Nuclear Physics B (Proc. Suppl.) 33A,B (1993) 113-125 North-Holland

,~wtq.r;~w,m..j[a. PROCEEDINGS SUPPLEMENTS

An overview about initial results from the Compton observatory GRO

V. Sch6nfelder

Max-Planck-Institut fiir extraterrestrische Physik, D-8046 Garching bei Miinchen, FRG

Gamma-ray astronomy provides a diagnostic tool to study high energy processes in the Universe. At present, NASA's Compton Observatory is performing the first complete sky survey in gamma- ray astronomy. The observatory was launched on April 5, 1991 by the Space Shuttle Atlantis into a near-earth orbit. An overview about the first highlight results from this mission is given. Special topics of presentation are: pulsars (especially the Crab and Vela pulsars), the diffuse galactic gamma-ray emission from interstellar space, the nuclei of active galaxies (especially quasars), the 511 keV and 1.8 MeV gamma-ray line emissions from the central region of the galaxy, the puzzle of cosmic gamma-ray bursts and finally, the Sun during solar flare acitvities.

1. INTRODUCTION

Gamma-ray astronomy is at present making a major step forward. This progress is due to NASA's Gamma Ray Observatory GRO, now called Compton Observatory, which was brought into a near-Earth orbit by the Space Shuttle 'Atlantis' on April 5, 1991. Gamma- ray astronomy is now about 25 year old: The first detection of cosmic gamma radiation was made by OSO-III in 1967.

Since gamma radiation represents the most energetic part of the electromagnetic spectrum it provides information about the most ener- getic processes and phenomena in the Universe. From previous gamma-ray missions we know that the gamma-ray fluxes from cosmic objects are extremely small. This is easily understandable, since a single 100 MeV gamma-ray photon has the same energy as 101° infrared photons together.

In order to make these low fluxes de- tectable, the telescopes must have large collecting areas, high detection efficiencies, and the exposure times must be long (weeks or even sometimes months). In so far it is not

surprising that the Compton Observatory is the heaviest scientific payload ever brought into space by a shuttle.

2. THE GAMMA RAY OBSERVATORY GRO

The Gamma Ray Observatory GRO is the first satellite mission that covers the entire space astronomy gamma-ray range from about 100 keV to 30 GeV, more than five orders of magnitude in photon energy. Therefore, simultaneous observations over the full dynamic range are possible. The coverage of such a broad spectral range cannot be achieved by one single instrument. Instead, GRO carries four different instruments with com- plementary properties.

A schematic view of GRO is shown in Fig. 1. The platform carries the three major instruments OSSE, COMPTEL and EGRET (from left to right) and the fourth instrument BATSE, which actually consists of 8 detectors - two at each corner of the spacecraft.

0920-5632/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.

114 V. Schfnfelder /Initial results from the Compton observatory GRO

The Gamma-Ray Observatory (lOO keV to 30 GeV)

OSSE

100 keV - 10 M c V

3 .8* x 11.4*

COMPTEL

1 to 30 M e V

l O t o 2 *

within 64 °

F W l / M

EGRET

20 M e V to 30 GeV

0 , 4 " to 2 °

wi thin 4 5 "

F W I / M

/

BATSE (8)

20 k~V to 30 M e V

/

Figure 1. Schematic View of GRO

OSSE is a collimated scintillation spectro- scopy experiment in the transition region be- tween hard X-ray and low energy gamma-ray astronomy, namely between 100 keV and 10 MeV. The collimated field-of-view is 4 x 11 degrees (Kurfess et al., 1983). COMPTEL is a

Compton telescope in the energy range 1 to 30 MeV. It has a wide field-of-view of about 1 steradian and an angular resolution within this field of 2.5 to 5 degrees FWHM (SchSnfelder et al., 1984, 1992). EGRET is a sparkchamber experiment in the high energy range from

V SchSnfelder/Initial results from the Compton obsen,atory GRO 115

20 MeV to 30 GeV. Its field-of-view is about half a steradian and its angular resolution of the order of 1 degree (Fichtel et al., 1983). BATSE is an all sky monitor for burst and

transient source events in the energy range 20 keV to 30 MeV (Fishman et al., 1985). Table 1 lists the institutes under whose re- sponsibilities the four telescopes were built.

