X-ray sky 1968.3 (the coordinates are galactic longitude and latitude, I" and bII).

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376 GIACCONI, GURSKY & VAN SPEYBROECK

SCORPIUS XR-\ •

10' +i'f'+< t ':!itt+.

-.fh' +' � 10

�,i!

i�t ... :> � I :

�I Me -"! � • FRIEDIiAN el 01 • I +" GRADER .. o\. - , CHOOIL elol.

10" -+- HAYAKAWA .101. I PETERSON a JACOBSON

I 10'

Ul

10' 10" • (keVI 10'

FIG. 4. A compilation by Gould (1) of measurements of the spectrum of Sco X-I. The measurements of Grader, Chodil, and Hayakawa are based on proportional

counter data obtained during sounding rocket flights . The data of Peterson were obtained during a balloon flight .

sociated with external galaxies. In particular, Byram, Chubb & Friedman (10), Friedman & Byram ( 1 1) , and Bradt, Mayer, Naranan, Rappaport & Spada (12) have reported a source within a fraction of a degree of M87, the well-known radio galaxy. T he intensity of t he X radiation as observed at the Earth ranges from 10-7 ergs/cm2-sec in the 1 to 10 keY range (�30 photons /cm2-sec) for the stronger sources, to somewhat more than 10-10 ergs/cm2-sec for the weakest reported sources. The X-ray spectra are characteristically decreasing functions of increasing energy as shown in Figures 4 and S. Only one well-known object, the Crab Nebula (13, 14), is positively identified as an X-ray source. The Crab, which is the 900-year-oJd remnant of a supernova explosion, is one of the most luminous objects in the Galaxy in t he optical

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+. GRADER.t ,I. + HAYAKAWA lI,t + PETERSON.t.1 • HAYMES a CRADDOCK T FROST "0'

CRAB NEBULA

,II,VI FIG. 5. A compilation by Gould (1) of measurements of the spectrum of the Crab

Nebula. The measurements of Grader and Hayakawa were obtained during sounding rocket flights. The others (Haymes, Peterson, and Frost) were obtained during balloon flights.

and the radio frequencies. On the other hand, the strong source Sco X-1 has been identified with a faint starlike optical object that had not been catalogued previously and from which there is no measurable radio emission (15). These two X-ray sources clearly represent two different classes of objects. In the Crab Nebula , the X-ray emission originates in a diffuse region of several arc minutes extent. The spectrum obeys a power law and extends to at least 100 keY. In Sco X-I, the X-ray emission region does not show any finite extent up to a limit of 15", and the spectrum appears to have an exponential shape. Optically, Sco X-I exhib its erratic variability and spectroscopically shows the characteristics of high-excitation phenomena (15-17). There are no definitive data on the X-ray variability of Sco X-I, but there is increasing evidence tha t the emission from several other sources does vary significantly with a time scale of the order of months. The source Cyg XR-l was reported to have undergone a fourfold decrease between two observations taken a year apart (10), and a source in Centaurus apparently appeared and disappeared in about 1 year, representing a change in intensity of about a factor of 100 (18-24).

The most startling aspect of these discoveries is the great power of the sources. For the galactic sources, the total emission in the X-ray region must be in the range 1035 to 1037 ergs/se�, which is as much as the brightest stars emit in visible light alone. The fact that for many of the X-ray sources there

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is no associated bright object in optical or radio wavelengths mcans that we are dealing with a phenomena in which X-ray emission is the dominant form of energy dissipation.

No generally accepted models exist for any of the X-ray sources, which is,

in part, a reflection of the relative crudeness of the X-ray measurements. The two physical mechanisms that seem to be capable of providing the X-ray emission are the synchrotron radiation from ultrarelativistic electrons gyrat

ing in a magnetic field and the thermal radiation from an extremely hot plasma. These and other possibilities are subject to experimental verification.

The X rays from the Crab Nebula could be the high-energy extension of the synchrotron radiation from that object; if so, they should exhibit a high degree of polarization, as is observed at the optical frequencies. Sco X-1, in visible light, appears to have many characteristics in common with objects

known to possess extensive hot gas emission regions. The X rays could be originating in a hotter region of the gas; if so, there should be X-ray lines originating from medium-Z elements, such as neon, magnesium, silicon, and sulfur (25, 26).

Another important aspect of observational X-ray astronomy is the presence of a diffuse background of X rays of apparent cosmic origin and of intensity about 10-8 ergs/cm2-sec-(sr) in the energy range 1-10 keY. The spectrum of the X-ray background is shown in Figure 6. Measurements of the background impose restrictions on instruments differing from those encountered in measurements of discrete sources. There is also a non-X-ray background consisting of cosmic rays, cosmic ray-induced interactions, and trapped radiation. While the diffuse X-ray background is reduced according to the field of view of the collimators, the non-X-ray background, of which there are both penetrating particles and l' rays, is proportional to the area or perhaps the volume of the detectors and is independent of the field of view.

EFFECTS OF THE INTERSTELLAR MEDIUM The absorption of X rays does not end outside the Earth's atmosphere.

The interstellar medium is a strong absorber of X rays ; its X-ray absorbing characteristics have been estimated by a number of investigators (7, 18, 27-

29). In Figure 7 we show the results of the calculation by Felten & Gould. In the X-ray energy region the absorption takes place predominantly in the higher-Z elements such as oxygen and neon. Since the cosmic abundance of these elements is not known to better than about a factor of two, there is a corresponding uncertainty in the X-ray absorption coefficient.

The absorptive effect of the interstellar medium can be expressed by the relation:

<f>'(E) = exp [-T(E) ]<f>(E) where cJ>(E) is the differential primary X-ray spectrum in units of number of photons per unit energy interval at an energy E, (1/ is the observed spectrum, and T(E) is the optical depth at the X-ray energy E along the line of sight to

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-I

-2

-6

-7

-8

DIFFUSE BACKGROUND

t LfIDfN·1IAOOYA i BlmER.tal. 'f NRl ASH f LOCKHEED t NIIOl)TA � �rw JPL t MIT

-9Q�--�----�2�---+I----�--� I09,fK,VI

FIG. 6. A compilation of measurements of the diffuse X-ray background by Gould (1).

the source. Except for the presence of absorption edges, r(E) varies approximately as the 8/3 power of the X-ray energy, and is a decreasing function of energy. Because of this strong energy dependence, and since the absorption enters as an exponential factor, the effect of the interstellar absorption is to produce a relatively sharp cutoff at some low energy.

The quantity r(E) is derived from more elementary constants through the following relation:

T(E) = 1 L N,u,(E) •

where l is the distance to the source, N; is the average number density of the ith element, and (Ji(E) is the cross section for X-ray absorption of that ele

ment at an energy E. The sum must, of course, extend over all elements ; but, as noted, the principal contribution to r(E) will originate in only several ele-

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10' '-:7---''--.l.-J--'-1...U�_''''''''-'--..ll..l...LL.li.J � � d PHOTON ENERGY. E (keVl

FIG. 7. A calculation by Felton & Gould (28) of the effective atomic cross section s for photoelectric absorption by interstellar matter. Contributions by atomic hydrogen and helium alone are shown and also the totals due to all elements with the hydro

gen in atomic and molecular form. The K edges for the heavier elements are indicateq.

A similar calculation by Bell & Kingston (27) indicates a less substantial difference in the photoabsorption by atomic and molecular hydrogen.

ments. It is frequently more convenient to quote the number Nal, the column density (H atoms/cm2) of hydrogen atoms to the source, since this quantity is sometimes directly available on the basis of 21-cm radio data. One can specify the energy Eo at which r(E) is unity, in which case r(E) can be replaced by (Eo/ E)8/3. There is some evidence for finite absorption effects in several X-ray sources. The group at Livermore Radiation Laboratory (18) report a turnover at about 1 keY in the X-ray spectrum of Seo X-I that could be due to absorption. More positive evidence is reported by Gorenstein, Giacconi & Gursky (30) in the case of Cygnus X-3, whose spectrum shows an Eo of about 2.5 keY. This X-ray source is situated at galactic longitude ZIT = 80° in Cygnus, which, according to radio measurements, is the direction of one of the densest observed spiral arms. Unfortunately, observation of a turnover in the spectrum cannot be unambiguously assigned to the effects of the interstellar medium; the absorbing layer could be near, or even intimate to the X-ray source itself. Several measurements can yield the X-ray absorption of the interstellar medium more or less unambiguously. One is simply the observation of low-energy cutoffs in sources for whieh the distance is known and for which there is some basis for specifying the spectrum at the source be-

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 381 fore the absorption. The only object for which this is now the case is the Crab Nebula. Another measurement is of the low-energy cutoff of the diffuse X-ray background. If that background is of extragalactic origin, the cutoff will vary systematically as a function of the galactic latitude. Recently, Bowyer, Field & Mack (31) may have observed this effect.

The presence of interstellar absorption severely limits observations at low energy; at energies below about 0.5 keV useful measurements can be made on objects only within about 100 pc of the Earth. One the other hand, the galactic disk should be transparent at energies above about 4 keY, and outside the galactic disk the density of matter is sufficiently low to permit the observation of extragalactic objects to cosmological distances .

In principle at least, the observation of absorption effects can yield the following information :

(a) The distance to the source, if there is an independent measurement of NHI.

