Low and medium energy gamma-ray astronomy — present status and future aspects

Ada. Space Ree. Vol.3, No.4, pp.59—69, 1983 02731177/83 $0.00 + .50Printed in Great Britain. All rights reserved. Copyright © COSPAR

LOW AND MEDIUM ENERGY GAMMA-RAY ASTRONOMY — PRESENTSTATUS AND FUTURE ASPECTS

V. Schonfelder

Max-Planck-Institutfür PhysikundAstrophysik,InstitutfurExtraterrestrischePhysik,8046Garching,F.R.G.

ABSTRACT

During the last few years quite some progress has been achieved in the field of low and mediumenergy ganina—ray astronomy below about 30 MeV. Gamma rays from the galactic center and anti—center region have been detected, which require a high interstellar electron flux in the 100MeV range, if they are predominantly diffuse in nature. Though the Crab pulsar and its nebulaare still the only galactic gamma—ray sources which definitely have been detected, some re-cently determined upper limits to the gama—ray fluxes of other radio pulsars are close tothe theoretically expected values. Active galaxies seem to have a maximum of luminosity inthe range between several 100 keV and a few MeV and, therefore, are of special interest. Firstobservational results have been reported on the Seyfert galaxies NGC 4151 and MCG8-11—11,and the radio galaxy CenA. The nature of the diffuse cosmic gamma—ray component at low gamma—ray energies is not yet solved. Unresolved active galaxies are good candidates for its origin.

Considering the present status of gamma ray astronomy the study of galactic sources like ra-dio pulsars and the unidentified high energy gamma—ray sources, the Milky Way as a whole,active galaxies and the diffuse cosmic sky seem to be the prime targets for broad band obser-vations below 30 MeV in the GRO area. An unexplored field like that of low energy gamma-rayastronomy, however, is always open for surprises.

INTRODUCTION

The exploration of the sky in the low and medium energy gamma ray range below about 30 MeVlags far behind the exploration of the sky in the X—ray and high energy gamma ray band. Duringthe last few years, however, quite some progress has been achieved and some very interestingresults have been obtained, which can be grouped under the following headings:

‘Total Galactic Emission”“Galactic Gamma Ray Sources’“Extragalactic Gamma Ray Sky”

The Galaxy has now been resolved below 30 MeV in the center and anticenter regio8, and spec-

tral measurements are available at those energies, where the contribution from ~r —decay gamma-rays can be neglected.

The only galactic source, which definitely has been detected and identified at low gamma rayenergies, is still the Crab. It is always a rewarding target. At present there are two ques-tions which make low energy gamma ray observations of its pulsar especially interesting:1. Does the pulsar show the same time variability as observed at high energies?2. What is the nature of the interpulse emission between the two main pulses?

Other source candidates for low energy gamma—ray studies are the Vela pulsar, rapidly rotatingnearby radio pulsars, and the unidentified COS-B gamma ray sources. For some of these objectsnew observational lower limits have recently been set, which all are close to the theoretical-ly expected fluxes.

In the extragalactic sky active galaxies have become of special interest to gamma ray astrono-mers. These sources seem to have a maximum of their luminosity in the range between several100 keV and a few MeV. First results at low gamma—ray energies have been reported on the Sey-fert galaxies NGC 4151, MCG8—11—11, and the radio galaxy CenA. The quasar 3C 273 is anotherpromising candidate.

The nature of the diffuse cosmic gamma ray component with its spectral bump at MeV energies isnot yet solved. However, active galaxies are good candidates for its origin.

In the following each of the three above listed topics will be discussed in more detail.

59

60 V. Sch6nfelder

Total Galactic Emission

During the last year two remarkable observations of the galactic plane at low and medium gam-ma ray energies have been published. Both observations were made with balloon borne instru-ments. A sparkchamber experiment with a very low threshold energy of 4 MeV was used by Agri-nier et al. [1] to study the galactic center emission, and a map of the anticenter region wasreconstructed from data of the MPI Compton telescope (Graser and Schonfelder, [2]).

