Future astrophysics space missions in gamma ray astronomy

Nuclear Physics B (Proc. Suppl.) 14B (1990) 35-50North-Holland

FUTURE ASTROPHYSICS SPACE MISSIONS IN GAMMA RAYASTRONOMY

V. Scbönfelder

Max-Planck-Institut für extraterrestrische Physik

35

An overview about presently approved space missions in gamma ray astronomy is given. Special emphasis is devoted to theGamma Ray Observatory GRO of NASA - a multi-instrument observatory which covers more than 5 orders of magnitude inphoton energy from about 100 keV to 30 GeV. Significant progress in the exploration and understanding of the gamma raysky can be expected in the very near future. Thenext generation of gamma ray missions will have to focus on high resolutionspectroscopy, on high resolution imaging and on broad band studies of gamiina ray burst sources.

1. INTRODUCTIONGamma ray astronomy from space is now more than 20

years old. The results obtained so far have widened our un-

derstanding of high enera phenomena in the sky con-

siderably. Still, the gamma ray part of the electromagnetic

spectrum belongs to one of the least explored spectral ran-

ges in atronomy. This becomes immediately clear, if one

compares the sensitivities which have been achieved in the

different spectral ranges with each other. The most sensitive

instruments in the radio, infrared, optical and X-ray range

are able to detect celestial objects which are about five or-

ders of magnitude weaker than the Crab - a remnant from a

supernova explosion in 1054 which very often is used as a

standard candle in the sky. In contrast, the most sensitive in-

strument in gamma ray astronomy flown so far - the COS-B

satellite of ESA - could detect sources which were only a few

times weaker than the Crab . The next gamma ray mission in

space - the Gamma Ray Observatory GRO of NASA - cer-

tainly will make a big step forward: it will be able to detect

objects which are 20- to 100-times weaker than the Crab .

Still, gamma ray astronomy needs tremendeous efforts to

develop instruments which will bring the sensitivity limits to

those levels that were achieved in the other parts of the

electromagnetic spectrum .

In the first part of this article a brief overview is given

about approved future space programs in gamma ray astron-

omy. The main topic of this article, however, will be devoted

to GRO - the most powerful gamma ray mission in prepara-

tion. First, a detailed description of its instrumentation will

0920-5632/90/$03.50 © Elsevier Science Publishers B.V .(North-11olland)

be given and then the scientific objectives of the mission will

be discussed. In the last chapter the question will

addressed, what kind of instruments will be needed after

GRO.

2. FUTURE SPACEMISSIONS IN GAMMA RAY

ASTRONOMYThree different space projects in gamma ray astronomy

are now near its completion, and the launches of their

instrumentations are expected during the next year . The

three projects are listed in Table 1 .

FutureTABLE 1

Approved Gamma Ray Telescope Missions

SIGMA French/Sovjet 30keV launchCoded Aperture to date :scintillation 2 MeV 1989spectrometeronboardGRANAT

GAMMA I French/Sovjet >50MeV launchsparkchamber date:telescope in 1989Conjv,rlctinnwith a codedaperture mask

GRO US-multiinstru- 100keV launchment Gamma Ray to date :Observatory 30GeV 1990with participa-tion fromGermany, Hollandand SSD-ESA

36

SIGMA is a French experiment which will be flown on

the Sovjet GRANATmission (1). Celestial gamma rays are

detected in an 800 cm2 large, 1.25 cm thick NaI-detector

after they passed through a coded mask of Tungsten located

2.5 m away. The NaI-detector is operated in a similar way as

the Anger Cameras used in nuclear medicin. SIGMA hasa

field-of-view of 4°45' x 4°20' and an angular resolution (pre-

cision with which two point sources can be distinguished) of

10 arc minutes. Source locations can be determined to an

accuracy of 2 arc minutes. The energy resolution is 5 % to

14 % throughout the energy range (30 keV to 2 MeV) . The

main objective of SIGMA will be to map the gamma ray sky

and to locate the hard X-ray - soft -y-ray sources with an un-

precidented accuracy.The Sovjet-French GAMMA-1 instrument (2) is a spark-

chamber telescope like the previous SAS-2 and COS-B in-

struments. For background rejection it uses a Cerenkov de-

tector and a time-of-flight system along with the classical an-

ticoincidence system . GAMMA-I can be operated with and

without a coded aperture mask in front of the sparkcham-ber. The use of the mask results in an angular resolution

above 50 MeV of 1 to 2 degree . With its sensitive area of

1400 cm2 GAMMAI takes a middle position between COS-

B (z 50 cm2) and GRO-EGRET (z 1600 cm2), which will

be described later in this article.

As said earlier, the Gamma Ray Observatory GRO ofNASA is expected to make the next big step forward ingamma ray astronomy. Its four instruments OSSE, COMP-

TEL, EGRET, and BATSE have complementary propertiesand cover more than 5 decades in energy from about 100

keV to 30 GeV. OSSE is a collimated scintillation spectro-meter in the transition region between X-and -y-ray astron-

omy. The imaging Compton telescope COMPTEL will ex-

plore the 1 to 30 MeV range with an angular resolution of afew degrees within a large field-of-view of about 1 steradian .The sparkchamber experiment EGRET is devoted to highenergy gamma ray astronomy (20 MeV to 30 GeV), andBATSE is a scintillation detector instrument for the study ofgamma ray bursts and transient events . The free flyingGamma Ray Observatory is now planned to be launched insummer 1990 by the Space Shuttle into a low earth orbit of28.5° inclination. During the first year of the mission a com-plete sky survey will be performed. After that time selected

V. Sch6nfelder/Future astrophysics space missions

objects will be studied in more detail.