(b) The abundance of certain elements in the interstellar medium. The observation of an edge discontinuity gives directly the number of atoms of the elements that contribute to the edge.

(c) Information about the nature of the source, if the absorption can be placed at the source rather than within the interstellar medium.

The X-ray absorption measurement is related to a number of other measurements of the properties of the interstellar medium, notably the 21-cm emission in radio, interstellar absorption lines at optical frequencies, and interstellar reddening ; and measurements of the absorption at the X-ray frequencies may clarify many present uncertainties, such as the distribution of the spiral arms.

An effect of the dust content in the interstellar medium is small-angle scattering, as was pointed out by Overbeck (32). At optical frequencies, the scattering angle is very large and leads to a diminution of the brightness of a focused image. At X-ray frequencies, the scattering angle is small (0£ the

order of an arc minute) and leads to a halo around the central image. However, on the basis of present estimates of the dust-to-gas ratio, the effect is small compared to the absorption in the interstellar medium.

EXPERIMENTAL SYSTEMS AND DATA-REDUCTION PROCEDURES

In spite of the variety of observations that have been made, the various techniques are essentially similar. The basic detection system comprises a collimator that transmits incident X rays arriving within a restricted range of angles and an X-ray detector that records the number of transmitted X rays. The narrowest collimation that can be achieved conveniently is about to, although the wire-grid collimator has been utilized to achieve less than l' collimation. The gas proportional counter, because of its high efficiency and low noise, has found widest application as a soft X-ray detector. No really satisfactory detector has been developed for very low energies ( < 1 keY), although several promising devices have been employed. At the higher energies

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(> 10 keY) it becomes possible to use the scintillation counter, which is the principal detector utilized in balloon-borne experiments. The collimator-detectOr problem is compounded by the requirement for large apertures. The early experiments by Giacconi et al. (9) utilized 10 cm2 of area; the more recent rocket experiments by Naval Research Laboratory, American Science and Engineering, and Massachusetts Institute of Technology have utilized around 1000 cm2 of detector area.

The expected counting rate per unit area of a given detector can be expressed by the relation

s = f </>(E)E(E)dE counts/cm2-sec 1.

where r/J(E) is the incident photon flux as a function of photon energy E, and feE) is the quantum efficiency which will also be some function of energy. The actual counting rate will result from a variety of radiation including charged particles as well as photons. Where a point source of radiation is present within the detector's field of view, the total expected counting rate can be expressed as

R = (Bl + B.p,)A + SAf«(J - (Jo) 2. where A is the detector area, S is the contribution of the point source as de. rived from Equation 1, andj(f}-f}o), which is less than unity, gives the transmission of the collimator as a function of the angle (f}-f}o) between the center of the field of view and the position of the source. The quantity BI + Bzn is the background counting rate per unit area and comprises two components: BI, originating from penetrating particles and 'Y rays, which is not a function of the size of the field of view n, but only of the size of the detector; and B2, originating from the diffuse X-ray background, which is directly proportional to n.

A typical experiment consists of scanning across a source. Its presence is indicated, in the case of strong sources, by a counting-rate profile that matches the collimator transmission function f«()-()o) or, in case of a weak source, by a statistically significant increase in counting raie in a region of angular width comparable to the size of the collimator. In general it is only possible to determine the background counting rate by making repeated traversals of a source region with a single detection system to obtain a number of source-on, source-off conditions. For only the several strongest sources is it possible to neglect background rates. It is clear from Equation 2 that to obtain source information from experiments of this kind it is necessary to know the rotational motion of the detector-collimator system. To evaluate the data in terms of a distribution of X-ray sources it is adequate to know the rotational parameters in an arbitrary inertial frame of reference, but to find the position of a source on the celestial sphere it is necessary to determine the absolute orientation of the vehicle. Typically one employs a device, such as star camera, magnetometer, or Sun sensor, that yields the orientation of the

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vehicle with respect to some known terrestrial or celestial parameter. For reasons of weight, cost, etc., instruments such as gyroscopes and accelerometers that directly sense changes in inertial attitude have not been employed. In certain cases, for example if the vehicle is spin stabilized, ordinary rigid body dynamics can be used to determine the rotational motion (8); however, in general there is only crude a priori knowledge of the torques and moments of inertia, and one must be prepared to make a point-by-point study of the attitude of the vehicle as a function of time.

A variety of techniques have been employed to increase the information available from these instruments. For example, the measurements may b e made with different absorbers which are effectively a set o f filters. A t least one experiment is planned in which the scattering pattern of the X-ray radia

tion would be used to determine the source polarization. All these techniques substantially reduce the nu mber of events detected and can be applied only to the strongest sources.

To obtain quantitative estimates of the observable parameters i t is customary to apply the conventional least-squares techniques in which o ne compares observed counting rates (Ro) with expected cou nting rates (R,,) according to the prescri ption:

X2 = L (ROi - R'i)2 i (1i2 3.

where O'i is the standard deviation of the ith measurement. The values of free

parameters in R. which minimize X2 are taken to be the best value of those

parameters, and the error in the parameters is found by studying the varia

tion in X2 around the minimum. The minimum value of X2 determines the goodness of fit; i.e., the probability that the expression used as Re would yield the observed data. The parameters determining R" include source location and i ntensity, spectral parameters such as effective temperature, possibly a source size description, and the background function. The values obtained for the parameters usually will have correlated errors; for example, a

high background estimate will normally be associated with a low intensity estimate.

In most. of the remainder of this article we shall discuss the individual experimental components. Much of the experimenters' time must be devoted to the peripheral portion of the equipment, such as the data-transmission system, vehicle mechanical requirements, limited electrical power, and thermal extremes; these difficulties limit one's experimental ohjectives and the reliability of the data. We shall not discuss the many challenging technical problems, but rather shall limit ourselves to the devices that most directly affect the data.

COLLIMATORS The simplest collimator consists of a set of parallel hollow rectangular

tubes or pipes such as the example shown in Figure 8. The theoretical and

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FIG. 8. Prototype mechanical collimator.

observed responses of this type of collimator are shown in Figure 9; the

theoretical curve shows the familiar triangular shape com mon to most mechanical collimators. The peak predicted transmission is the ratio of actual

open area to total face area, which includes the tube walls and necessary s u pports. The discrepancy between the theoretical and experimen tal curves is

primarily a result of the alignment errors in the individ ual cells of the collimator. In this case some of the discrepancy is caused by the finite angular width (,,-,4') of the X-ray beam used to make the measurement of transmis-

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 385 90%

BO%

70%

GO%

'0%

40%

20% --

10%

.

.loOlerlo.!. 17.7"

- - - ---I

A 63% ABSOLUTE i---

---c� �"S"iSSION

,� 14'

� !

\ 1

\

:----1100

I \

1 I fo'r -

T

JB" W. OF LOBES -

J. F.VI. OF rLN�LE 40 a za " a 2·

FIG. 9. Theoretical and measured transmission of the mechanical collimator shown in Fig. 8.

sian. The theoretical responses to some extended sources are shown in Figure 10.

The variety of fabrication techniques indicates that the apparently simple problem of collimator design has not been satisfactorily solved within the limitations of space vehicles. Some typical designs are: (a) collections of rectangular tubes as discussed above; (b) sheets of hex-celled honeycomb material (which results in an approximately round field of view); (c) parallel sheets of material which define the field of view in one direction only; (d) a combination of (b) and (c) in which the parallel sheets determine a wider acceptance angle in both directions. Aluminum, beryllium-copper, stainless steel, latex rubber, and probably other materials have been used in collimator construction. Slit-type collimators have been used in preference to round or hexagonal field-of-view collimators because they are statistically more efficient for isolated sources. The round collimator may have advantages in dense source regions such as the galactic center and permits a more direct superposition of data obtained in different traversals of the source region. An example of the use of slit collimators in a scan experiment is the survey of the galactic equator by Fisher, Johnson, Jordan & Meyerott (33), and of the Cygnus region by Giacconi, Gorenstein, Gursky & Waters (34) . Bowyer,

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386

.9

z .8 Q � .7 :IE � ,6 ..: 1= .5 � .4 � ..J .3 UI a: .2

. L

GIACCONI, GURSKY & VAN SPEYBROECK

FIG. 10. Predicted response of an ideal collimator to circular sources with uniform surface brightness. The parameter X which labels the curves is the ratio of source diameter to L which is the FWH M of the response to a point source.

Byram, Chubb & Friedman (35-36) have used round fields of view to advantage in all-sky surveys.

As discussed in the data-reduction section above, the resulting data are fit by an assumed smooth background function and a trial set of sources. The trial sources may be suggested by the data, other experiments, or theoretical considerations. The source parameters, such as total intensity, angular position, and angular size, are selected so that the predicted data are in best agreement with the observed data. The experimental errors are determined by testing the sensitivity of the data fit to the source-parameter values .

The errors can be estimated for planning purposes by expressing the moments of the source distribution in terms of the expected moments of the data and then calculating the expected statistical fluctuation in the measured values of these quantities. If we assume an isolated source and a collimator with an ideal triangular response in one dimension and a wide acceptance in the other, then in the small-angle approximation the expected counting rate cf/ (8) is:

ql(O) = B + f O+L dO' (1 - 1/1- /Ill) </>(0') 6-L L 4.

where B is the background rate, L is the narrow collimator acceptance angle, and c/>(8') is the source function folded with the wide collimator acceptance function in the orthogonal direction. The expected value of the nth moment of the data is:

,..,. = f d89n(ql(O) - B) 5.