Fig. 1 shows the galactic center emission in the three energy bands 4-10 MeV, 10—26 MeV, and> 26 MeV as a function of galactic latitude. The plane is clearly resolv

8d in the two upperenergy bands. Due to the limited angular resolution of the instrument (9 to 14° HWHM), nodiscrete sources can be resolved in the excess flux.

_______________________________________ 1 - 10 MeV

7,... 10~~•‘‘ .........r.f.. 410~ _______ _____MeV 42 ___

~ io-3 10-26 1

- LT1tV ;:; ~4 6 110 106 102 98 9~ 90 86 02 78 74 70 66 62 58 51, 5010 •~i.i.. .11111

— 900 00 + 900 IIi~0,75J.= (O5-0,75)J,~, (025-OSli,,, 0-025)J,,, 50

Fig. 1 Observation of the galactic center re— Fig. 2 Sky map of the galactic anticenterre—gion above 4 MeV by Agrinier et al., Lii. gion in the energy range 1.1—10 MeV from Gra—

ser and Schönfelder [2]

In Fig. 2 the image of the ant~center region of the galaxy between right ascension 50° to1100 and declination 100 to 50 is shown. A significantly enhanced gamma—ray emission is ob-served along the plane from 1 = 160° to 1 = 197°. Part of the emission is due to the Crab. Theangular resolution of the telescope is indicated by the circle around the Crab position. Atpresent it cannot be decided, whether the remainder of the emission is due to further unre-solved sources or whether it is diffuse in nature. The lack of emission along the plane atthe two edges of the image may favour a contribution from localised regions rather than adiffuse origin. In addition to the Crab there are six Uhuru sources within the bright regionof the image. All six sources are very weak: their strength s lie between 2.4 to 4.5 Uhurucounts only. One of these sources(4U 0515+ 39) is in the list of high energy X—ray sources(80-180 keV) observed by HEAO-1 (Levine et al., [3]). No significantly enhanced emission isobserved from the

0direction of the high energy gamma—ray source Geminga (2CG 195+04) at a =

97~950, ~ = 17.95 and from the Seyfert galaxy MCG 8—11—11 at a =89.5°, ~ = 46.3°, which re-cently was reported to be a soft gamma—ray source (Perotti et al., L71)•

In Fig. 3 the spectral information derived from the center and anticenter region are compa-red with each other. In both cases the total emission of the plane is shown over a wide spec-tral range from hard X—ray energies to high gamma—ray energies; only clearly resolved sourceslike Crab and Geminga are excluded.

The spectra of Fig. 3 are supposed to contain four different components. The first one is dueto unresolved gama—ray sources. The other three components are diffuse in origin. Th8y areproduced by interactions of high energy cosmic ray nuclei with interstellar matter (a —decaygamma—rays), and by bremsstrahlung or inverse Compton collisions of cosmic ray electrons withinterstellar matter or photons, respectively. The contribution of the latter three componentsto the total galactic emission has been estimated by many authors. For the center region thecalculations of Kniffen and Fichtel [8J have been added to the data points above 10 MeV. Thea —decay gamma—rays cause a smooth bump in the spectrum around 100 MeV. At lower energiesaround 10 MeV the spectrum is totally determined by electron induced processes, if the contri-bution of unresolved sources can be neglected. Whereas the inverse Compton component at 10 MeVis caused by electrons with energies above several 6eV, the bremsstrahlung component atlOMeVmainly results from typically 30 MeV electrons. Our knowledge of the interstellar electronflux at these low energies is very uncertain. Direct electron measurements with cosmic rayexperiments have to be corrected for solar modulation, and non—thermal radio—measurements inthe galaxy only go down to about 0.7 MHz,_~h~hcorresponds to typically 70 MeV electrons.Kniffen and Fichtel [83 have used the = E .~ —electron spectrum derived by Webber et al. [9]to calculate the bremsstrahlung spectrum, which together with the other two components givesa reasonable fit tQ jhe data. The high data points below 100 keV are caused by discrete X—raysources (Wheaton, L6J).