In addition to the three projects of Table 1, which all

contain gamma ray "telescopes" in the real sense of the word

"telescope" (they allow to locate sources in the sky), there

exist a few additional approved space missions which are

devoted to gamma ray burst astronomy, only. In most cases

these instruments are omnidirectionally sensitive and do not

provide directional information.A cosmic gamma ray burst

is identified by a sudden increase of the count-rate in the

burst detector . Such burst detectors a priori provide infor-

mation on the burst intensity, on the burst spectrum, andon

the development of the burst with time. Directional infor-

mation on the burst can only be derived by triangulation, if

the burst was seen by more than one of these detectors si-

multaneously .

An example ofsuch ornnidirectionally sensitive burst de-

tectors is the HUS-instrument onboard of the ESA mission

Ulysses (formerly known as the International Solar Polar

Mission), which consists of a small CsI-crystal and which will

allow the study of bursts in the energy range 15 to 200 keV

(3). The HUS-instrument is a French-German-Dutch colla-

boration.

Other scintillation burst detectors will be onboard of the

Sovjet GRANAT mission: these are the French Phebus in-

strument, which consists of 6 BGO detectors around the pe-

riphery of the spacecraft to cover the entire field-of-view

(energy range: 100 keVto 100 MeV), and the Sovjet Konus

instrument consisting of 7 detectors again distributed

around the spacecraft (energy range 20 keV to 2 MeV) .GRANAT also contains a narrow field burst telescope, call-

ed Sunflower, consisting of 4 X-ray proportional conters (2

to 20 keV) with a field-of-view of 6° x 6°, and two optical te-

lescopes with 5° x 5° fields-of-view . Sunflower can be slewed

automatically into the direction of any burst localized by

Konus.

Omnidirectionally sensitive cooled Germanium burst de-

tectors, which will provide the fine spectral resolution which

is necessary to perform line spectroscopy of burst spectra,

will be flown on the WIND-spacecraft - a component of the

Global Geospace System which is part of the International

Solar-Terrestrial Program (ISTP), and on the Mars Orbiter

Mission (to be launched in 1992). The latter detector is pri-

marily designed to perform gamma ray line spectroscopy of

the Martian surface in the 40 keV to 10 MeV region froman orbital altitude of 360 km, but can be used for burstspectroscopy as well .

In addition to these omnidirectionally sensitive burst de-tectors there will be a Danish rotation modulation collima-tor instrument (WATCH), which allows burst locations, on 2different spacecrafts: the first one will be on the first Eu-reca spacecraft to be launched by the Shuttle, and the sec-ond one will be again on GRANAT.

3. THEGAMMA RAYOBSERVATORY GRO:ITS INSTRUMENTATION

The Gamma Ray Observatory GRO is one of 'heGreat Observatories" which presently are in the phase ofplanning and development in the USA. The Hubble SpaceTelescope and GRO, both are in the final phase of deve-

lopment and hopefully will have been launched by 1990 bythe Space Shuttle. The other two observatories (TheAdvanced X-Ray Astrophysics Facility: AXAF, and theSpace Infrared Telescope Facility: SIRTF) both are still inthe planning phase. The four great Observatories will coverthe electromagnetic spectrum from infrared to gamma rayenergies and will lead to an enormous progress in spaceastronomy.

Since the Gamma Ray Observatory GRO is the first sa-tellite mission that covers the entire gamma ray range fromabout 100 keVto GeV(more than five orders of magnitudein photon energy), simultaneous observations over the full

dynamic range will be possible. The coverage of such abroad spectral range cannot be achieved by one single in-strument . Therefore, GRO will contain the 4 different in-

struments listed before, which have complementary proper-ties .

Experience has shown that the gamma ray fluxes fromcelestial objects are in general very small. This is easily un-

derstood, because a 100 MeV gamma ray combines thesame energy in one single photon as about 1010 infrared

photons. Therefore, gamma ray telescopes must in general

be large in size, and long observation times are required .GRO will be one of the largest and heaviest astronomy

space projects ever built.

An artists view of GROwith its four instruments OSSE,

COMPTEL, EGRET, and BATSE is shown in Fig. 1 .


OSSE

COMPTEL + Z

EGRET

37

FIGURE 1Artists View of GRO. OSSE is on the left, EGRET on theright side, COMPTEL is in the middle. BATSE consists of Sdetectors, two at each corner ofthe observatory .

OSSE is the "Oriented Scintillation Spectroscopy Experi-ment" of the Naval Research Laboratory, and the North-western University (4). Its highest sensitivity is in the transi-tion region between hard X- and soft -y-radiation (0.1 to 10MeV).

COMPTEL is an imaging "Compton Telescope" in the

MeV-range. It is built by an international collaboration be-tween the Max-Planck-Institut für extraterrestrische Physik

in Germany, the Laboratory for Space Research in Lei-

den/Holland, the University of New Hampshire, USA, and

the Space Science Department of ESA (5) . Its nominal en-

ergy range is 1 to 30 MeV.

EGRET ("Energetic Gamma Ray Experiment Telescope")

is a sparkchamber experiment for high energy -y-ray astron-

omyabove 20 MeV. It is built by an international collabora-

tion between the Goddard Space Flight Center, USA, the

Stanford University, USA, and the Max-Planck-Institut fi~c

extraterrestrische Physik in Germany (6).

BATSE is the "Burst AndTransient Source Experiment" of

the Marshall Space Flight Center, the Goddard Space Flight

38

Center and the University of California, San Diego. It con-

sists of 8 single detectors - one at each corner of GRO, andoperates above 20 keV.

OSSE is a scintillation spectrometer with a collimated field-

of-view. A schematic view is shmvn in Fig. 2 .OSSE consists of 4 identical phoswich detectors of NaI

(T1) and CsI (Na) . A NaI annular shield together with theCsI portion of the phoswhich form the active shield of eachdetector . A Tungsten passive collimator within the NaIannular shield defines the field-of-view ofeach detector .

Each detector is mounted in a single axis orientationcontrol system which provides offset pointing over a rangeof 192°. The detectors are generally operated in co-axialpairs. While one detector of a pair is observing the source,the other one can be offset to monitor the background. Af-ter typically 2 minutes the detectors will interchange obser-vation directions by opposite rotations .