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 387 If the scan line is long enough so that the source does not affect the data at the endpoints of the scan, we can express the source parameters in terms of the moments of the data as follows;

Source strength:

1= f dO'</>(O') � 1-'0

Source position (centroid) :

Source size parameter:

f dO'O'</>(O')

f dO'</>(O') 1-'1 1'2

16 f dO'(O' - 0)2</>(0') d2 = = 16 [/0<. _ L2 - (I"YJ

f dO'</>CO') 1-'0 6 1-'0

6.

7.

8.

The factor 16 has been chosen so that for a uniform circular source d is the source diameter. If the source dimensions are small compared to the collimator acceptance angle, the expression for d2 becomes the difference of two almost equal quantities and is subject to large fractional errors.

The variances of these quantities can now be calculated. In the weak, point-source limit the error expressions are :

Source intensity:

Source position:

Source size:

(11'1) = "';NB "" L-I/2 I N8

L"';NB 1 lTO = --- -- "'" LI/2 Ns "';3

lTd2 = Li"'; NB 11792"", Li/2 Ns 60

9.

10.

11.

where Ns and No are the total numbers of events expected from the source and background in the region of source contribution. The dependence of these quantities on the collimator width L is also indicated for the case of narrow collimators in which the solid angle-dependent background is negligible. In this case both Ns and No are proportional to the time spent in the source region which is proportional to L. If the collimator width is too narrow the

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TABLE I

INTENSITY, POSITION, AND SIZE ERRORS FOR A ROCKET ORSERVATION OF M87

AS A FUNCTION OF COLLIMATOR FWH M L

uo (deg) (Ud2)112 L(deg) (urll)a Angle error along (deg)

either scan line Circular source

4 0.043 0.14° 2.31° 2 0.06 1 0.0° 1.38° 1 0.086 0.071° 0. 82° 0.5 0.122 0.049° 0.490 0.25 0.173 0. 035° 0.29°

a The quantity (uI/I) is reduced by a factor of y2 from Equation 9 because of the two scans.

source contribution will be small compared to the background fluctuations. The condition Ns =3UB =3 V NB is often used as a source criterion; the sensitivity by this definition increases as £1/2 until the effects of other sources and the solid angle-dependent background become important.

Consider, for example, a rocket flight designed to investigate a single important source, such as M87. The pointing accuracy of the rocket is about ± 2°, so a reasonable experiment plan would be to scan across the source in two orthogonal directions with scan lines extending 5° on either side of the source. We will assume that 100 sec are available for each scan, an effective counter area of 800 cmz, and a residual background of 120 counts/sec. The intensity of the source, according to Bradt et al. (12), is =0.05 counts/cm2-sec (observed in the counter); the expected statistical errors for various choices of collimator dimension are given in Table I. These error estimates are for the idealistic condition in which statistical fluctuations are the only error source, and probably would be exceeded in practice.

We have assumed that the background is known with sufficient accuracy so that the background subtraction error is small compared to background statistical fluctuations in the source region, and that the effect of other sources in the field of view during the measurement can be subtracted with less error contribution than the uncollimated background fluctuation. The latter approximation is not usually justified when the source separations are smaller than the collimator acceptance angle. The difficulty of data analysis in this case is illustrated in Figure 11 in which an early survey with a wide collimator is compared to later investigations with narrow collimators (33, 37,38).

Wire-grid collimators.-The necessity of using very narrow collimators for precise position and significant angular-size measurements was recognized quite early in X-ray astronomy. For example, in Sco X-1, the absence of a

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I I

�saE OCT 1966 GJlj'0.7 GJI-Z.5 GX-12.4 I GX-S.6 I t I H GX-I4.1

LOCKHEED SEPT 1965

I ." ... . ,' ., \ . .. . I \ •••• •• -

��IL -

r t \ .. ..,... •

.. ., \ ... .. .,' \ • -. I \ ... :.: .. J \

1965 �

·t\:·· ·,' \ . : . . / \ '-- � \ I \ /1\ .. .. " ( \ /\_. _ , ," \, II I, I \ I ) ' • ,-. , \ ,- . .:. " �. \/ \ I \ I I \ \.. ..!! f- .i 1\ r \ I / \ \ / .... , � I \ I \ \� / \ >/ 1-. �I / \ ,/ \ /\ I \ ,/ \ -1 ____ L __ �_; __ � __ �_�\_!._ ___ 2� __ \.

I I I +20' -20·

FIG. 11. Comparisons of three surveys (33, 37, 38) of the Sagittarius-Scorpius region of the Milky Way. In all three surveys, the scan was along the galactic equator. In the AS&E and Lockheed experiments a slit-shaped field of view of 10 and 1.8° width at half-maximum response, respectively, was used, and in both cases the slit was approximately normal to the galactic equator. In the NRL experiment an 80 round field of view was employed. The dotted triangles under the data points were attempts by the NRL group to decompose the data into discrete sources.

conspicuous optical object led to speculation that the object might be of large angular size and of too low surface brightness to be observable in visible light. This might be the case if the object were a much older version of the Crab Nebula. However, the difficulty of fabricating fine collimators and their use with on ly crudely oriented vehicles was also recognized. These considerations led Oda (39) to devise the modulation or wire-grid collimator in which many of these difficulties disappear. The device can be practically fabricated

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-----' fon-' k(2n-I)(,td) I I D :--

>- 1.0 r----1,i---T-----::;oO+-...",..-l � I C/) k = INTEGER OIl: n = NUMBER OF � WIRE GRIDS � � 0.5 ... ... ::IE C/) Z « It:

s

... O ��L-��L-__ -W __ �JL __ ��L-�� 60° 60·

FIG. 12. Response of a modulation collimator shown schematically in the upper right . The quantity d is the wire diameter, s is the wire to wire spacing, and n is the number of wire planes. In the four-grid modulation collimator used by the AS&E/M IT group to observe the Crab and Sco X-1 (40-42), s=d=O.OOS inches, D=24 inches, which yielded a width at the base of each of the triangular transmission bands of 80 arc sec.

with angular resolutions of less than 1 arc min. Its angular response function comprises a series of narrow transmission bands covering a wide field of view. The typical construction consists of two or more planes of wires as shown in

Figure 12. In the normal construction, which results in a particularly simple response function, the spacings between wires are equal to the wire diameters. The distance from one outside plane of wires to the other wire planes is:

D where J' = 0 1 . .. n - 2 for an n grid 2i " ,

system, and the separation of the outside planes is D. If the angle between the normal to the plane of the wires and the projection of the radiation di. rection onto the plane perpendicular to the individual wires is t, the transmission is given by:

• = cos·�[ 1/2(2 -

co��) - \ � tan � - (2n-2)k \ ] 12.

(d = wire diameter = wire separation)

if an integer k exists such that the bracketed quantity is positive, and zero otherwise. For small angle if; the FWHM of the individual transmission band

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 391 is "" 2d/ D and the response function is approximately periodic with period -(2n-2) arc tan 2d/D. The number of grids should be large enough to separate transmission bands more than the expected dimensions of the sources.

If a modulation collimator is placed in series with a coarser ordinary collimator, the effect on the error quantities in Equations 9, 10, and 1 1 is to replace L by 2d/D and reduce Ns by a factor of 2n. Therefore, more accurate measurements of source position and angular size are obtained at the cost of decreased sensitivity and larger intensity errors.

A four-grid collimator with an FWHM of 40" was used to determine the angular size and position of Sco X-1 and the Crab Nebula (40-42) in a single' rocket experiment. The data obtained when Sco X-1 was in the field of view are shown in Figure 13. Angular-size information was obtained by superimposing the individual source traversals (taking account of the variation in the rate of scan) and comparing the observed distribution wit,h the kno",n angular response function. Because of the multiple-response function, the data from a single collimator are consistent with a source position on any of a series of bands on the celestial sphere. In the actual experiment two modulation collimators with slightly different response periods were used; this resulted in two sets of possible location bands and a kind of vernier effect that was used to reduce the source position ambiguity to the relatively few positions consistent with both sets of data. These remaining possibilities were adequately separated so that a simple collimator experiment could determine which is the actual position. Though this experiment yielded a wealth of new information (it made possihle the optical identification of Sco X-I), it has not proven possible to find a second set of targets on which to repeat the observation; this illustrates the limitations imposed on observations by the use of sounding rockets. As noted from the data in Figure 13 on Sco X-1, the peak signal/background ratio was only about 3: 1, and in the case of the Crab Nebula, the signal level was about i that of the background. Although some improvements can be made in the signal/background ratio, particularly through the new background rejection techniques discussed later, it is difficult to increase the net signal, which depends only on the size of the detectorcollimator. The basic limitation has been the short time duration of the rocket flight; in satellite experiments it will be possible to study many additional sources with this technique.

Lunar occultation.-The Moon occasionally will occult a source and can be used for accurate location and angular-size measurements. The technique, in principle, can yield extremely high angular resolution since diffraction around the limb of the Moon is not a limiting factor as in radio occultations.