Low and Medium Energy Gamma—RayAstronomy 61

ANTI — CENTER GALACTIC CENTER102 1111111 11111101 11111119 11111101 1111111 102 lIlllI,9IIIIIIIIIIIIIIIITIIII!IlllIIIlIII

101 . 1 - -0- [OS — ~‘? 320~t‘40’- L 050—7 Wheaton, 1976) 10 4 SAS — 2

t4. toZ4O’ .~_ 4 Agrinier et at 33O’~t’30’1 - - 1 - f 4- Apollo 16 310’ L’ 22’

.4- ~— Mandrau et at 315’’ t 45’

10 1 Graser and Schönfelder1901 10 1 -

1o160° — 197’

10~ - - - 10-2

- ~

=1S1’-160’

~ - ~

10 .~10 Bremsstrahtung CE,°1”t

~ 10~ - “~\.,~,lo1i6’—i36’ ~~_6 inn. Compton effect

• Bremsstrah(unq IE,°-”t ~ i~—~ Kniffen and Fichtel.

,f II

/“ 10~ /

It i~—~- It -

// /

10_tO -

10_to . -

10_li 111,11 11,1~1 1,1.1 ~i 10~” ~ l~ll~lt 11101

001 01 1 10 100 1000 0.01 0 1 1 10 100 1000

E7[MeV] — 1Mev]

Fig. 3 Comparison of the energy spectra from the galactic center and anticenter region.COS—B data are from Paul et al ., [4] , Apollo—16—data from Gilman et al ., [5] , and OSO—1—data from Wheaton, [6]. SAS-2 data are from [8] and Mandrou et al . from [10].

For the anticenter region the a°—decayspectrum and the bremsstrahlung spectrum of the centerregion were both scaled down by a factor of 2.6. The contribution from inverse Compton colli-sions is supposed to be negligible. In this case the data points between 1.1—10 MeV lie afactor of 5 to 10 above the calculations. The OSO—7—data, too, are too high to be explainedby bremsstrahlung. In the 10 key—range the energy loss of electrons is dominated by ionisa—tion losses. Therefore the bremsstrahlung ~o~ponent at these energies should be significantlysmaller than the extrapolation of the - E~L’1 power—law. The apparent discrepancy may indi-cate that the observed emission is mainly caused by localised regions. There is, however,another possibility.

Based on COS—Bdata above 50 MeV Lebrun et al. [123 have suggested to interpret the gama-rayspectra from high galactic la~i~udes 10° ~ Ibi ~ 200 as being composed of a a —decay and abremsstrahlung component E . only, no additional inverse Compton component was considered.Such a fit is indeed possible at energies above about 10 MeV for the anticenter direction,which more or less represents the local gamma—ray emission. The fit exactly goes through theCOS—8points and the Compton—telescope data (Graser and Schönfelder [14]). The spectrum must,however, flatten towards lower energies in order to match the OSO—7—datapoints (Fig. 4). Theflattening is caused by the high ionisation losses, which dominate over all other energy lossprocesses at these energies.as is discussed in more detail by Sacher [ii].

Recently the contribution of unresolved radio pulsars to the total galactic gamma—ray emis-sion was reestimated by Harding and Stecker E131 . The contribution was found to be comparableto the bremsstrahlung intensity at 100 MeV. It seems that the true intensity of the diffusegalactic gama—ray component will only be known after much more sensitive source measurementshave become available.

Galactic Gamma Ray Sources

Among the 25 high energy gama-ray sources listed in the second COS—Bcatalogue there are atpresent only two compact galactic objects which definitely have been identified. These arethe Crab (pulsar plus nebula) and the Vela pulsar. Recently, new data on these two sourceshave been obtained at low gamma ray energies as well.

62 V. Sch~nfe1der

ANTI- CENTER102 I I 11111 I 11119 I 11119 III!