Table 1 The four GRO telescopes

OSSE (Oriented Scintillation Spectroscopy Experiment)

Naval Research Laboratory (USA) North Western University (USA)

COMPTEL (Compton Telescope)

Max-Planck-lnstitut L extraterrestrische Physik (FRG) Laboratory for Space Research (The Netherlands) University of New Hampshire (USA) Space Science Department of ESA (The Netherlands)

EGRET (Energy Gamma Ray Telescope Experiment)

Goddard Space Flight Center (USA) Max-Planck-Institut f. extraterrestrische Physik (FRG) Stanford University (USA)

BATSE (Burst & Transient Source Experiment)

Marshall Space Flight Center (USA) Goddard Space Flight Center (USA) University of California, San Diego (USA)

The mission concept of GRO is illustrated in Figure 2. The Shuttle Atlantis carried the Observatory into a 450 km orbit (28.5 day inclination). During the second day of the Shuttle mission the Astronauts onboard re- leased the Observatory. It will stay in orbit for at least 2 years. GRO has a self-contained propulsion system to maintain the 450 km or- bit. In addition, the system will allow the 15 ton spacecraft to undergo a controlled re- entry at the end of the mission.

The GRO mission started with an all-sky survey, the first in gamma-ray astronomy. The survey began on May 16, 1991 and will last until November 15, 1992. After the survey, selected celestial objects will be studied in more detail. The gamma-ray observations are complemented by ground based observations in other spectral ranges. In the next chapter a description of some of the most important results of the Compton Observatory is given.

116 V. SchiJnfelder / bzitial results from the Compton observatory GRO

At TI TUDE ~ GRO MAINTAINS ORBIT' ' 45Ditto \ FOR SCIENCE MISSION i 243 ~ ~ ~ ' ~ 460-440 km .... ..... \ /----"

OBSERVATORY x . i AND AI~PENOAGES ~ ] OPTION B: REPAIR/

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Figure 2. GRO Mission Concept

3. INITIAL SCIENTIFIC RESULTS FROM THE COMPTON OBSERVATORY

3.1. Gamma-ray pulsars Prior to the launch of GRO, radio pulsars

were the only class of compact objects within the Milky Way that were known to contain gamma-ray sources. Pulsed gamma radiation from the Crab and Vela pulsars had been seen by the previous gamma-ray astronomy projects SAS-2 and COS-B and a few balloon-borne instruments. The question was, whether GRO would be able to detect pulsed emission from more than these two pulsars. The answer is "yes". Up to now (July 1992) GRO has de- tected pulsed emission from three further pulsars. These are:

PSR 1509-58 PSR 1706-44 Geminga

(P = 151 msec) (P = 102 msec) (P = 237 msec).

PSR 1509-58 was discovered at gamma-ray energies by BATSE below ~ 1 MeV. OSSE has confirmed this detection. COMPTEL and EGRET were so far unable to observe any pulsed emission from this object above I MeV.

PSR 1706-44 is a radio pulsar, which was discovered only 2 years ago during a radio pulsar survey. Its pulsed gamma-ray emission was discovered by EGRET. This object coincides with one of the so far unidentified COS-B sources.

The Geminga source is one of the most intense high-energy gamma-ray sources in the sky (at energies above several 100 MeV it is more intense than the Crab). It was discovered in 1973 by SAS-2 and later studied in great detail by COS-B. The gamma-ray source had been tentatively identified with the Einstein X- ray source 1E 0630+178 (Bignami et al., 1983), and this object was recently found to be

14. Schfnfelder /Initial results from the Compton obsen,atory GRO 1 1 7

a 237 sec X-ray pulsar (Halpern and Holt, 1992). With the knowledge of the X-ray pulsar period the EGRET scientists (Bertsch et al., 1992) were then immediately able to confirm the pulsed emission also at gamma-ray energies above 100 MeV. In the meantime, the pulsed gamma-ray emission has also been found in the archival SAS-2 and COS-B data.