FIGURE 2The Oriented Scintillation Spectrometer Experiment(OSSE) .

The characteristics of OSSE are summarized in Table 2.

V Sch6nfelder/Future astrophysics space missions

o

Energy range:

0.1- 10 MeV

o Collimatedfield-of-view :

3.8° x 11.4° FWHM

o Energyresolution :

8%FWHM at 662 keVand 3.2 % at6.13 MeV

o Background :

Measuredsimultanously by anidentical seconddetector with offsetpointing direction .

COMPTEL A schematic view of the COMPTEL detectorassembly is shown in Figure 3.

T

EE00.oN

TABLE 2Characteristics of OSSE

IMAGING COMPTON TELESCOPE'COMPTEL'

1700mm- ~{

FIGURE 3Detector Asât~mbly of the Imaging Compton Telescope(COMPTEL) .

COMPTEL consists of 2 detector arrays: an upper one ofliquid scintillator cells and a lower one of NaI-detectorblocks. Both arrays are entirely surrounded by anticoinci-dence shields of plastic scintillator. Infalling -y-rays are de-tected by a Compton collision in the upper detector and asecond, subsequent interaction in the low detector. Me-asuring parameters are the location of the interactions inboth detector arrays and the energy losses in these interac-tions. From these parameters the arrival direction of the in-falling -y-ray can be determined to lie on an event circle onthe sky.

In a certain sense COMPTELcan be compared with anoptical camera: the upper detector replaces the lense, inwhich the light is scattered, the lower detector replaces thefilm, in which the scattered light is absorbed. The charac-teristics ofCOMPTELare summarized in Table 3.

TABLE 3Characteristics of COMPTEL

o

Energy range:

1 - 30 MeV

o

COMPTEL is an imaging telescope with a wide field-of-view of 1 ster

o

Angular resolution within fov: 1D to 2.2° dependingon energy (1 o width ofGaussian profile)

o

Energy resolution :

6 to 9 %FWHM

o

COMPTEL is a low background instrument. Like ineach imaging system COMPTEL measures the back-ground simultaneous to the source observation.

EGRET is a sparkchamber experiment like the previous(much smaller) SAS-2 and COS-B experiments . Aschematicview of EGRETis shown in Figure 4.

The sparkchamber consists of two assemblies : an upperone of 28 chamber modules interleaved with Tantalum foilsof 0.02 radiation length within which electron-positron pairs

are produced, and a lower one with larger spacing, in whichthe electron trajectories can be further followed . The spark-chamber is triggered, if at least one of the electrons of the

electron-positron pair is detected by the directional time-of-flight coincidence system (consisting of an upper and a


lower scintillator plate, see Figure 4), and if there is no sig-nal from the large anticoincidence of plastic scintillatorwhich surrounds the sparkchamber assemblies . The tracksof the pair particles are registered electronically and areused to determine the direction of incidence of the primaryhigh-energy photon. The energy of the gamma ray will pri-marily be measured in a big Nal (TI) crystal below thesparkchamber assembly, where the pairs are absorbed. Thecrystal is 8 radiation lengths thick and has a size of 76 x 76cm2. The main characteristics of EGRET are summarizedin Table 4.

Energy range:

20 MeV to 30 GeV

Field-of-view:

maximum opening angle 45°

Angular resolutionwithin field-of-view:

2D at 100 MeV, 0.4" at 1 GeV

Energy resolution :

15 %, in the central part ofthe energy ranges .

TABLE4Main Characteristics of EGRET

ANTICOINCIDENCESYSTEMIMP) RESPONSIBILITY)

SPARK CHAMBER

TIME OF FLIGHTSYSTEM

BULKHEAD PEDESTAL

TOTAL ABSORPTION5HOIP!ER COUNTER(STANFORD UNIV 1

GAS REPLENISHMENTSYSTEM

FIGURE 4High Energy Gamma Ray Telescope (EGRET)

39

40

MIME is the only ORO instrument which is not a real te-

lescope, but an all sky monitor for bursts and other transient

events. It consists of 8 uncollimated scintillation NaI (TI) de-

tectors, one at each comer of the spacecraft (see Figure â).

A schematic view of one ofthe detector modules is shown in

Figure 5.BATSE

DETECTOR MODULE(1 OF 8)

FIGURE 5Burst and Transient Experiment (BATSE) Detector Module

Each module consists of a large area and a spectroscopydetector. The large area detector is a 20" 0, 1/2" thick NaI(TI) disk. A light collector housing collects the scintillationlight onto three 5" photomultipliers. A veto plastic scintilla-tor shield on the front side reduces the background. The lo-cation of a -y-ray burst on the sky is possible by comparingthe responses (count rates) of individual detector diskspointed towards different directions (see Figure 1). Thespectroscopy detector of each module is a 5" 0, 3" _thick NaI(TI) crystal, which is optimized for good energy resolutionand a broad energy range coverage.

BATSE will provide a burst trigger signal to the ocher 3GRO instruments, whenever a burst is detected . ThoughBATSE is concepted as an allsky monitor for burst and tran-sient events, observation and location of steady sources willalso be possible by means of Earth occultation .


The main characteristics of BATSE are summarized in

Table 5.

TABLE5Main Characteristics of BATSE

Large Area

SpectroscopyDetector Detector

Energyrange:

50keV-lMeV 20keV-30MeV

Field-of-

Full unocculted skyview:

Burstlocation

1° to 10° (depending onaccuracy:

intensity) :

Energy

6energy

7%FWHMresolution " bands at 662 keV.. .... .

Timeresolution:

2lusec (minimum)

The Gamma RayObservatory GROwill be a free flying

satellite. The total weight of GRO is about 15 000 kg; it fills

half of a shuttle payload bay. The instrument requirements

to the spacecraft are listed in Table 6.