101 - CRAB, PSR 0531+21- j 050—7 Wheaton, 1976)

t0260’

Oamma10_i Graser and Schönfetder,1982 >200MeV

~ L—160’ - 197’ ~ 50-200MeV

to151’-160° ri_ fl r~i~i I L~, 1-20MeV

~ io4 ~ ~‘j.—-u- -

—~ io5 - BremsstrahLunq g,~28~

106 - after Lebrun et at, 1981 \\t~1i~i36’ - iOO-1,00keV

::i; ______ ________________10-v I I ~

10_11 I 1111111 III~1II II’’”” II~I~ III I

0,01 0_i 1 10 100 1000

E7EMeV]

Fig. 4 Fit to the gamma-ray data from ~he Fig. 5 Crab pulsar light curves at differentanticenter direction by combining the a — photon energies. Adopted from Buccheri [15].decay component with the steep electron spec-trum suggested by Lebrun et al . [12] for thelocal gamma—ray emissivity. A flattening ofthe electron spectrum by one power is as-sumed at electron energies < 10 MeV.

Crab

The Crab pulsar has been observed at practically all wavelengths. A compilation of light cur-ves from radio to gamma ray energies is given in Fig. 5. The 1.1—20 MeV curve was obtainedwith the MPI Compton telescope during measurements in 1977 and 1979 (Graser and Schönfelder,1982 [14]). The high energy gamma ray curves are from an early COS—Bobservation in 1975(Bennett et al., [16]). Though the overall picture with the two main peaks at a distance 0.4of the period is the same at all wavelengths, there actually are two marked differences:

1. Recently the COS—Bcollaboration (Wills et al., [17]) has reported that the gamma ray emis-sion above 50 MeV shows time variability. The second peak was much reduced or nearly absentduring observations between 1976 and 1979. This variability was not observed at gamma rayenergies below 20 MeV. The second peak of the light curve obtained with the Compton telescopeduring this period has an intensity comparable to the first peak at a statistical signifi-cance of 5.5 o.

2. A very clear interpulse emission exists between the two main peaks at X—ray energies be-tween about 1 keV and several 100 keV. This emission is much reduced in the sub key—range andabove 1 MeV. Several models have been discussed by Knight [183, in which the interpulse emis-sion is interpreted as an additional pulsar component of somehow thermal origin. These modelswill have to be revised, since COS-B has now determined a definite interpulse flux of 15 ± 4%of the total pulsed flux above 50 MeV (Wills et al., [19]).

In Fig. 6 and Fig. 7 energy spectra of the pulsed Crab emission are shown. Fig. 6 applies tothe total pulsed emission (main pulses plus interpulse). Above about 50 keV the spectrum can

Low and Medium Energy Gamma—RayAstronomy 63

be well fitted by a single power law with index —2.2. Towards lower energies the spectrumflattens smoothly. The power law fit of the HEAO—1data (Knight, [18]) is not consistent withthe rest of the data around 1 MeV. No break of the spectrum exists at 1 MeV as suggested bythis author. Therefore, the pulsar does not have its maximum of luminosity near 1 MeV, butaround 50 keV.

10_I 11111111 111111111 11111111 111111111 111111111 11111111

102 - CRAB PULSAR

io~ -

10’

i0~ ‘

i~-~ __

~i08

I-. I\D \

0. ~ • ~i’i.i,-c’,__L_c22 2 \ II~

• 1MeV C S \eu this experiment \\ ~

‘+~OrwIg et at. 19711010 + Flshmon et al 1969

+ Kurfess 1971

10° - + Wilson et at, 1977‘~ Kniffen et at.i974 \\

+ [OS-B Observations1012 - (Bennett et 0)1977 \\

+ Walroven et at, 1975—13 11111111 111111111 111111111 11111111 1111111

U 10’ 102 ~ 10’ ~ 106 10’

• Ey [keVi

Fig. 6 Energy spectrum of the total pulsed Crab-emission (main pulse plus interpulse)Adopted from Graser and Schönfelder [14

In Fig. 7 the interpulse emission spectrum is compared with the total pulsed emission. Thespectrum reflects the steady increase of the interpulse emission at keV—energies as seen inthe previous light curves. The upper limits between 200 keV and 3 MeV are consistent with aconstant interpulse intensity level of 15% as at high energies.