Apart from these new discoveries, the

Compton observatory has made observations of the previously known two pulsars Crab and Vela with unprecedented accuracy. As an example, Figure 3 shows the lightcurves of the Crab pulsar as measured by BATSE, OSSE, COMPTEL and EGRET, and Figure 4 the en- ergy spectrum of the pulsed Crab emission as measured by OSSE, COMPTEL and EGRET.

i

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C o m p t o n C r a b P u l s a r L igh t Curves ' ' l + ~ ~ i ' ' i ' + i '

J ~ [GRIT ) 50 M e V

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.~_~_~ . . . . . . . . . . . ~ _ _ ~ . .

: i --711 . . . . . . : 2 ( . . . .

0.0 0.2 0.4 0.6 O.t Phase

Figure 3. Crab pulsar lightcurves as measured by GRO

118 V. Schfnfelder ~Initial results from the Compton observatory GRO

i

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O. IO 1 10 1 0 0 1000 10000 MeV

Figure 4. Energy spectrum of pulsed Crab emission as measured by OSSE, COMPTEL and EGRET.

Radio pulsars are known to be isolated rotating neutron stars. The gamma radiation is supposed to be produced by relativistic elec- trons, interacting with strong magnetic fields or other photon fields, either near the mag- netic polar caps or in more outer regions of the magnetosphere of the neutron star. Crucial for an understanding of the pulsar radiation mechanism is the fact that at least for the two previously known gamma-ray pulsars, Crab and Vela, their maximum of luminosity is located at gamma-ray energies. The gamma- ray luminosity of these two pulsars is 4 to 5 orders of magnitude higher than the radio- luminosity. It is important to note that also the other three newly discovered gamma-ray pulsars do have their peak luminosity at gamma-ray energies.

3.2. Diffuse galactic gamma-ray emission The diffuse galactic gamma radiation is

produced in interstellar space by interactions of cosmic rays (primarily cosmic ray protons and electrons) with interstellar matter (mainly interstellar hydrogen). Questions to be ad-

dressed by the study of the diffuse gamma-ray emission are: 'How are cosmic rays dis- tributed throughout the Galaxy?' and 'What can we learn from the distribution of cosmic rays about their sources?' These questions had already extensively been discussed after the SAS-2 and COS-B data became available; but still many questions remain open. EGRET is now repeating the galactic survey of COS-B with about ten-times higher sensitivity and somewhat improved angular resolution. COMPTEL at present is surveying the galactic plane at low gamma-ray energies between 1 and 30 MeV. This MeV-survey will provide unique information about low-energy (< 100 MeV) cosmic ray electrons, which cannot be studied through direct particle measurements (because of solar modulation) or non-thermal radio observation (because of synchrotron self-absorption). The results from EGRET and COMPTEL on the diffuse galactic gamma-ray emission are not yet available for publication. Preliminary results from the galactic center region are of excellent quality and show great promise.

V Schfnfelder/bdtial results from the Compton observatory GRO 119

10 -1

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10 -5

Figure 5.

OSSE , , , , , , , [

l i_ i i i J ~ - t - - - -

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6 ~ l,ongitude : 25"

- 4 - L o n g i t u d e = "1"19"

10.0

OSSE measurement of the energy spectrum from the galactic center region (from Johnson et al., 1992).

3.3. Gamm/t-ray line spectroscopy The field of gamma-ray line spectroscopy is

closely related to the question of nucleo- synthesis in the Universe. During the forma- tion of the chemical elements, not only stable, but also radioactive elements were formed. Some of these are gamma-ray emitters; these can be studied by gamma-ray spectrometers. Prior to the launch of GRO, the following lines had been detected: the 511 keV-annihila- tion line from the galactic center region, the 1.8 MeV line from radioactive 26A1 (also from the galactic center region), and a series of lines from SN 1987a resulting from the 56Ni - > 56Co -> 56Fe decay chain. First - still un- published - results on gamma-ray line observa- tions by OSSE and COMPTEL have been pre- sented at various conferences.

Fig. 5 shows the OSSE-spectrum of the galactic center region. The 511 keV annihila-

tion line is clearly visible together with the three-photon continuum. At 1 = O ° the line is much stronger than at 1= 25* and 1 = 339 °. A longitude distribution of the line flux over the entire galactic plane is not yet available. This distribution will be crucial to answer the question about the origin of the annihilation line.