The angular velocity of the Moon is about � arc sec/sec, and the accuracy of this type of measurement depends upon the time required to detect the source or source feature of interest. In addition, one must have a reas��bly precise location of the object before the measurement, particularly if the experiment is to be conducted from a rocket with a total observation time of about 300 sec.

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392

60

.0

0 ""

7D

60

'"

! 40 '" e. § 30

2<>

292 ...

GIACCONI, GURSKY & VAN SPEYBROECK

'00 304 316 320 324 TIME AFTER LAUNCH (.oc)

320 324 TIME AFTER LAUNCH (IIC)

'" ".

FIG. 13. Histogram of actual counts in the energy range from 2 to 20 keV observed from X-I during the modulation collimator experiment that led to the optical identification of the SOl

(15,41,42). Each peak is the result of the X-ray source transitting an allowed transmission direc' of the collimator. The separation between successive transits was ",,5 arc min. The observed va

tions in the time separations result from nonuniformityof the drift of the rocket; e.g., in the intej 304-316 sec, the vehicle virtually stopped. The width of the individual peaks agrees with the n

sured response of the collimator (40 arc sec at half-maximum response), and indicated that the ang size of the X-ray emitting region could not exceed 20 arc sec.

The NRL group (13) observed the Crab Nebula by this technically exact

ing method during the lunar occultation of July 7,1964 and obtained the first evidence of the extended nature of the Crab X-ray source. The number of

sources suitable for this type of measurement is limited, but perhaps some

use of this technique can be made during the present series of galactic center

occultations.

DETECTORS

Proportional counters.-The proportional counter i n various forms has been the principal detector for X rays with energies between 1 and 10 keY

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FIG. 14. Thin beryllium window proportional counter.

and has been discussed extensively in the li terature (43-45) . The counter consists of a gas and electrodes which are arranged so that a high electric field gradient is obtained near the positive electrode. The cathode normally also serves as the gas container and main structural member, and contains a thin entrance window for the X rays. A typical construction is shown in Figure 14.

An X-ray photon that is absorbed in the counter gas usually ionizes an atom of the gas by ejecting a photoelectron from the tightest bound level allowed by energy conservation. The photoelectron will interact with the gas and result in additional electron-ion pairs ; eventually its energy will be dissipated and a cloud of electron-ion pairs will remain along its path. The original ion usually will de-excite by emitting Auger electrons, although it may emit its characteristic line radiation (fluorescence yield) . The Auger electrons also will interact with the gas and result in electron-ion pairs. If no fluorescence photons escape the counter the total n umber of electron-ion pairs is proportional to the incident photon energy, but is subject to fluctuations as the distribution of energy among competing processes such as nonionizing collisions is determined statistically.

The initial electron cloud then drifts toward the anode because of the electric field in the counter. When the electrons enter the high electric field region near the anode they receive sufficient energy between collisions so that further ionization of the gas occurs, the secondary electrons create additional secondary electrons, and the number of electrons continues to increase until they are collected by the anode. Gas gains of 103 to 1 05 are readily achieved in this manner. According to Zastawny (46) the gas gain is given by:

A = exp { (Prasal [ K + B (In (�:) + �: - 1) ] � 13.

Where K, B, and So depend upon the gas, and P, ra, and Sa are the pressure, anode radius, and (EI P) at the anode respectively. For the commonly used

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394 GIACCONI , GURSKY & VAN SPEYB ROECK

90 per cent argon, 10 per cent methane mixture K = (0.5 ± 0. 1 ) X 10-3 V-I ; B = (3.00 ± 0.03) X 10-2 V-I ; and So = (25 ± 1) V cm-1 torr-I.

A noble gas is usually chosen for the primary counter-gas constituent because of the higher obtainable gas gain and initial ionization ; this is a consequence of the fact that nonionizing energy loss mechanisms, such as excitation of low-level rotation or vibration states in 'a polyatomic molecule, are absent or less probable in the noble gas. A pure noble gas results in a very unstable counter, however, since a trace of impurity may dominate the behavior of the gas, and, in addition, the noble gas is transparent to the ultraviolet light emitted by the excited noble-gas atoms; the ultraviolet photons are sufficiently energetic to eject electrons from the cathode and start a new multiplication sequence. The noble-gas atoms also may be excited to metastable states in the electron multiplication process, these states have long lifetimes and the atom often de-excites by ionizing collisions with the counter walls. A polyatomic quench gas, such as methane, carbon dioxide, or alcohol, is usually added to the counter to absorb the ultraviolet light from the noble gas and allow the noble-gas atoms in metastable states to de-excite by collisions with the quench-gas molecules. The mean-free path for these events is small and the electrons ejected from the quench gas are ejected near the anode at the time and place of the gas mUltiplication occurring during the original pulse and therefore do not start a secondary avalanche at a later time.

The intrinsic efficiency of the counter is th e probability of the photon not being absorbed in the window and then being absorbed in the gas. Since the photoelectric cross section varies approximately as (Z4/E8/3) in the few-keY region, it is important to have a thin, low-Z window and a thick, high-Z gas. Beryllium and metallized Mylar have found widest application as window materials, but other substances such as teflon or aluminum which can be useful for spectral filters have also been used. For soft X rays and low- to moderate-Z materials, we can neglect scattering compa�ed to absorption and the efficiency is approximately :

.(E) = exp (-tw/Aw) (1 - exp (-tg/Ag) 14. where € is the counter efficiency, tw(tg) is the window (gas) thickness, and AW(Ag) is the absorption mean-free path in the window (gas) at the energy E.

The efficiencies calculated for various typical counter configurations are shown in Figure 15. The absorption edges and the effect of window thickness on the efficiency at low energy are particularly noticeable.

The charge collected at the anode is proportional to the number of initial electrons and the gas gain of the counter, and can be used to measure the photon energy. The spectral resolution primarily depends upon the fluctuations in the number of initial electrons and the gas gain.

The energy resolution may be degraded by gain dependence on the location of the event in the counter. This may be caused by nonuniform fields near the ends of the counter, variations in anode finish and thickness, and

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 395 EFFICIENCY

1.0 LIIIIIIIT1Tr-III-r.:;;C;�FE9®l 0.8 O.B

0.'1

0.2

0. 1 .08 .06

. 0 4

. 0 2

2 3 'I 5

0.01 l.-__ W-_.J---'_--IL....L....L.-.J.JI..._....L.....,... ___ .!--_ ....... _....J 0.1 0.2 0.4 0.6 0.8 1.0 12 1.4 1.6 10.0

E N E RGY ( K E V )

FIG. 1 5 . Calculated efficiencies of various proportional counters. The effect of the metal coating on the normal Mylar window has been ignored.

anode misalignment. The latter effect is usually negligible. The end-field distortion can be minimized by proper shaping of the ends of the counter, but usually some effect will remain, and so the end region must be shielded from the X-ray flux if optimum resolution is to be obtained. Charles & Cooke (47) found that 1 per cent variations in the anode wire thickness resulted in an 8 per cent change in the gain of their counter. These authors have performed an extensive energy-resolution study and find that, under optimum conditions, the fractional energy resolution in their data region of 0.277 keY <E < 10 keY can be expressed as 0. 14 E;-1/2, where E is in keY. They also found that the pulse-height distributions are Gaussian over several standard deviations and that the resolution was independent of counter voltage in their test region of gas gains between 3 X 102 and 105• This resolution function yields a FWHM of 14 per cent at the 5 .9-keV line obtained from Fe55 and is slightly better than the resolution typical of flight instruments.

The measured spectrum from a line source will include an "escape" peak as well as the primary peak. The escape peak results from those events in which the original ion de-excites by emitting an X ray which escapes from the

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3 9 6 GIACCONI, GURSKY & VAN SPEYB ROECK

TABLE I I PROPERTIES OF COMMON PROPORTIONAL COUNTER GASES

Property Neon Argon Krypton -----

Atomic number 10 1 8 36 K fluorescence yield 0 . 01 0 . 1 0 . 60

L fluorescence yield 0 . 10 Absorption edges (keV)

K 0 . 84 3 . 20 1 4 . 32

L I 0 . 287 1 . 92 L I I 0 . 296 1 . 729

L III 0 . 244 1 . 674

M I 0 . 035 0 . 289

N I

Relative intensities of K

fluorescence lines Ka1 1 00 1 00

Ka2 50 50

KIH 20 22

Kf32 0 . 5 2 Relative intensities of L

fluorescence lines Lad 100

La2 1 3

L(31 62

L(32 8 L(33 14

Xenon

54

0 . 84

0 . 23

34 .55

5 . 448

5 . 103

4 . 783

1 . 14 0 . 208

1 00

50 30

7

100 12 58

20 9

counter and is located at the photon energy less the energy of the escaping fluorescent X ray. The fluorescent X-ray yield increases with Z and is negligible for He and Ne. The fluorescence properties of common proportional counter gasses are given in Table I I (48) . The fluorescent yield of krypton and

xenon is seen to he a dominant effect above the K edges of those elements.