The total Crab emission consisting of the pulsar plus nebula contribution is shown in Fig. 8.

Again it can be described by one single power law with index —2.4 above about 50 keV.

Vela Pulsar

Though the Vela pulsar is the strongest source in the sky at high gamma ray energies, no pul-sed emission so far has been detected at X—ray energies. For an understanding of the pulsaremission process it would be highly desirable to have spectral measurements next to the highenergy gamma ray range. New upper limits from the HEAO—1A4—experiment (Knight [18]) in theenergy range 20 keV to 6 MeV indicate that the spectral shape must deviate from the high ener-gy power law dependence already above 200 keV (see Fig. 9). The theoretical curve in Fig. 9was derived by Fawley [20] for a special polar cap model.

64 V. Sch~nfelder

10~i ~ i Ii Ii II Unidentified Gamma Ray Sources

Among the unidentified high energy gamma raysources there are two which have been investi-

102 - - gated at low gamma ray energies as well, namelyGeminga (2C 195+04) and 2C 135+01. While onlyupper limits have been reported for Geminga,which are still consistent with a power law ex—

- - trapolatiori from the high energy range (Haymes+ et al. [21] and Graser and Schönfelder, [2]), a

positive excess is reported by Perotti et al.+ 22] in the region of 2C 135+01 at energies±~ between 100 keV and 1 MeV.

10~’ -

Other Radio Pulsars

‘B— ~ Radio pulsars are supposed to be good candidatesN,

~ ia-° - for gamma ray sources. Therefore, attempts fortheir detection have been made at high and low

‘~ +050-8 4-606eV IT -~-22 gamma ray energies. Theoreticians have developed

l~1lEA01 I 20 various models to understand the gamma ray emis-LEO1 o LEO2 15-20n6,v I sion of pulsars. The so—called polar cap and

I --4- HEAO-1LEOl 50—7006ev

liFt COfIPT0N-TELE5COPE ÷1.1—3 MeV Fig. 7 Energy spectrum of the Crab interpulse

emission. Adopted from Knight [18]io-’ - 20 - component in comparison with the total pulsed

Fig. 8 Energy spectrum of total Crab emission108 ,,,,,, III III i Ill (pulsar plus nebula). Adopted from Graser and

10~ 10’ 102 ~ 10’ Sch’dnfel der [14]Energy (key) +

io2 ~° TOTAL CRAB EMISSION0

10-’ -

- -

4. this experiment 1981\

> 1O~- ~ Penningsfetd et ul.1979

°l~. Ling et al 1979W io~ - Q Toor and Seward 1974In

‘-‘~‘-‘ Dolan xl al, 1977 \\

10 0 + Carpenter et al 1967(I) -

~ Helmken and Hoffmann, 1973 3 ~ MeV~Gruber and [ing, 1977

=°~ io~ - ~Z’1I. P. Mandrou et al,1977

+ Baker et al 1973\0 Parlier et aL 1973

1010 ‘-4-’ Wilson et al, 1977

~ 1-laymes et al 1968

io~ ~~çr~SchónfeLderet al 1975~ Kniften et aL 1974

\

1012 + LOS-B Observations (putsar) ‘—0-i total Crab- (Bennett et aL 1977) (Linhti et al, 1980) \

+ Watroven et at, 1975 —c--’ White et at, 1980

1 ~13 111111111 11,11111 111111111 111111111 11111111

10’ 102 i03 10’ io~ 106— Ey [key]