TABLE6Spacecraft Support Requirements

Scientific Pay_ loadWeight

6000 kilog-sms

Instrument Power

750 watts

Experiment DataRate

23 kilobits/sec

Pointing Accuracy

±0.5°

Attitude Determina-tion

2 arcminutes

Absolute TimingAccuracy

0.1 milliseconds

The spacecraft must be capable of accommodating the6000 kg of the instruments and must supply 750 watts of in-strument power. The 23 kilobits per second of experiment

data will be supported by NASA's Tracking and Data RelaySatellite system . GRO is 3-axis stabilized. The pointing ac-curacy is ± 0.5°; however, the pointing direction will be

known at any time to an accuracy of 2 arcminutes. Absolutetime will be accurate to 0.1 msec to allow pulsar studies .

OROwill have a nominal circular orbit of about 450 kmand 28.5° inclination . This orbit guarantees a mission lifetime of at least 2 years and at the same time provides a lowbackground environment. On the other hand about 50 % ofthe observation time will be lost due to occultation of thefields-of-view of the instruments by the Earth. ORO has a

self-contained propulsive system to enable the spacecraft to

maintain the 450 km orbit for even more than 2 years. in

addition, this system will allow the spacecraft to undergo acontrolled re-entry at the end ofthe mission.

As can be seen from Figure 1, the two instrumentsCOMPTEL and EGRET are fixed to the GRO platform

such that both have the same viewing direction (telescopeaxis is in Z-direction) . OSSE's prime target normally also is

in the Z-direction . However, due to its orientation controlsystem OSSE can move the field-of-view of the detector to

secondary targets, while the GRO platform remains fixed.

The observation time of GRO will be 2 weeks per viewing

direction. During the first 15 months of the mission a com-plete sky survey is foreseen . For this task 32 different point-

ings are foreseen . The sequence of the survey is constrainedby various aspects like sun position, visibility of secondary

source candidates of OSSE, and effective observation time

of COMPTELwhich is limited by the influence of the bright

shining horizon. During the second and further years of the

mission detailed observations of selected objects are fore-

seen. The GRO observations will be complemented by

ground based observations, which already now are planned

for some promising targets at radio and very high/ultra high

gammaray energies.

4. ASTROPHYSICS WITH GRO

Due to the 10- to 100-times higher sensitivity of the

GRO instruments in comparison to previously flown in-


struments a significant progress in the exploration and un-derstanding of the gamma ray sky can be expected fromGRO.

Themain targets ofinterest will be :

1 . Discrete sources in the Galaxy that show steady con-tinuum gammaray emission.

2. The diffuse continuum gamma-ray emission from inter-stellar space.

3. Discrete and extended sources which show gamma rayline emission.

4. External galaxies, especially Seyferts and Quasars.5. The diffuse cosmicgamma-ray background .6. Cosmic gamma ray bursts (their localization and the

study of *L--*---------

11ectra and time histories).7. TheSun (solar flare gamma ray and neutron emission).

These topics are now discussed in more detail . Based onthe sensitivities ofthe GRO instruments and on our presentday's knowledge it is estimated what might be expected fromGRO.

Galactic Gamma-RaySources

At present the key question in this field is: "What kind ofobjects do we see as gamma-ray sources in the Galaxy?"

The most promising candidates are radio pulsars and X-ray binaries. Both kinds of objects contain stars in the end-

stage of their evolution, namely neutron stars, and in case of

binaries perhaps black holes. Other known sources are the

Galactic Center and the unidentified COS-B sources.

log OAv/k*V)

-1 .6 -3 0 3 6-, ,

- vas-pid.a

T"rT

7 10 13 %

22 2S

log WHO

9

-920

4 1

FIGURE 6Luminosity spectra of Crab and Vela pulsars . From Bignamiand Hermsen (8)

4,

Figure 6 shows the luminosity per decade of the Crab-

and Vela pulsar. Both pulsars radiate about 5 orders of

magnitude more power at gamma ray energies than in the

radio band. If this is true in general for radio pulsars, then

the key for an understanding of the pulsar radiation me-

chanism may be found at gamma ray energies. GRO should

be able to see more than these 2 pulsars . Predictions about

the most promising pulsar candidates have been tried by

many authors. They are very difficult and strongly depend

on the pulsar models choosen. In order to perform the pul-

sar analysis with theGROdata :he pulsar parameters like p

and p have to be known. These will be derived from

correlated pulsar observations at radio wavelengths which

have been agreed with radio astronomers in Australia, in

England, and in the US .X-ray binaries are known to be very common X-ray

sources which are powered by accretion. A few of them

have already been observed at hard X-ray energies around

100 keV, especially by HERO-1 . Among these are CygX-1-

the best black hole candidate, and Cyg X-3 - the source

which attracted so much attention by its possible TeV and

PeVemission .Except for Cyg X-1 none of these sources so far could be

detected at MeV gamma ray energies. Cyg X-1 itself is, ofcourse, an extremely interesting object . The HEAO-3 obser-

vations (9) have shown that its gammaray emission is timely

variable, and Liang and Dermer (10) have recently propos-

ed a scenario, in which the MeV gamma ray emission is

produced in a 400 keV hot pair dominated central sphericalplasma around the black hole, whereas the hard X-ray emis-sion is supposed to come from a more extended disk struc-

ture with an electron temperature of 90 keV.Another extremely interesting binary system is PSR

1957+20 which recently attracted much attention. This sy-stem is supposed to contain an evaporating very low massivestar (11). It has been suggested that MeV gamma ray emis-

sion - powered by the neutron star in this system - is respon-sible for the evaporation process. This hypothesis makes thissystem to a very interesting object for gamma ray studies .

Another object of interest is the Galactic Center - one ofthe other most promising black hole candidates . First detec-tions of continuum gamma ray emission have been made


from the Galactic Center, already quite some time ago. The

2 spectra obtained by HEAO-3 (12) at 2 different times se-

parated by 1/2 year are shown in Figure 7. A strong MeV-

emission was seen only once (in fall 1979); in 1980 and 1982

no detectable MeV continuum emission existed. The

HERO-3 authors have interpreted their measured spectrum

by a 4-component model - consisting of a power law

spectrum, the 511 keVannihilation line, a three-photon con-

tinuum from positronium decay anda Comptonized thermal

component which accounts for the variable MeV-emission.