The time resolution of the proportional counter is necessary for planning coincidence or anticoincidence requirements. Since the initial photoabsorption can occur anywhere in the counter, the time resolution corresponds to the drift time of an electron from the cathode to the anode. For a counter with a 2-inch-square cathode and a .002-inch-diameter anode operated at 2200 V this time was found to be O.8 J.Lsec for a 90 per cent argon, 10 per cent CH, gas filling and 2.5 J.Lsec for a 90 per cent argon, 10 per cent CO2 gas filling

(49). Geiger counters.-The early experiments (9, 31 , 34) in X-ray astronomy

were conducted with thin-window Geiger counters. The Geiger counter is similar to a proportional counter but is operated at such a high gas gain that the characteristics of the pulse depend on the saturation characteristics of

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 3 9 7

the counter and associated circuitry rather than upon the initial event. A large standard pulse that simplifies electronic requirements is obtained, but the energy information is lost and the recovery time is increased. The effective background will include events that would fall outside the selected X-ray energy region in the proportional counter.

Experimental necessity for data at higher and lower energies.-For most X-ray sources the bulk of the significant X-ray data falls within the energy interval of approximately 2 to 8 keY. The upper energy limit is caused by decreasing source spectra and the lower limit by absorption in the counter window. An accurate source description requires data outside the energy region for the following reasons :

(a) Although the models normally employed to describe the X-ray sOurces usually assume a single dominant generating mechanism with uniform parameters such as effective temperature over the entire X-ray source, the actual objects may have a variety of generating mechanisms with distributions of parameters such that the dominant portion of the source depends upon energy. For example, radio sources often have different power indices at different frequencies, and similar behavior can be expected in the X-ray region.

(b) Even if a single dominant mechanism and one set of parameters accurately describe the source it may not be possible to determine which mechanism or parameters accurately by observations in the restricted energy interval above. Absorption effects are more important and therefore easier to study at lower energies, while a cutoff in the energy distribution of electrons in a synchrotron source is better observed at higher energy. As the energy interval 2 to 8 keY is only a few energy-resolution widths wide, the data yield only a few constraints upon a source model. Frequently, an attempt is made to fit the experimental spectral data with several simple models, each model having perhaps three adjustable parameters. The typical result is that more than one model can be made to agree satisfactorily with the data, but the parameters of each model are defined within narrow limits. The spectrum of Cygnus X-3, for example (30), can be fit with either a blackbody spectrum without absorption or a bremsstrahlung spectrum with appreciable absorption. The parameters of either model are defined to an accuracy of about ± 10 per cent by the required statistical agreement with the measured integrated intensity, energy at peak intensity, and measured spectral width. The maximum difference between these two models is less than 10 per cent between 2 .5 and 9 keY. However, the predictions of the two models are quite different at lower energy where absorption is important; the ratios of intensities from the two models at 2, 1 .5 , and 1 keY are 0.6, 0. 12, and 1 . 2 X 10-4• Similarly, at higher energy the bremsstrahlung yield i s relatively greater; the ratios at 10, 20, and 30 keY are 1.2, 1 . 1 X 102, and 2. 1 X 102•

These considerations indicate the necessity of extending the X-ray measurements to higher and lower energies. Better spectral resolution is also

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n ecessary to observe source characteristics such as emission lines and absorption edges ; this information will not be available until :spectral measurements can be performed with dispersive techniques.

SPecial window proportional counters.-Proportional counters can be constructed with a thin window of an organic material, such as Mylar or teflon, which has been coated with a thin metal layer to eliminate sensitivity to ultraviolet light and reduce the gas leakage. These counters are not completely gas tight, are more subject to contamination, and have other operational difficulties, but are sensitive at lower energies. Counters with l mil Mylar

windows are sensitive down to about 0.7 keY and then have a narrow region of 10 per cent or less efficiency below the carbon K edge at 0.28 keY. Such counters have been used extensively by the NRL Group ( 1 1 , 34, 35) and by Bowyer et al. (3 1) . The teflon windows have a similar transmission band at energies less than the fluorine K edge at 0.7 keY.

Bradt et al. ( 1 1) have flown a proportional counter with a i mil aluminum window. This window has a transmission region from about 0.7 keY to the aluminum K edge at 1.5 keY and then i s essentially opaque between 1.5 and 3 keY as shown in Figure 15. Since the energy resolution of the proportional counter is adequate to separate these two energy regions it is possible to measure the flux at the lower energy accurately.

Photoelectric detectors.-The photoelectric detector is useful for lower energy measurements where an essentially windowless detector is desirable. In 1960 a group of Russian scientists (50, 5 1) discovered that the external photoelectron yield of several alkali halides is anomalously large for soft X rays. A soft X-ray detector utilizing this effect has been constructed and flown on the satellite OSO-4. The device consists of a esl photocathode and an electron multiplier (EM R-541-A). The first element of the electron multiplier is at a potential of about 1000 V above the photocathode in order to collect the photoelectrons. A thin window consisting of 1 J.L aluminized Formvar supported by a 90 per cent open screen was used to eliminate u ltraviolet radiation and residual ions. The total area of the device is about 40 cm2•

The photoelectric detector does not have intrinsic energy resolution but energy selection can be accomplished by means of filters. This technique can also be used with Geiger or proportional counters; the first discovery of an X-ray source depended upon the different counting rates observed in 1 .4 mg/cm2 and 7.0 mg/cm2 mica-window counters (9) . On a subsequent flight (52) spectral information was obtained by comparing the transmission through a 7.04 mg/cm2 Be filter and a 1 .72 mgjcm2 Mylar filter. The absorption in the upper atmosphere observed during rocket ascent and descent has also been used to obtain spectral information (9, 34) . If two filters of adjacent atomic number are used, the resulting transmissions can be made essentially identical everywhere except for the region between absorption edges. This technique yields the flux in a narrow band by comparison of the two event rates, but requires a large number of events as the result is the difference of two larger numbers. The statistical intensity error is approximately

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and this technique is only useful for strong sources. Scintillation detectors.-NaI (Tl) scintillation counters were first used in

1 964 by Clark to observe X rays from the Crab Nebula from a balloon (53), and by Giacconi, Gursky & Waters (52) to observe X rays from Sco X-I from a sounding rocket. Scintillation counters were utilized well before this time to measure high-energy solar X rays from the Sun by Chubb, Friedman &

Kreplin (55) . There have since been additional observations of the Crab Nebula (56-59) , Sco X-1 (60, 61) , and the Cygnus sources (62-66) .

The device, discussed extensively in the literature (67-70) , consists of a scintillating crystal and phototube. The scintillator must include some moderate- to high-Z components to be efficient at the upper end of the effective energy range and also so that the photoelectric effect will be the dominant energy-absorption mechanism. The latter property is necessary for optimum energy resolution since the competing process, Compton scattering, does not result in a unique energy deposition in the scintillator. For these reasons sodium iodide and cesium iodide have been the most useful scintillator materials for hard X rays. Sodium iodide yields more light and consequently better energy resolution, but is somewhat more difficult to handle and package as it is extremely hydroscopic.

Aitken and co-workers (71) have investigated the light yields as a func

tion of X-ray energy for various scintillating crystals; they find that the light yield per unit photon energy varies by approximately 15 per cent between 10 and 200 keY, and observe changes in response at the X-ray absorption edges of the scintillator material. Their energy resolution (FWHM) for N aI crystals is about 40, 1 7 , and 13 per cent at 5, 10, and 100 keY, is proportional to about £-0.4, and has a slightly different energy dependence in different energy regions. Their results with N aI are consistent with one photoelectron from the photocathode for every 350 eV deposited in the scintillator. Their system was unusually efficient, and more typically � 1 keY i s required per photoelectron ; thc energy resolution wou ld then be worse

by a factor of about 1 .7 . The light yields of the other materials compared to N aI (Tl) are approxi

mately CsI (Na) , 0.75 ; CsI (TI) , 0.25 ; and CaFz (Eu), 0.4. These light ratios

depend upon the X-ray energy and are different for other types of radiation. Al! of the light yields above are temperature dependent; NaI (Tl) and CsI (Na) have decreased yields at lower temperatures whereas the light yield of CsI (TO is strongly enhanced at low temperatures.

Stein & Lewin (72) have emphasized the importance of correcting the observed spectrum for the possible escape of the iodine K X ray which results in a measured energy 29 keY less than the initial photon energy. The prob

ability of the effect depends upon the counter geometry and photon energy ; it is abou t 27 per cen t at the K edge threshold for most geometries ; at higher

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energIes the probability is smaller and depends on the particular counter geometry.

BACKGROlJND REJECTION TECHNIQUES

Because of the very low source counting rates, it is desirable to utilize some means of reducing the background, which as discussed earlier consists of penetrating particles and "I rays. The "I rays result from cosmic-ray interactions both in the atmosphere and in the vehicle. In experiments with poor signal/background ratio, reduction in background is equivalent to an increase in aperture.

The rate of background flux is highly variable and depends on the instrumentation and the geographic location of the vehicle. The cosmic-ray flux in the vicinity of the Earth varies between about 0.2 and 1 particle/cm2-sec, depending on the geomagnetic latitude, and the total "I flux is comparable to these numbers. We ignore for the moment the cosmic X-ray background, which of course cannot be rejected but is reduced to almost negligible proportions by narrow field-of-view col limators .