Low and Medium Energy Gamma—RayAstronomy 65

VELA PULSARTINE — AVERAGED PULSED FLUX AND UPPER LIMITS

UCSD/MIT HEAO—I 1055-52 1642-03

I ‘‘hi’’~l’’I’’’’i’’’I’’’1’’’I’’’’i’’I’ ‘~ ‘‘‘Il’’’~l’lI’’’i’’’I’’ ~

Crab Veta 1822-09 1929.10 0950.08‘I, 4 4 44i0 I I I I I I I

10’ 50 -1eV - 10 ii,V [05-61

1 - ii MeV HObO-it ilel~ 10’ - ~1~ __•~__ - i•i — 20 Me’.’ Il-IPI — Batio,ni

: 10’ - ~ 113-1,126MeV198n1

T :~. ~-‘ /}~t -

10’ H, _r0,5 carding, ~ 130

~::~ 1101 IIIOT~d4~VOL

ICY’ - y Pr..dO SI Si, I916 - ~ IRISk,, CI Ii, 1913 ~,

-r V~0i~Si at. 1919 ‘-4-f’. ,o-’ -7 IkIl ~0(k -i

(0 -

+ 118.1. II IL 914

Lickli it IL 980 I I I I

i,,,] ,1~,i=1 lllIL~~i ,,,I,,,i ,,~i,,,i ,,, t0~ ix’ 10’ 10’ 10’ 101 10’

ICY’ 10’ ICY 0° (0’ 10’ 10’ (0 0 3- I years 1

ENERGY(keV(

Fig. 9 Upper limits to the pulsed emission Fig. 10 Theoretically predicted pulsar effi-of the Vela pulsar at X- and low gamma ray ciency for converting rotational energy intoenergies. Adopted from Knight [18]. gamma ray luminosity (Harding, [25]) is compared

with upper limits to 5 pulsars derived by KnightL18] and Graser and Schönfelder [26].

outer gap models are able to describe many properties of the Crab and Vela pulsar. They werealso used to predict gamma ray luminosities of other pulsars. In these models electrons andpositrons are accelerated along magnetic field lines by strong electric fields, which are in-duced by the rotation of the neutron star. The particles are believed to be accelerated in apolar gap near the poles (Ruderman and Sutherland, [23]) and may be accelerated or reaccele—rated in an outer gap at about 0.2 of the light cylinder radius (Cheng and Ruderman, [24]).The gamma radiation is produced as curvature radiation in a direction tangent to the fieldlines, which then is attenuated by electron—positron pair production in the strong magneticfield. These electron-positron pairs may produce further radiation by the synchrotron process,since they are emitted at an angle to the field lines. Based on these models Harding [25] hascalculated the gamma ray luminosity of a large number of radio pulsars. While the gamma >ayluminosity of a pulsar is found to decrease with increasing age, its efficiency n~= L /E forconverting rotational energy into gamma—ray luminosity should increase with age up to cer-tain limit.

Recently some radio pulsars have been investigated for pulsed gamma ray emission by Knight[18] using HEAO—1 data and by Graser and Schbnfelder [26] using balloon flight data of theirCompton telescope. Upper limits from these investigations are now available and can be com-pared with the theoretical values. This comparison is made in Fig. 10. For gamma—rays of ener-gy above 100 MeV the predicted dependence between o~and the apparent pulsar age ‘r = 1/2 P/pis indicated by the solid line; for normalisation the Crab—value was used. For the energyrange 1 to 20 MeV the same dependence is assumed to be correct as indicated by the dashed line.This ad hoc assumption must not necessarily be correct; at present the gamma ray production inpulsars has only been studied above 25 MeV (Daugherty and Harding, [27]).

The upper limits to the pulsed gamma—ray emission of 5 radio pulsars studied by HEAO—1and theMPI—Compton telescope give n~—va1ueswhich are close to those expected from theorte. -All 5pul—sars are nearby and have relatively short periods as can be seen from Table 1.

If our present understanding of gamma ray pulsars is correct, these candidates should be easi-ly detectable in the next generation of gama—ray telescopes.

A 3.4~ detection of pulsed gamma-ray emission between 300 keV and 1.126 MeV from PSR 1822—09has been claimed by Mandrou et al., 1281. Based on the reported flux an ny—value much largerthan 1 is derived (see Fig. 9). This result needs further confirmation.