OROwill be able to study the MeV-emission with high pre-

cision. Monitoring the variations of the gamma ray

spectrum will be crucial for an understanding of the object

behind the source. This will be especially true, if GX 1+4 is

the source as recently suggested by McClintock and Le-

venthal (13) .

10W

.

.

y 10 -2

10 -3

s

10-4 1- . ..001

-HE A0-Ctall 1979

---HEAD - C

spring 1960 i

upper limit of 1this experi - rTment 1991

0

. . . . . I

. 1 . . .I

E y 1MeV1

FIGURE 7Gamma ray spectrum of the Galactic Center Region asmeasured By HEAO-3 (12) ; see (16) .

Finally, of course, the puzzle of the unidentified COS-B

sources has to be solved. Though, about half of the originally

22 objects contained in the COS-Bsource catalogue are now

kn-~wn to be simply regions of enhanced interstellar matter,

about half a dozen of these sources remain unidentified till

now. Nobody at present knows the nature of these sources!

Do they represent a new class of celestial objects which

mainly radiate at gamma ray energies? Observations of the

unidentified COS-B sources with higher sensitivity and atlower gamma rays mayhelp their identification .

In addition to the kinds of objects discussed so far, alsoother objects - like Supernova remnants and molecular

clouds will be studied by GRO as well . From previous ob-

servations we know already that both these kinds of objects

are very promising targets.The prospects of GRO for studying galactic gamma ray

sources can be judged from Figure 8. Here the sensitivities

of the 3 instruments OSSE, COMPTEL, and EGRET over

their energy ranges are compared with the fluxes of knowngamma ray sources, e.g. the Crab, Cyg X-1 in its low and

high intensity state, and the COS-B source Geminga. The

intensity level of the weak X-ray binaries detected by

HERO-1 around 100 keV is also indicated.

Around 100 keV OSSE will be able to detect sources

which are 103-times weaker than the Crab . OSSE will study

the energy spectra of X-ray binaries detected by HEAD-A4,and will certainly detect newones.

Between 1 and 30 MeV COMPTEL will be able to de-

tect sources which are about 20-times weaker than the Crab.

Cyg X-1 and the Galactic Center can be observed with high

precision . Intensity variations of the high intensity states can

be recorded on a very short time scale . The prospects are

good that COMPTEL will see some of the X-ray binaries

and the unidentified COS-B sources. The sensitivity of

EGRET to some degree depends on the location of the

source within the Galaxy. On average, EGRET is 10-times

more sensitive than COS-B.

Diffuse Galactic Continuum Gamma Ray Emission

The only complete survey of the galactic plane in the

gamma ray range was performed by COS-B at energies

above 50 MeV. From these measurements and those by

SAS-2 it was possible to derive the gamma ray emissivity in

interstellar space throughout the plane. It is now generally

agreed that the diffuse galactic gamma radiation above 35

MeV mainly consists or a 7r=decay component from nuclear

reactions of cosmic ray protons (and heavier nuclei) with in-

terstellar matter, and an electron-induced component which

is produced as bremsstrahlung with interstellar matter and -

to a smaller extent - by inverse Compton collisions with

2.7 K

blackbody,

infrared

and

optical

photons.

By

V. Schönfelder/Future astrophysics space missions

correlating the gamma ray measurements with the galacticinterstellar matter distribution it was possible to infer thedistribution of cosmic rays (electrons and protons)throughout the Galaxy. We now know that the cosmic raydensity is not constant throughout the plane, but higher in

the inner pe.rt and lower in the outer part of the Galaxy

(14) . COMPTEL will for the first time extend the survey:owards lower energies down to about 1 MeV. This willallow a detailed study of the electron induced gamma raycomponent.

N

YN

10

\ 103

10 -5

43

0.1 1 10 102 103

Energy 1 MeV

FIGURE 8

Sensitivities of OSSE, COMPTEZ, and EGRET to detectgalactic gamma ray sources during 2 weeks ofobservation.

So far, only a few measurements of the galactic emission

at low gamma ray energies exist towards special directions.

Figure 9 shows acompilation of measurements between 100

keV and 2 GeV towards the center region of the plane. the

spectrum can be fitted by a three - component model con-

sisting of a 7r=decay, a bremsstrahlung and an inverse

Compton component, neglecting, however, a possible con-

tribution of unresolved sources. Below - say 20 MeV - the

gamma rays according to this model are mainly produced by

electrons only . From such measurements it is, therefore,

possible to derive the energy spectrum of cosmic ray elec-

trons in interstellar space in an energy range (below 70

MeV), which is unaccessible to radioastronomy and to di-

rect particle measurements (because demodulation theories

are too uncertain to derive the demodulated spectrum in in-

terstellar space) . Our present day's knowledge of the local

44

interstellar cosmic ray electron spectrum as derived fromthe few existing gamma ray measurements is illustrated inFigure 10. The present upper limit in the 1 to 10 MeV rangeis still above the theoretical predictions, e.g. from Ip and Ax-ford (15).

The expected contributions by GRO to this researchfield are two-fold: first EGRET will repeat the high energysurvey with about 10-times higher sensitivity and somewhatimproved angular resolution, and second, COMPTEL willestablish the first survey of the plane at low gamma ray en-ergies down to 1 MeV - thus allowing a detailed study of theelectron-induced gamma ray component .

Measurements at different galactic latitudes will help toseparate the two electron-induced components: the brems-strahlung and the inverse Compton component .

v

c0

0

a

10 2

10 0

10 -2

10 -4

10 -6

10-0

10 -10

0.01 0 .1 1 10 100 1000y (eV)

FIGURE 9Broad band gamma ray emission of the galactic plane fromthe Galactic Center Region (16) .