Initial COMPTEL results on the 1.8 MeV- line from radioactive 26A1 were presented by Diehl et al. (1992) at the Toulouse Conference in March 1992. Figure 6 shows the back- ground-subtracted energy spectrum from a 6 °- wide longitude interval at the galactic center: the 1.8 MeV stands out clearly; the measured line width corresponds to the instrument en- ergy resolution.

Similar measurements exist from other regions of the galactic plane as well. The resulting still preliminary - longitudinal

120 V SchiJnfelder /Initial results from the Compton observatory GRO

distribution of the line-flux shows a peak at or close to the Galactic Center and a broad ridge of about + 20 ° around the galactic center. However, also in other parts of the galactic plane the line-flux does not drop to zero either. It is expected that the longitudinal and latitudinal distributions of the line-flux will finally allow one to identify the objects in

which the 26A1 is produced. The most promising candidates are supenovae, novae or special stars like Wolf-Rayet stars. The 26A1, which is produced within these objects, is ejected into interstellar space and then remains detectable for its mean decay time of about one million years.

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, , i I , [ , i , i l l I i i l l * i l l * i * I l l l * i . l * i l l l I J i i l . * l l l l l l * " i l l I I I *

4000 6000 8000 { k e Y )

Figure 6. Background subtracted energy spectrum from a 6°-wide longitude range around the galactic center region as measured by COMPTEL. The 1.8 MeV lines stands out clearly.

3.4. Active galactic nuclei Active galactic nuclei are the most powerful

compact objects in the Universe. It is sup- posed that a supermassive object - probably a black hole - is located in its center and that it is powered by accretion of matter from its sur- rounding. Due to the angular momentum of the accreted matter, an accretion disk de-

velopes, and often a jet-like stream of plasma is visible perpendicular to the plane. For the formation of the relativistic jet, strong mag- netic fields play a fundamental role.

One of the great surprises of GRO was the discovery of many quasars and BL Lac objects by EGRET that show high-energy gamma-ray emission. Table 2 lists 11 of these objects. All

V Schi)nfelder ~Initial results from the Compton observatory GRO 121

of them are radio-loud emitters; about half of them show superhminal motions. Many of them are time-variable in the gamma-ray emis- sion. The redshifts of these objects range from nearly zero to more than 2. Obviously we are

not only seeing the nearest quasars. If the gamma-ray emission is assumed to be emitted isotropically, the luminosities are as high as 1048 erg/sec (see last column of Table 2).

Table 2 Egret detections of active galactic nuclei (Apr. 91 - Feb. 92)

Characteristics of Active Galaxies detected by EGRET

Name 1 b Flux Spect. z Luminosity ( > 100 MeV) Index ( > 100 MeV) 10 -6 cm -2 s -1 f ' 10 4s erg/s *

0202+149 (4C+ 15.05) 148 - 4 4 1.0(0.3) 0208-512 276 - 6 2 1.0(0.1) - 1.7(0.3) 1.00 2.0 0235 + 164 (OD+ 160) 157 - 3 9 0.8(0.1) -1 .8(0 .2) 0.94 1.2 420-014 (OA 129) 195 - 33 0.4(0. I) 0.92 0.4 0454-463 252 - 3 9 0.3(0.1) 0.86 0.3 0528+134 191 - 1 1 0 .4 to 1.6 -2 .4(0 .1) 2.06 4 t o 13 0537-441 250 - 3 1 0.3(0.1) 0.89 0.2 0716+714 144 +28 0.2(0.06) 0836+714 (4C +71.07) 144 +34 0.15(0.04) 2.17 1.I 1101 +384 (Mrk 421) 180 +65 0.11(0.03) - 1.96 0.031 0.0001 1226+023 (3C273) 290 +64 0.24(0.06) -2 .4(0 .2) 0.158 0.008 1253-055 (3C'279) 305 +57 0.6 to 4.9 - 1.81 to -1.98 0.54 0.3 to 2 1606+ 106 (4C+ 10.45) 23 +41 0.35 to 0.56 1.23 1633+382 (4C+38.41) 61 +42 0.8(0.1) -2 .0(0 .1) 1.81 6 2230+114 (CTA 102) 77 - 3 8 0.24(0.07) -2 .4(0 .1) 1.037 0.5 2251 + 158 (3C454.3) 86 - 3 8 0.8(0.1) -2 .0(0 .1) 0.859 0.5