Discrimination in energy-sensitive detectors.-If the detector signal is proportional to the energy deposition, it is possible to design the detector so that a large fraction of the background signals fall outside the energy region in which the X-ray signals occur. Much of the background is close to minimum ionizing and yields an energy deposition of about 2 MeV /g/cm2• In any practical scintillation counter (e.g. , 1 mm thickness of N aI) , the total energy deposition will be in excess of 1 MeV, well outside the X-ray energy range. But in gas proportional counters (e.g . , 5 cm-atm of argon) the energy deposition is in the keV energy range, which makes this technique inapplicable.

Use of independent guard counters. -Penetrating particles can be detected by counters adjacent to the primary X-ray detectors and one can electroni

cally eliminate that portion of the signals from the X-ray detectors originating in events that traverse the two counters ; plastic scintillation counters find

wide use in this application because of the flexibility in shaping the m ; proportional and Geiger counters are also used because of their simplicity. Almost 100 per cent rejection efficiency can be achieved, especially when using scintillation counters ; but since a substantial fraction of the background is in

the form of "I rays that are not rejected by these guard counters, such a high efficiency is not usually essential. Scintillation counters as "I-ray shields as well as guard counters have been used by Haymes (56) at Rice I nstitute and by Peterson (59) of the University of California, San Diego. Cesium iodide, which has a high cross section for "I-ray conversion, is used as the scintillating material : "I rays interacting in the scintillation material are detected, and time-coincident signals in the X-ray detector, such as could result from the scattered "I-ray or recoil electron, are rejected. Such a scintillation counter also can be used as the collimator for the incident X rays.

Pulse-shape discrimination.-This technique, yet to be widely utilized, is an efficient means of rejecting both 'Y rays and penetrating particles in gas

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 40 1 proportional counters. The counter pulses originating from these sources of background differ significantly in their shape (notably , the rise time) from X-ray pUlses. Relatively simple electronic circuits that enable one to dis

tinguish the two kinds of pulses can be devised. The apparent cause of the difference in shape is a difference in pathlength.

An X ray in the 1-10 keV energy region gives rise to an electron which has a range of less than . 132 cm in argon at STP, whereas energetic particles must traverse several centimeters of gas in order to deposit the same energy. The

increased tracklength characteristic of background radiation results in a spread in the times required for the initial electrons to drift to the multipli-

TABLE I I I

BACKGROUND REJECTION B Y PULSE-SHAPE DISCRIMINATION

Energy deposited in X-ray 'Y-Ray Counter Reference the counter (keV) acceptance acceptance gas

1 . 7 0 . 8 0 . 16 90% Ar, lO%CH. 74 3 0 . 8 0 . 12 90% Ar, 10%CH. 74 5 .9 0 . 8 0 . 025 90% Ar, lO%CH. 74 1 . 45 0 . 9 0 . 08 90% Ar, 1O%C02 75 2 .9 0 . 9 0 . 05 90% Ar, 10%C02 75 5 . 9 0 . 9 0 . 01 90% Ar, 10%C02 75

10 0 . 9 0 . 05 90% Ar, 10%C02 75

cation region near the anode ; the resulting pulse shape has a slower rise time

than that obtained for localized initial ionization .

By using a discrimination system based on proportional-counter rise time, Mathieson & Sanford (73) obtained a rejection of 90 per cent of the background-type events that deposited 5.9 keV in their detector without appreciable loss of true X-ray events. Gorenstein & Mickiewicz (74) and Van Speybroeck & Kellogg (75) , using a similar technique, have obtained the background rejections listed in Table I I I . The superior performance of the argon-C02 mixture is partly a consequence of a slower pulse shape that is less demanding of the electronics, and may also indicate that electron diffusion is less important in the argon-C02 counters. The diffusion of the original ionization naturally limits the usefulness of the technique, particularly at lower energies where a short tracklength is sufficient for a background particle to deposit the same energy in the counter as the X ray in question.

Kocharov & Naidenov (76) have reviewed the selection of counter materials for reducing the background from radioactive counter components. They also review several counter schemes in which an array of counters is packaged in a single envelope. The individual counters are separated by wire grids or other thin materials, and the counters adjacent to the walls are

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placed in anticoincidence ; thus events originating from interactions in the counter walls are eliminated.

.

ASPECT MEASUREMENTS A variety of aspect-measuring devices have been proposed for X-ray

astronomy experiments, but only star cameras, electronic star sensors, and

Sun sensors presently have the required resolution for precise experiments. Most recent rocket experiments have actually used star cameras for this purpose. A 16-mm exposure, once per second, can normally provide attitude data more precise than other sources of error in these experiments.

The choice of camera lens and film depends upon the resolution required and the rotation rate of the vehicle. As resolution requirements increase, it is necessary to increase the focal length which narrows the field of view ; consequently, fainter stars must be detected in order to obtain a required number of stars. Two stars are adequate for aspect determination, but three or more are highly desirable to avoid possible ambiguity. If one assumes a random

distribution of stars, an average of 5.25 are required, to be 90 per cent confident of obtaining three stars in the field of view.

As an example of camera performance, we will consider the scan of the Cygnus region by Giacconi et al. (30, 34, 38) in which a 25-mm FL, £0.9 lens was used. This results in a field of view of about 380 square degrees and according to Allen (77) a sensitivity to 4th mag stars is required to obtain the desired average of 5.25 stars/exposure at low galactic latitudes. The sensitivity was measured in ground-based star photographs and agreed with the film calculation below. The measured image diameter was about 4 arc min or 0.029 mm with a fast, low-resolution film similar to Kodak series 2475. The exposure time in flight was determined by the motion of the vehicle as the shutter was left open, and corresponds to about 0.3 sec for 4 arc min motion during the slow portion of the scan. The energy density necessary for a photographic density of 0 .1 is about 0.01 ergs/cm2 in this film. A typical m. = O star results in about 1O-6 ergs/cm2-sec in the wavelength interval of 0.4 to 0.7 Il, the approximate lens and film cutoffs. This results in a calculated limiting magnitude of 6, which was actually observed and resulted in an adequate number of star images. The individual stars were located with a precision of about 2 arc min, and the accuracy of camera axis location was about 1 arc min after averaging the positions obtained from several star images.

Normally, the camera axis is aligned with the X-ray collimator axis ; in the modulation collimator experiment in which the size and position of Sco X-I were measured, the camera was placed behind the X-ray col1imator which was illuminated by a light source (40-42) . This resulted in an image of the star field and collimator on the same film and eliminated some of the possible collimator/camera misalignment errors.

Electronic star sensors were widely used in early sounding rocket experi-

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 403 ments and still must be used in those experiments, such as from satellites, in which film recovery is not feasible. The star field is usually imaged onto a reticle by a lens and the transmitted light is detected by a photomultiplier. The uniformity of response is improved by placing a Fabry lens behind the reticle so that the photocathode is uniformly illuminated regardless of the portion of the reticle transmitting the light. A typical design has a reticle in the shape of the letter N. As the vehicle rotates, three pulses will be received from each star ; the relative timing of these pulses will provide the necessary attitude information.

Star sensors are difficult to use when observing near the Sun because, even though the Sun-shading devices may be quite efficient, the sunlight scattered from the dust around the vehicle will simulate star images. This background is easily eliminated from photographs but can be quite troublesome in analyzing an electronic star sensor record. For this reason it is useful to have a solar sensor for obtaining attitude information when the vehicle is in sunlight. This functions on the same principle as the star sensor except for the angular extent of the solar disk. Because of the great intensity of the

Sun, a solid-state detector can be used as the light-sensitive element.

Various other aspect devices, such as magnetometers and the vehicle guidance system, have been or will be used for attitude control during experiments but are generally not accurate enough for the final aspect deterrni

nation.

ADVANCED TECHNIQUES

X-ray telescopes.-The fact that X rays will reflect with high efficiency at grazing incidence allows the construction of optical systems for X rays. The first such device to be built was the X-ray microscope of Kirkpatrick (78). A general discussion of the conditions for constructing focusing X-ray optics has been given by Wolter (79) .

The first proposal for using X-ray optical systems for astronomical observations was made by Giacconi & Rossi (80) who suggested various kinds of collectors for concentrating the X rays entering a large-aperture device into a small focal spot. A one-dimensional slat arrangement of paraboloidal reflectors has been described by Fisher et al. (81), and a low-resolution form of the paraboloid-hyperboloid combination using microscope slides to approximate the correct figure has been described by Kantor (82). These collectors provide about the same angular collimation as the best mechanical devices, but since they allow the use of small detectors, a superior signal-tonoise ratio can be achieved, compared to conventional mechanical collimatordetector arrangements.

Giacconi's group constructed the first paraboloid-hyperboloid (Figure 16) combination of grazing surfaces for producing astronomical images ; this instrument was first used in a joint experiment with Goddard Space Flight Center to obtain photographs of the Sun in X rays (83) . The optical systems

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f--- L _-----l I--L --1 -I

HYPERBOLOID

�------ 2c ------+�--

FIG. 16. Schematic cross section of an X-ray telescope.

themselves have been described by Giacconi et al. (84) and by Zehnpfennig et al. (85) . An example (83) of an X-ray photograph of the Sun is shown in Figure 17 .

Focusing telescopes will clearly be important in studying nonsolar X-ray sources, since they aliow measurements qualitatively different from those which can be accomplished with conventional mechanical collimators. Because of the high intrinsic spatial resolution, source structure can be studied in detail and celestial position can be precisely established. Focusing devices also allow the use of certain dispersive techniques to obtain high spectral resolution.