V. Schdnfelder/Future astrophysics space missions

The high sensitivity of both instruments combined withthe good angular resolution will hopefully separate the sofar unresolved source component from the really diffusecomponent. It is expected that the measurements will leadto a better understanding of the relationship between thedynamic structure of the Galaxy, the interstellar matter dis-tribution, and the cosmic ray density.

NelY

W

0

0

100

0'

10-3

this experiment

(based an best fit model cfSather A Schonfelder.641

10 100 1000--

energy 1 nev 1

FIGURE 10

Local Interstellar cosmic ray electron spectrum between 1and 1000 MeV (16).

Galactic Gamma Ray Line SpectroscopyORO will for the first time provide the opportunity to

perform surveys of the galactic plane in the light of gammaray lines. Lines are expected from interstellar space andfrom discrete sources . The line surveys will allow to separatethese two components and therefore put the discussion onthe origin of the line-emission onto solid grounds . This isespecially true for the two so far most widely discussed linesat 511 keV from electron-positron annihilation and at 1 .8MeV from radioactive decay of 26A1, However, it is ex-pected that other gamma ray lines will be found in addition.The gamma ray lines to be investigated are either produced

by nucleosynthesis products or by nuclear reactions of en-ergetic particles with matter. The most important isotopicdecay chains from nucleosynthesis processes are those listedin Table 7. Whereas the first four decay chains may be ob-served in single events (novae or supernovae), the last twoare observable only goibally in interstellar space. Gammaray lines from nuclear reactions of energetic particles withinterstellar matter have been extensively discussed by Ra-maty, Kozlovsky, and Lingenfelter (17). The most promi-nent expected lines are at 4.4 MeV and 6.15MeV. Gammaray lines from nuclear reactions are also produced in dis-crete sources . Candidates are interstellar clouds (18), su-pernovae in clouds (19), and accreting compact objects (20) .The predicted line fluxes are generally low, of the order of10-5/cm2 sec or even lower.

TABLE 7Isotopic Decay Chains from Nucleosynthesis Processes

The sensitivities of OSSE and COMPTEL for detectinggamma ray lines from point sources are shown in Figure 11 .The sensitivities are nearly identical near 1 MeV, namely 210'5/cm2 sec . Below 1 MeV OSSE is more sensitive, above1 MeV COMPTEL has a higher sensitivity . The sensitivitiesof OSSE and COMPTEL for detecting diffuse gamma raylines in interstellar space are also given in Figure 11 . Thesesensitivities strongly depend on the angular bin sizes in ga-lactic longitude. For OSSE the largest possible angle (whichgives the highest sensititvity) is 11 .4°. For COMPTEL anyvalue within the field-of-view can be selected. Again, at 1MeV the sensitivities of OSSSE and COMPTEL (10° longi-

V. Schônfelder/Future astrophysics space missions 45

tude bin) are comparable:

10-4/cm2 sec rad. In case ofCO

L the sensitivities can be improved by a factor of3 for a40°-longitude bin (see Figure 11) .

With these sensitivities, CO

Lwill be able to mapthe 1.8 MeV gamma ray line from radioactive 26A1 alongthe entire galactic plane from the center to the anticenter(21). OSSE is more suitable for mapping the 511 keV anni-hilation line, however, due to its small field-of-view it willconcentrate predominantly on mapping the region ar,the Galactic Center.

The next most promising gamma ray line is probably the1.156 McWine from the 44Ti = = > 44Ca decay. Due to its68 year decay time this line should be visible from the fewmost recent supernovas. With the GRO point-source sensi-tivity of 2 * 10-5/cm2 sec a map ofthe galactic plane at 1.156MeV therefore should show a few - say half a dozen pointsources.

External GalaxiesIn the extragalactic sky there is a good chance that a few

nearby normal galaxies will be seen by GRO. In case of thesmall and the large Magellanic Cloud the gamma ray emis-sion may even be strong enough to allow a crude measure-ment of the structure.

The most interesting objects in the extragalactic sky,however, are the nuclei of active galaxies . At least some ofthem seem to have their maximum of luminosity in therange between several 100 keV and a few MeV, and gammaray observations therefore seem to be essential for an un-derstanding of the central source in AGN's . Detection ofMeV gamma radiation has been reported from the Seyfertgalaxies NGC 4151 and MCG 8-11-11, and the radio galaxyCentaurus A. All three nuclei show spectra which seem tobe highly variable in intensity as well as in spectral shape. Athigh energies around 100 MeV so far only the secondnearest quasar 3C273 has been seen in gamma rays. An in-terpolation between the existing X-ray measurements andthe COS-B gamma ray measurements suggests that the qua-sar 3C27? also has its maximum of luminosity at a few MeV.

The situation is illustrated in Figure 12, where the lumi-nosity spectra of Cen A (22) and 30273 (23) are comparedwith the sensitivities ofOSSE, COM

L, and EGRET.

Decay Chain Mean Life (yrs) Emission

36Ni -->36Co -06Fe 0.31 e+0.847 MeV1.238 MeV

57Co ->37Fe 1.1 0 .122 MeV0.014 MeV

22Na ->22Ne 3.8 e+1 .273 MeV

44Ti ->44sc ->44Ca 68 e+1 .136 MeV0.078 MeV0.068 MeV

60Fe _>60Co _>60Ni 2 .2 x 106* 1 .332 MeV1 .173 MeV0.039 MeV

26A1 ->26M g 1 .1 x 106 e+1 .809 MeV

46

u

10-1

10 -2

10 -3

10 -4

3T- POINTSOURCESENSITIVITY

(

T = 106 sec )

OSSE

610 102 10 3 10 4ENERGY (K eV)

Also indicated are the hard X-ray spectra of the 12AGM's (mostly Seyferts) detected by FIEAO-1 (24) up toabout 100 keV . Clearly, all three GRO-instruments are verypowerful telescopes to study these AGM's. The 12 AGM'sdetected by HEAO-1 and numerous still undiscoveredactive galaxies will be seen by OSSE around 100 keV. Theirdetectability by COMPTEL and EGRET will depend on thecontinuation of their energy spectra towards higherenergies . If they have spectra similar to Cen A and 3C273,then COMPTEL will be able to study - say a dozen or evenmore of them . We may get the first measurement of theluminosity function of AGM's at MeV-energies . Thisfunction together with the measured properties of individualgalaxies may lead to a better understanding of the enginethat powers these objects.