* f = beaming factor

Two of the objects (3C273 and 3C279) were not only observed by EGRET, but also by OSSE and COMPTEL. Fig. 8 shows the energy spectra of both quasars (top: 3C273, bottom: 3C279). Both quasars do have their peak luminosities at gamma-ray energies, and both quasars show spectral breaks in the COMPTEL energy range. These breaks may provide a key for the understanding of the gamma-ray emission.

In case of a spherically isotropic and central emission region of a few light-days to - weeks diameter photon-photon interaction would be an unavoidable consequence of the compactness; they would cause a break in the power spectra at that energy, at which the

optical depth becomes "1". At least, in case of 3C279, the observed break at about 10 MeV cannot be the result of such photon-photon interactions, otherwise the absorbed part of the gamma-ray spectrum would have to be re- emitted in other parts of the electromagnetic spectrum with the same luminosity. This is not the case. Therefore, it has been suggested that the gamma radiation must be produced in more outer, optically thin regions.

One possible 'outer' origin are the knots in the relativistic jets of these objects (as sug- gested by Dermer et al., 1992, Camenzind & Dreisigacker, 1992, and Mannheim & Biermann, 1992). Relativistic electrons or protons, which are accelerated within these

122 V. Sch6nfelder ~Initial results from the Compton observatory GRO

knots are able to explain the entire emission from radio to gamma-ray energies.

Due to the strong beaming of the jet emission, the luminosity is reduced by the Doppler-Lorentz factor D0+a) ( where D =

( ~ ) / ( 1- p co, e ) and a is the spectral power index ), which turns out to be a factor of 104 for 3C279. Such jet-models can be tested by contemporary observations from radio to gamma-ray energies.

1014 . . . . . . ~ . . . . . . . . , . . . . . . . . , . . . . . . . . , . . . . . . ~ . . . . . . .

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Frequency [ H z ]

Figure 7. Power spectra of 3C273 (top) and 3C279 (bottom). (From Hermsen et al., 1992.)

I4. Sch6nfelder ~Initial results from the Compton observatory GRO 123

-1110 -136 -gO -4S 0 45 IO 135 laO

G m ~ mngmm (deg.,e)

"',, " a 1"]'~°°° N (> S) " R 3

I ~ s R- 2 , , 100

I , , ~ 10

10 11~ c , ~ c ~

Figure 8. BATSE results on gamma-ray bursts. Top: distribution of bursts in the sky. Bottom: log N - log S diagram.

3.5. Gamma-ray bursts The gamma-ray burst sources are the

brightest gamma-ray emitters in the sky during the time of their short outburst. Prior to the launch of GRO most people had thought that in some ways neutron stars might be the sources of these bursts, though the trigger mechanism was not known. After the first results on bursts from BATSE became available, this hypothesis got in serious diffi- culties, and the question of the burst origin is entirely open again.

BATSE is at present measuring the global distribution of bursts in the sky. First results were published by Meegan et al., 1991. Fig. 8 shows the celestial distribution of 153 bursts,

measured by BATSE. It does not show any statistically significant deviation from isotropy. Fig. 8 (bottom) shows the log N - log S diagram of these bursts. Instead of the fluence S (erg/cm 2) the relative count rate C is plotted on the abszissa. Since the number of bursts N(> S) scales like ~ R 3 for a spherical distribution and since the fluence S ~ 1/R 2, the log N - log S curve should have a - 3/2 slope for a spherical distribution. This is not found in the measurements, the curve bends towards weaker bursts. The surprising point was that also these weaker bursts are distributed isotropically.