The reflectivity at grazing incidence is shown for nickel in Figure 18. These curves show the characteristic short-wavelength cutoff of the reflectivity which forces one to utilize shallower angles at the higher energies. There is no long-wavelength cutoff, and except for diffraction effects, X-ray optics function equally well in visible light. Ray-tracing studies and tests of working telescopes have demonstrated the potential to achieve high angular resolution (several arc seconds) over adequate fields of view (several arc minutes) . The total field of view is limited by the grazing angle ; vignetting occurs off axis because of rays failing to strike the second surface.

The fact that grazing incidence must be utilized introduces certain disadvantages into X-ray optical systems. In the first place, the collecting area comprises only a small fraction of the total polished area, namely, only the projected area of the first surface ; and in the second place, the devices have large f-numbers and correspondingly large focal lengths. These considerations lead to telescopes longer, heavier, and more expensive per unit collecting area than devices that operate at longer wavelengths. However, given the extended observation time permitted from satellites, a wide variety of

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FIG. 1 7 . X-ray photographs of the Sun obtained during a sounding rocket flight of 17 March 1965, compared to the Sun photographed in Hex at about the same

time (83) .

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406 GIACCONI , GURSKY & VAN SPEYBROECK

>u z w � u. U. W 0.1 I'--hf---.J---,f, � i= &l ...J u. � K EDGE

o A FIG. 18. Calculated reflection efficiency of nickel at various grazing angles. I n

actual telescopes, the specular component o f the reflectivity is observed to be less than the above values and depends critically upon the surface finish.

significant experi ments can be performed with modest-sized telescopes of no more than 50 to 100 cm2 of collecting area. The AS&E group is preparing an instrument for solar stndies as part of N ASA's Apollo applications program that will contain a pair of mirrors with a total of about 40 cm2 collecting area. The design specifications of that telescope system are given in Table I V along with the specifications o f the small telescopes used i n sounding rocket experiments.

The problems involved in an X-ray telescope observational program are similar to those encountered in visible lighl, b u l compounded by the necessi ty of flying the experi ment. If fi l m is used as the recording medi u m , the relevant parameter describing the telescope sensi tivity is th� fl u x density of X rays in the focal plane, given by ;

rA ¢'� ¢ (Fil)' photons/cm2-sec

w here A is the collecting area of the telescope, r is the reflectivity of the surfaces, F i s the focal length, 0 i s the angular resolution, and c/> i s the incident flux of X rays within the wavelength range to which the telescope and film are sensitive. The linear diameter of the resolution element in the focal plane

( Fa) is about 60 J,tm, thus film resolution is not a serious factor. For extended X-ray sources, such as the Crab N ebula, the quantity c/> m u s t be replaced by (dc/>/dQ)!52 where dc/>/dQ is the surface brightness of the X-ray source. For the

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 4 0 7

larger telescope described i n Table IV, t h e factor rA / (FO)2 is about 105. Since an X-ray flux of about 1 07 photons/em' is required to produce an image o n film, this telescope could produce an i mage of the stronger X-ray sources (4) ;: 1 photon/ cmZ-sec) i n less than 2 min exposure ti me, provided that the source size is smaller than the angular resolution of the telescope. It i s not

clear what is the maximum possible exposure time. The sky brightness as set

by the diffuse X-ray background is extremely low ; and other factors, such as

fogging caused by trapped radiation , will probably limit exposure times. However, film is less desirable and also less efficient as a recording mediu m

than an electroimaging system. T h e latter permits o n e t o record individual events and to sum data sequentially. In contrast, the i ntegration of

TABLE IV PROPERTIES OF Two SOLAR TELESCOPES

Property

D iameter of mirror

Focal length

Total collecting area

Angular resolutional

field of view center Of/

" Two concentric mirror system.

3-inch telescope

7 . 6 cm

83 6 cm

1 . 6 cm'

,-......, 1 arc Inin

NASA-ATM telescope

22 . 86 em, 30 . 48 cm" 2 1 3 . 36 cm

42 cm'

3 arc 'sec

photons must be done on the fi l m , which forces one to poin t the telescope with high stability for long periods o f time. T here are also operational prob

lems with storing film in space and the necessity of either returning the film to Earth or processing and analyzing the exposed film i n orbit. The parameters determi ning the sensitivity of an electroimaging device, assuming it to be a q uantum detector, are the signal-to-noise ratio i n an image element and the time available for accumulating ·events. The total signal rate is cpfr A cou nts/sec where � is the photon detection efficiency. The state of develop

ment of X-ray i maging devices is changing rapidly and it is difficult to esti

mate the noise rate in the devices which will actually be used. As is true i n

optical astronomy generally, the ultimate potential of telescopes cannot be realized u ntil high-quality imagi ng devices are available.

Bragg spectroscopy.-As has been discussed, only low-reso l u tion, nondispersive techniques have been utilized for obtaining spectral information concerning the X-ray sources ; the high-dispersion techniq ues for X rays that are commonly available i n the laboratory have been applied to solar studies and inevitably will be used in stellar X-ray astronomy.

Bragg scattering from crystals is the basis of most such spectrometers. A crystal lattice forms a three-dimensional diffraction array which will reflect

X rays of wavelength X with high efficiency within a narrow range of angles

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408 GIACCONI , GURSKY & VAN SPEYB ROECK

t:.0 centered at a grazing angle 0 that satisfies the Bragg condition :

n"A = 2d sin 0 n = 1, 2, 3, . . .

where d is the crystal spaci ng . A given type of crystal will reflect X rays shorter than a long-wavelength l imit ).. = 2d, at w hich poi nt 0 = 90°. I n principle there is no short-wavelength limit, but at very small grazing angles the reflectivity becomes high at all wavelengths. A list of crystals that have been used in solar studies i s given in Table V.

The quantity t:.0 is a property of the crystal ( in the l imit i t is the width of the diffraction pattern for monochromatic radiation) and is typically of the order of 1 arc min i n the soft X-ray range. The existence of a finite t:.0 means that a range of t:.0 will reflect efficiently at a given angle 0, where t:.)" and t:.0 are related by the derivative of Eq uation 1 7 , namely

ntl"A = 2d cos Otl() The wavelength resolution of the crystal is defined as )"/11),, which is j ust

tan 0/110. At the X and 0 combination that satisfies the Bragg condition, the reflectivity is quite high, typically between 20 and 40 per cent. Since at a given 8 only a narrow range of wavelengths is recorded, the crystal must be "rocked" (rotated with respect to the incident beam) i n order to analyze a spectrum of radiation.

I t is feasible to apply scaled-up versions of solar X-ray spectrometers of the kind flown by B lake, Chubb, Friedman & Unzicker (86) , and by N eupert, Gates, Swartz & Young (87) , to the study of X-ray sources; but because of its exceedingly high spectral resolution, the device works best in the study of line emission. I f the X-ray emission process i n these sources is thermal

TABLE V

PROPERTIES OF SOME CRYSTALS USEFUL IN SOLAR X-RAY EXPERIMENTS

KAP ADP Calcite

Grating spacing (2d) 26 . 4 A 10 . 648 A 6 . 048 A Integrated 2 . 1 0-5 radians 10-5 radians 1 0-5 radians reflection coefflcienl (typical)

FH'HM of diffraction curve at 1 . 54 A - - 5 arc sec 4 . 94 A - - 20 arc sec

8 . 3 A 40 arc sec

lO A 2 arc min 20 A 10 arc min

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O BSERVATIONAL TECHNIQUES IN X-RAY ASTRONOM Y 409

DETECTOR ARRAY

ROWLAND CIRCLE

� �CON SYSTEM � / '-\

�--==-= -�-�(if=:::::J

DOUBLE REFLECTION GRAZING INCIDENCE OPTICS

APERTURE PLATE� FILTER WITH WHEEL

RASTER DRIVE

BRAGG CRYSTAL AND CRYSTAL DRIVE

FrG. 19 . Grazing-incidence optics with Johann Mount crystal.

bremsstra hlung, a certain fraction of the emitted power wil l be in the form of

l ine emission. However, whereas in the Sun a large fraction of the emitted power is in the form of lines, in the X-ray sources t his may not be the case. Tucker & Gould (25) have estimated that only a few per cent of the power emitted from a 5 X 107hr 0 K plasma with normal stellar abu ndances (as might be the case i n Sco X-I) will end u p as lines. The dominant l ines would

result from highly ionized states of neon, silicon, magnesi u m , and iron. The reason for the low yield of l ine emission is sim ply the h igh temperature, w hich means that the ionic states of the atoms that give rise to l ines i n the severalkeY energy region are rare. X-ray telescopes can be utilized as collectors for crystal spectrometers. In a configuration proposed by Sc hnopper shown i n Figure 19, the crystal is placed tangent to the Rowland circle which contai ns

the focal point. I f the crystal is curved with a radius twice that of the Row

land circle, the reflected rays will form a l ine i mage on the Rowland circle. Curving of the crystal is necessary, not only to proouce an i m age, hut also to preserve the Bragg condition across the face of the crystal.