V. Schönfeider/Future astrophysics space missions

3cr- GALACTIC PLANE

SENSITIVITY

(T =10 6 sec )

103 10 4ENERGY (KeV)

FIGURE 11COMPTE. and OSSE sensitivities to detect gamma ray line emission (2 weeks observation period). Left side: for point sources.Right side : for extended emission in interstellar space .

FIGURE 12Sensitivities of OSSE, COMPTEL, and EGRET to detectActive Galactic Nuclei .

It has been suggested that the 511 keV annihilation linemaybe produced in AGN's at a detectable level. For distant

galaxies the line will be redshifted towards lower energies.

In case of 3C273 (Z = 0.158) the line would be centered at430 keV, well detectable by OSSE .

With some luck the nucleosynthesis line at 847 keV from

the 56Ni = = > 56Co = = > 56Fe decay chain (Table 7) may

be seen from Type I supernova explosions in the Virgo clus-

ter. Within 10 Mpc typically 1 Type I SN per year is ex-

pected (25), and the predicted peak flux of the line at this

distance is 6 - 10 -5/cm2 sec. In case of SN 1987a in the large

Magellanic Cloud the 122keV line from 56Co-decay has the

best prospects of being detected, if the GRO launch is in

1990.

N 10-1

10-3

10-4

10 -1

Fukada et al.Schönfelder et al .White et al.

/Trombka et al.

Annihilation

Fichtel et al . SAS-1

HEAD A-1 Garmire et al .Marshall et al.;

610

1keV

10

100

1MeV

10

10,0 ,Energy normal

galaxies

FIGURE 13Cosmic Gamma RayBackground Spectrum.

TheCosmic Gamma RayBackground

The study of the diffuse cosmic gamma ray background

belongs to the main objectives of GRO. Its origin is not yet

really understood. At present there seem to be two classes

of models to explain the gamma ray background . Either the

observed background is simply the sum of unresolved active

galaxies. As indicated in Figure 13 by the curve labelled

"Seyferts", this seems to be possible above a few hundred

keV. In the other classes of models the gamma ray back-


ground is really diffuse in nature. The probably most fasci-nating model of this kind is the one proposed by Stecker in(26) which is based on the idea that the bump at MeV ener-gies is explained by matter-antimatter annihilation in abaryon surnmetric universe containing superclusters ofgala-xies ofmatter and others of antimatter.

47

Adecision among these two alternatives at present is notpossible . GRO may provide the answer . First of all, GROwill perform a precise measurement of the cosmic back-

ground spectrum . Secord, if GRO will detect a few dozensof active galaxies at gamma ray energies, a realistic estimateof the contribution of unresolved galaxies to the cosmic

gamma ray background should be possible . After subtractingthis component from the measured spectrum one will see, if

a really diffuse component is left, and if it still has the shape

required by the annihilation model of a baryon symmetric

universe. Observations of angular fluctuations should also

help to decide between the two alternatives: the fluctuation

from unresolved point sources are expected to be different

from the ridges expected from matter-antimatter bounda-

ries.

On top of the cosmic gamma ray background continuum

there may be broad cosmologically reshifted features with

relatively sharp edges at the rest energies of the gamma ray

lines generated by the decay of 56Ni = = > 56Co == > 56Fe

during 56Fe formation in the universe (27). OSSE and

COMPTEI. have the capability to detect some of these fea-

tures.

Gamma Ray Bursts

Though discovered already 15 years ago the gamma ray

bursters are still mysterious sources of gamma ray emission .

The outbursts of these sources are only very short: a few

tenth of a second to tens of second; however, during the

short outbursts the bursters are the brightest gamma ray

sources in the sky. The nature of these objects is not yet re-

ally understood, but here is strong evidence that a neutron

star is somehow involved. The evidence comes from the

spectra of the bursts . Many bursts show an absorption fea-

ture in their energy spectra near 20 to 50 keV, which is in-

terpreted as a cyclotron line . Furthermore, a few gamma ray

bursts (= 7 %) show an emission line somewhere between

48

400 to 450 keV, which is interpreted as redshifted annihila-

tion line. The physical trigger for the outbursts of the neu-

tron star is not yet really understood . Various possibilities

are presently discussed. Among these are starquakes, ma-

gnetic instabilities near the surface of the neutron star or

accretion of matter onto the surface of the neutron star

either from interstellar space or from a companion star. In

:he latter case the material is heated by the accretion pro-

cess and may lead to an explosion after some reservoir of

accreted matter has reached a critical mass (nuclear flash

model).It is quite clear that further observations are needed to

confirm the neutron star hypothesis and to distinguish

among these models. GRO is expected to provide this in-

formation. The main GROburst instrument BATSEwill:

locate the bursts within a few degrees, so that an identi-

fication maybecome possible

measure the celestial distribution of bursts

detect weaker bursts than has been possible before

(down to fluences of 10 -7 erg/cm2)performgamma ray line spectroscopy ofburst spectra

measure short time fluctuations and spectral variations .

The capability to perform gamma ray line spectroscopyofgamma ray bursts was considered to be so important thatthe additional spectroscopy detectors of BATSE were intro-duced still years after the definition of the 4 main GRO in-struments. The sensitivity of BATSE for detecting gammaray lines is illustrated in Figure 14 .