66 V. Sch~5nfe1der

TABLE 1 Upper Limits to oi of 5 Radio Pulsars

pulsar period (m sec) distance (pc) (1—11 MeV) (1.1 — 20 MeV)(HEAO-i) (MPI)

0833-45 89 500 10~

1055—52 197 800 2.6x io21929+10 226 100 2.3 x 10_i 5 x io_20950+08 253 100 2xi0~

1642-03 388 200 3 x 10_i

Extragalactic Gamma Ray Sky

The interest in extragalactic gamma-rays has always been large. For many years the extragalac-tic gamma-radiation was considered as one single component only which was called the “diffusecosmic gamma-ray component”. Its origin was widely discussed because it was supposed to con-tain signatures from very early epochs of the universe. After gama—radiation from the quasar3C 273 had been detected by COS-B, and after spectral measurements of active galaxies likeSeyferts and BL—Lac—objects at hard X—ray energies had indicated that these objects may evenhave their maximum of luminosity at low gamma-ray energies, a wide interest in extragalacticgamma-radiation arose among many astrophysicists.

Essentially, there are two properties which are typical for active galaxies that are supposedto be gamma-ray emitters: first, they show time variability on time scales of years and shor-ter; second, they have their maximum of luminosity between several 100 keV and a few MeV.These properties may be illustrated for the case of the radio galaxy CenA.

_______________________________________________________ -5 0 5 10I I I 1111111 F III I I I 1*1 I I I III I I I ILl I I

n CON A I • QI0f~ ~~~~ iO20Hz iO211Hz -

Fin, 01100 6eV +8 - .i~I0~ IO43erq/s CEN A -20 1 T 4.4Mpc .‘+~ -

I is ~i 6 ~ I0~9 ,~9 - ~~‘10-I0- .~ ~. .n

I ~ - ~ 0-I2 - . -

‘9’ Au T - . RADIO DR V X—RAY 66660 RAY -IL I 1’ 1i3 — a I I I I I I I I I I I I I I I I I I

1 1 —5 0 5 0

In~ I I I I I I I I I - LogE(eV(— I960 i969 i970 i97i i972 1973 1974 i975 976 iS?? ISiS i979

YEAR

Fig. 12 CenA spectrum from radio to gamma—rayFig. 11 Long term variation of the CenA-flux energies. The intensity scale is given in powerat 100 keV (from Baity et al. [29]). per decade (from Baity et al. [29]).

In Figure 11 the long term variations of CenA near 100 keV are shown. Between 1973 and 1975the galaxy was nearly 10 times brighter than before or after that period. At low X—ray ener-gies variations on the time scale of days have been observed.

Fig. 12 shows the spectrum of CenA from radio to gamma—ray energies. The intensity scale isgiven in power per decade. The X—ray spectrum follows a power law in the hard X—ray range,HEAO—i data indicate that there might be a break at - 140 keV. The upper limits at gamma—rayenergies above 1 MeV place the luminosity maximum into the several 100 keV region.

The spectra of other active galaxies look very similar. In case of 3C273 the maximum is sup-posed to lie between 1 to 3 MeV as can be derived from the spectrum shown in Fig. 12, if thehard X—ray and the high energy gamma—ray data points are extrapolated into the MeV—range.

In case of the Seyfert galaxy NGC 4151 controversal results have been obtained in the lowenergy gama—ray range, which, however, may very well reflect time variabilities of the source.Whereas theMISO-exper-iment (Perotti et al. [31]) observed positive signals from the galaxyduring two balloon flights between 180 keV and 20 MeV at intensities which differed by a fac-tor of 4, several other observers placed upper limits well below the MISO—flux (Fig. 14). Oneshould notice that above i MeV the MISO-results are of low statistical significance only andfurther measurements with much higher accuracy are needed to determine the actual spectralslope in the region where the luminosity maximum is expected. A similar spectral shape wasfound by the same group (Perotti et al. [7]) for the Seyfert galaxy MCG8-11—il. The limits