Obviously, these weaker bursts are not from a disk-distribution (as would be expected

124 V. Sch6nfelder / h~itial results from the Compton observatory GRO

from neutron stars). So the question is open again: what is the origin of gamma-ray bursts? If they are produced in the solar system (e.g. in the Oort's cloud), the amount of energy required to explain their luminosity, would be modest (~ 102s erg/sec). If the bursts are galactic, the sources would have to fill a halo at least up to 50 kpc, and an energy of --- 1043 erg/sec would be required per bursts. A new population of galactic objects would be needed. This hypothesis is not sup- ported by any excesses in the number of bursts from the Magellanic Clouds and the Andromeda nebula (though the statistical significance at this time is not yet conclusive).

The third possibility is an extragalactic or cosmological origin. The energy release would be tremendeous: 1051 to 1053 erg/sec. The bending of the log N - log S curve would have to be explained cosmologically. Neutron star -

neutron star or black hole neutron star merging are presently discussed as possible sources, though there are serious difficulties with such models as well (e.g. time structure of bursts, how to explain cyclotron lines, how to explain repeaters).

Perhaps the most attractive models to ex- plain the bursts are multi-component models, like e.g. the two-component model of Lingen- felter and Higdon (1992), in which two lumi- nosity classes of galactic sources are assumed to exist (one releasing 1039 erg/sec, the other 1044 erg/sec). The first class would be visible up to 300 pc, the second one up to 100 kpc. Lingenfelter and Higdon propose that both these classes are represented by neutron stars, but that the bursts in both classes are produced in different ways (thermonuclear run away in the first case and starquakes in the second c a s e ) .

GRB 910503

lO

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5 - t

o l U

,..d t~

o

180 175 170

gal. longitude (degrees)

Figure 9. The location of GRB 910503 from the COMPTEL image and the triangulation circle.

V Sch6nfelder/Initial results from the Compton observatory GRO 125

A major step forward in solving the question of the origin of gamma-ray bursts would be the identification of a burst source in other spectral ranges. GRO was able to locate a few burst sources very accurately and, there- fore, a counterpart search after the burst seemed promising. For illustration, Figure 9 shows an image of the burst GRB 910503. The image was produced by COMPTEL. The source location accuracy from this image is + 1". Also shown are six high-energy events from this burst as measured by EGRET (the six open circles) and a one-dimensional loca- tion of the burst by triangulation, using the burst arrival times aboard GRO and Ulysses. The combination of the COMPTEL location uncertainty with the triangulation uncertainty defines an error box of about 3 arc m i n x 2 degrees. Attempts to find a counterpart in the ROSAT data so far remained unsuccessful (J. Greiner, private communication). But further attempts of counterpart searches for other bursts are continuing.

Dermer, C.D., Schlickeiser, R. and Mastichiadis, A., 1992, A&A 256, L27

Diehl, R. et al., 1992, submitted to A&A Suppl.

Fichtel, C. et al., 1983, Proc. of 18th Int. Cosmic Ray Conf., Vol 8, 19

Fishman, G.J. et al., 1985, Proc. of 19th Int. Cosmic Ray Conf., Vol 3, 343

Halpern, J.P. and Holt, S.S., 1992, Nature 357, 222

Hermsen, W. et al., 1992, submitted to A&A Suppl.

Johnson, W.N., 1992, submitted to A&A Suppl. Kurfess, J.D. et al., 1983, Space Research,

Vol. 3, No. 4, 109 Mannheimer, K. and Biermann, P.L., 1992,

A&A 253, L21 Meegan, C.A., et al., 1991, Nature 355, 143 Lingenfelter, R.E. and Higdon, J.C., 1992,

Nature 356, 132 Sch6nfelder, V. et al., 1984, IEEE-Transactions

on Nucl. Science, NS-31, No. 1,766

4. CONCLUSION

Exciting results have been obtained from the early phase of the GRO-mission, which have stimulated many discussions and new theories. Whereas gamma-ray astronomy for many years was a field restricted to a small group of specialists, it has now found broad attention within the whole community of astronomers and astrophysicists.

REFERENCES

Bertseh, D.L. et al., 1992, Nature 357, 306 Bignami, G.F. et al., 1983, Ap.J. 272, L9 Camenzind, M. and Dreisigacker, O., 1992,

submitted to A&A C.amen~nd, M. and Kroekenberger, M., 1992,

A&A 255, 59