The slitless spectrometer.-Gursky & Zehnpfennig (88) originally suggested a slitless spectrometer consisting of a grazing-incidence telescope and transmission diffraction grating ; a working device has been described by Zehnpfen nig (89) . The presence of the grati ng produces a vi rtual source at some

diffraction angle for each wavelength ; these virtual sources are then focused o n to the film by the telescope. I f a grating is used i n which the width of the open portion is equal to that of the blocked portion, about 1 0 per cent of the power will be diffracted into each of the two first-order spectra. Gratings

have been constructed by evaporating gold at a grazing angle onto a thin plastic replica of a grati ng. The gol d deposits o n only one side of the ridges in the grating to form a set of X-ray opaque lines, and the plastic backing i s thin enough t o allow transmission of the X rays through the remainder of the grating. This technique for making gratings was first suggested by Bird & Parrish (90) .

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4 1 0 GIACCON I , GURSKY & VAN SPEYBROECK

X rays passing through the grating are diffracted by an angle () given by the relation

(j = I1l1jd 11 = 1 , 2, . . .

w here d is the line spacing. For a typical grating d is 1 jJ. and (J is about 3 arc mi n for 10 A radiation. A telescope angular resolution of 5 arc sec will thus yield a spectral resolution of about 0.25 per cent at 10 A. As with other slitless spectrometers, the device works best with poin t or highly structured sources of radiation. An example (88) of an X-ray spectrum obtained with an early, low-resolution, slitless spectrometer i s shown in Figure 20. The device offers the advantage of the high-speed recording of spectra ; a large fraction of the incident power is diffracted ; and an entire spectrum of radiation can be recorded simul taneously.

FUTURE DEVELOPMENTS There are now very few precise measuremen ts in X-ray astronomy-a

situation to be expected i n a field i n which each major experimental group is l imited to about 10 min of observation a year. The majority of experiments have been surveys of a substantial portion of the sky rather than detailed studies of a few sources. Experiments of this type will continue to yield i nformation, b u t the emphasis in the field will gradually shift to more accurate measurements of individual objects. The remainder of this article is directed to the requirements of these future experiments.

Gursky et al. (38) argue that the ten known sources (excluding Sco X- 1) in the Sagittarius-Scorpius region may be in the Sagittarius arm at a distance of abou t 2 kpc. The average intensity of these sources is 0.62 photons/cm2-sec in the 2 to 5 keY interval with a factor of about ten between the i ntensities of the weakest and strongest sources. I f Seo X- 1 were one of these average sources, it would be about 350 pc away-a posi tion well wi thin the uncertainty i n the location of that object.

T hese sources, i f located in the most distant parts of our Galaxy, would yield Huxes of abou t 4 X 10-3 photons/ cm2-sec if absorption is negligible. This is a factor of ten less than the flux of the weakest presently detected source, which is a measure of present experimental sensitivity.

Friedman (3) estimates that there are rv 1 250 similar sources in our Galaxy. Many of the sources will be obscured ; the total number is not large for statistical studies such as the n u mber of classes of these objects, the distribution among classes, the velocity distribution of the X-ray sources, the separation of absorption effects due to the interestellar matter from absorption effects in the sources, the association with other types of objects in the Galaxy, the types of time variation, and the possible association of t ime variations with other properties such as spectra and intensity. Experiments should have adequate sensitivity not only to detect these sources b ut also to measure the several source parameters.

The Crab Nebula has an intrinsic X-ray luminosity of about three tim es

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2.0

� iii , !I z w 0 � 1: ol. 1 .0 '" a: C) 5 :x: O !l Q.

0

OBSERVATIONAL TECHN IQUES IN X-RAY ASTRONOMY 4 1 1

.... -,..

DIRECT PHOTOGRAPH ( MO + AI Taro t )

MICRODENSITOMETER TRACING OF ABOVE PH OTOGRAPH

I • t I I 4 • ,

2 3 4 SCAN DISTANCE ( mm )

FIG. 20. Spectrum of X rays obtained with the slitless X-ray spectrometer de

vised by Gursky & Zehnpfennig (88) . The device consists of a transmission diffract:o:1 grating mounted in front of a grazing-incidence telescope. The dispersion in the above spectrum in Cl"lo./"Io. is about 5 per cent at 10 A which was determined by the angular resolution of the telescope.

the typical Sagittarius source, th u s an experimental sensitivity comparable

to that above is also req u i red for the study of su pernova remnants similar to the Crab Nebula .

Eventually the most informative meas u rements m ay be those of sources

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4 1 2 GIACCON I, GURSKY & VAN SPEYB ROECK

.� I I . � _ .

FJ(�_ 2 1 . The three small telescopes above are early developmental u n its similar to thc first telescopes used for solar X-ray studies_ The u nit below is a prototype of a

telescope which will be used for solar X-ray studies during the NASA Apollo Applica

tions Program.

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OBSERVATIONAL TECHNIQUES IN X-RAY ASTRONOMY 413 in other galaxies where the distance is not such a confusing parameter. The flux expected for a representative Sagittarius-type source located in the Magellanic Clouds would be about 10-3 photons/cm2-sec. A sensitivity to fluxes of order 10-5 photons/cm2-sec would be required to study these sources in the tenth nearest galaxy. This flux is a factor of 5 . 103 less than that received from the weakest sources presently detected.

A sensitivity to a flux of 10--3 photons/cm3-sec would permit us to observe the total flux from the five brightest galaxies if the ratio of X-ray flux to visible light calculated for our Galaxy is typical. Gould & Sciama (91) have suggested that the actual observation might show the external galaxy as a shadow on the diffuse background rather than as a source. An observation of such a shadow would demonstrate the extragalactic origin of the background.

Although a source has been detected near M87 and other extragalactic sources are suggested, this is not adequate information for estimating the sensitivity required to study peculiar galaxies in general. A number of extragalactic radio sources have harder radio spectra at high frequencies than M87 and a study of these objects may determine if M87 is unique or one of many similar sources.

Absolute-intensity calibrations are required for comparison with data taken at other frequencies and at other times. Unfortunately, the accuracy achieved in most measurements has not been sufficient to determine variability except in the most extreme cases. As noted, there is evidence of variability in the X-ray intensities of several sources, and in the optical luminosity of the two optically identified point sources.

The optical counterparts of both Sco X-1 (1S-17) and Cyg X-2 (92-96) display erratic variations of intensity that are characteristic of old novae. There is also good evidence that Cyg X-2 is a short-period binary (94) . The presence of optical variability immediately suggests a search for simultaneous X-ray-visible light studies. Intensity accuracies of about 5 and 1 per cent would be required to detect short-term variations in X-ray flux comparable to the observed short-period optical variations of Sco X- l and Cygnus X-2. The actual X-ray fluctuations may be considerably less, and correspondingly more accurate observations would be required. The individual measurements should be accomplished in a time shorter than the characteristic fluctuation interval-perhaps ! min-and should extend over a period of several hours. Simultaneous X-ray and optical observations could determine the possible phase relations between the X-ray and optical fluctuations. This program clearly requires observation times that can be achieved in satellites, or in a series of rocket or balloon flights.

Although most X-ray source locations are known only to about 1 deg, which is required to avoid confusion with other X-ray sources, most of our understanding of X-ray astronomy has resulted from the few precisely located sources that also can be studied at other wavelengths. On the other hand, if a source position is not known accurately, the tendency to identify the X-ray source with the most peculiar object in the general vicinity results in an exciting state of confusion. At low galactic latitudes an angular resolu-

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4 1 4 GIACCON I , GURSKY & VAN SPEYBROECK

tion of a few arc minutes is required for a point-source optical identification in which most stars are excluded by plausible spectral models, and a resolution of a few arc seconds is necessary for an identification without spectral assumptions. These numbers are based on the source's being in an unobscured region where optical observation is possible. The requirements are significantly less stringent at higher galactic latitudes.

Angular resolution of several arc seconds will be necessary to investigate the structure of most extended sources. The Crab Nebula, for example, would subtend only about 5 arc sec if it were in a distant part of our galaxy. The X-ray telescope will become the most useful device for these high angular resolution experiments; some examples of early telescopes and a model of the NASA-Apollo program telescope are shown in Figure 2 1 . I t is, of course, quite useful to discover the extended nature of a source even if structure within the emitting region cannot be determined. Many important sources can be expected to be larger than the above limit; for example, the relation between the peculiar jet of M 87 and the X-ray emission from that source can be profitably studied with a resolution of only ! arc min. A location accuracy of t arc min has been achieved for only two sources.

The present spectral information, which is limited by proportionalcounter resolution, can be compared to UB V observations in optical astronomy. The X-ray telescope and diffraction-grating spectrometer will allow us to observe X-ray emission lines and the predicted absorption edges resulting from the interstellar media. The Bragg crystal spectrometer will enable us to study line displacements reSUlting from relative motion, and possibly line profile changes due to nonuniform velocity distributions in the source. The X-ray observations then can be expected to yield information comparable to that obtained in the optical region.

SUMMARY X-ray astronomers have succeeded in utilizing virtually every technique

available to the earthbound X-ray researcher, and have invented several new devices as well. Although several of the techniques described here have not actually been used in space experiments to date, there seems little doubt that they will be important sources of new information within the next several years. As in most other branches of the space sciences, the experimenter is limited by the availability of space vehicles rather than by his instrumentation. X-ray astronomy is now in the last stages of simple exploration and perhaps is j ust entering the era of accurate measurements.

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Documents

Observational Techniques in X-Ray Astronomy