On the ordinate the line intensity is expressed in units ofthe fractional equivalent width (equivalent line width nor-malized to continuum devided by centroid energy of theline). BATSE will be able to detect gamma ray lines whichare 1 to 2 orders of magnitude weaker than previously de-tected lines (see Figure 14).

All the other GRO instruments have burst capabilities aswell . After receiving a burst trigger from BATSE they alsorecord the energy spectra of bursts. Of special importanceare the capabilities of COMPTEL to detect bursts in thedouble scatter mode, which happen to be in the COMPTELfield-of-view, and thus locate the burst sources to the order

V. Schônfelder/Future astrophysics space missions

of one degree. EGRETwill be able to measure the high en-

ergy end of burst spectra in its big NaI-spectrometer up to

about 40 MeV.

W

0

0

WJ icr ~âvWJe0

14-

sQ SENSITIVITY To LINES13 SECOND INTEGRATION)

ENERGY-10-60r9/C01"t00/y1)

Io'®®r®/eo81-50/yd

10'°or®lao"('10/yr) ,

\10''ero lcotIp 1 /yr I

OMOMMeue fi

10'1110

10"

lo'ENERGY DOW

FIGURE 14BATSE sensitivity to spectral lines. Expected frequency ofbursts above the indicated strength is given in parenthesis.

Study of Gamma Ray- and Neutron Emission of the Sun

during Solar Flares

The Sun is not a prime target of GRO. However, GROstill has several capabilities to study the Sun during solar fla-

res. If the Sun happens to be within about 30° from the

GRO Z-axis, it will be within the COMPTEL and EGRETfield-of-views. If the BATSE onboard location algorithm lo-

cates a burst as a solar one, a solar burst trigger is sent to

OSSE and COMPTEL: OSSE can then make use of its off-set capabilities and directly look at the Sun (if possible), and

COMPTEL can switch into a so called "solar-neutron-mode", which allows recording of solar neutrons in additionto gamma rays (28) . Also OSSE has neutron detection ca-pabilities: it separates neutron-induced from gamma rayinduced events by means of pulse-shape-discrimination .Finally, EGRET and COMPTEL are able to record high re-solution spectra of the flare gamma ray emission in theirbottom NaI detectors.

Gamma ray- and neutron emission of the Sun allow to

study the acceleration and subsequent reactions of high en-

ergy nuclei produced during flares. Whereas the continuum

X- and gammaray emission is produced by electron-inducedprocesses (bremsstrahlung) mainly, the gamma ray line- and

neutron emission originates from interactions of the

nucleonic component. During the last 8 years more than 100

flares with gamma ray emission have been observed by the

gamma ray spectrometer of the Solar Maximum Mission

(SMM). Many flares show gamma ray line emission in addi-

tion to the continuum emission . Themost intensive lines are

the 2.2 MeV neutron capture line of deuterium, the 511 keV

annihilation line, and the4.4 MeVand 6.15 MeV lines from

excited Carbon and Oxygen nuclei . Whereas the latter two

lines are emitted promptly and thus reflect (like the conti-

nuum) the temporal dependence of the acceleration mecha-

nism, the 2.2 MeV- and 511 keV-lines are emitted some-

what later (because the neutron first has to be captured to

produce the 2.2 MeV-line, and the positron has to annihilate

with an electron to produce the 511 keV line).

The sensitivities of the GRO- experiments for detectinggammaray emission from solar flares are comparable to the

sensitivity of the SMM-spectrometer . During the GRO

mission we therefore can expect to study a large number of

additional flares. The gamma ray- and neutron measure-

ments in conjunction with those of other instruments (inter-

planetary particle detectors, ground based neutron moni-

tors) provide the best possible observation of acceleration,

production and propagation aspects during solar flares.

5. THEFUTURE AFTERGRO

From the previous discussion it is quite clear that a very

significant improvement in the exploration and under-

standing of the gamma ray sky can be expected during the

next few years. Certainly, also unforeseen phenomena will

be discovered as was always the case when a new field was

explored. The new results will raise new questions, and cer-

tainly improved instrumentations will be needed then to get

these questions answered .

There are already quite firm ideas and concepts what is

needed for the next generation of gamma ray astronomy

telescopes . First of all, a high resolution spectroscopy mis-

sion is needed. This field is neither covered by SIGMA nor

by GRO. Therfore, an additional effort is needed here. High

resolution spectroscopy is important for two reasons: first,


it allows to resolve and identify gamma ray lines, andsecond, it allows a precise measurement of the gamma rayline profiles and the central energy of a line. These lattertwo parameters are essential to interprete the physical en-vironment of the emitting region . Parameters like gas tem-perature, gas turbulence, expansion or orbital motion of theemitting region, recoil energy of excited nuclei, all have animpact on the line profile or the central energy. For the un-derstanding of the physical environment the shape of the

line profile is at least as important as the flux itself. In thelight of this discussion efforts are being made in the US, inEurope, and in the Sovjet Union to get a high resolution

gamma ray spectroscopy mission approved .A general problem of all presently existing gamma ray

telescopes is its poor angular resolution. Efforts are needed

- and actually are being made - to develop instruments

which have resolutions in the arc minute range or even bet-

ter. After the survey results of SIGMA and GRO will be

available there will be a strong need of such telescopes for

the identification of the sources.Finally, there is also a strong need for a mission which

will be dedicated to the broad band study of gamma ray

burst sources - covering simultaneous observations in the

optical, X-ray and -y-ray range. The source location accuracy

provided by such a multi instrument mission will be crucial

for an understanding of the objects behind the gamma ray

bursters .

In summary, it can be concluded that gamma ray astron-

omy hasnowreached a stage where it becomes attractive to

the whole community of astronomers and astrophysics. It

can be expected that the results from the forthcoming mis-

sions - especially when combined with information from cor-

related observations in other spectral ranges from both

space- and ground based observatories - will stimulate ex-

tensive theoretical work in the 1990's .

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Future astrophysics space missions in gamma ray astronomy