Low and Medium Energy Gamma—RayAstronomy 67

set by Graser and Schbnfelder [2] in the 1.1-10 MeV range do not confirm their observation,but are not in severe contradiction. i~-~ ~

— 1 ‘‘I’’!’’ I’ll’’

-x i0~1 10’ -.7

I0

B 101 -

1 101 102 i03 10’ 10~ 106 io~Energy IkeV) —

Fig. 13 Spectrum of 3C 273 from X— to gamma— ~ -

ray energies (from Hermsen et al. [30]). Thedata are from HEAO-1 (broken and solid line), ~the MIT-Leiden balloon experiment (broken anddotted line), and from COS—B. -

i 0 Mushotoky et at 1978° OSO. 8 May—June 1977

1 °‘E2’7 IKinzer et al, 1978) —Auriemma it at 1978a- -‘I— 10-8 809 76 July 1976

C ‘‘ T ,~Coeet al 1980ARIEL 5 December 1976

lOl - - _~_±Meegan and Haymes 1979io~- October 1977 -

Perotti et at 1979titSO May 1977

1O_2 - - ~ White it at 1980October 1978 -

~ Schonf eIder 1980Octxber 1977

~ ~ —

10_li f B~gnam~et al 1979SAS 2 May 1973

extrapolated Perotti it at 1981 Ja.’ contribution MISO September 1979

of x—ray Seyferts

~ —4 ,~/‘ - 10_i? i .1 I i“i 10 lOl io

2 io~ 10’ io5

ENERDY (keV(+ Schonfetder et at, 1981 7’ Fig. 14 Energy spectrum of the Seyfert galaxy—f, Mondros etaLl979 - NGC 4151 from X-ray to gamma-ra~’ energies.~ 10~- — Fichtel et xl, 1977 Adopted from Perotti et al. [31j. The MPI data

Kraushaar et at (—E~’i 1972

- ~ Hopper etat(—031,1973 ~ points are from Schdnfelder •L32]1 10 + Herterlch etat, 1973 - X- and low gamma—ray energies combined with the

1’ Share etat,1974

- Purlier etat, 1975 rapid intensity variations suggest that these- * Mazetsetat,1974 photons originate within the nuclei of these ga—

‘I’ Daniel and Lavakare,1975 laxies. Because of the small size of the emitt—• ‘~ Kuo et ai,1973 The high luminosities of active galaxies at hard

Vedrenne etol,1971 - ing region photon-photon interactions between X-Fukadoetal, 1975 and gamma-rays leading to electron-positron pairs

~ Wh~teetai,~977 may affect the shape of the spectrum depending• ~ Tronubka etal,1977 on whether the emission is beamed or not (Bassa—- 4-’ Schonfelderetai,1977 ni and Dean, [33]). The actual engine responsib-

100 —J I il I I I I I 11,111 I 1111 le for the powerful emission is not yet known.0,1 1 10 100 1000 However, it can be expected that hard X- and low

~?‘ [MeV] energy gamma-ray observations will give insideinto the mechanism involved.

Fig. 15 Energy spectrum of the diffuse cosmicgamma radiation (from Schönfelder et al. [34]). The existence of such powerful extragalacticAlso shown is the contribution of unresolved gamma—ray sources must be taken into accountSeyfert galaxies as expected from an extra— when asking for the origin of the diffuse cos-polation from X-ray energies. mic gamma—ray background. The background spec-

trum is shown in Fig. 15. It is very steep be-tween 5 MeV and 200 MeV. Towards lower energies a flattening of the spectrum is necessary inorder to match the X—ray measurements in the 100 key-range. This flattening - often describedas a bump — is indeed observed in all experiments between about 500 keV and 5 MeV.

Unresolved active galaxies may explain part of the emission, especially in the hard X- and lowenergy gamma-ray range. Mushotzky et al. [35] had estimated that 20% of the diffuse cosmic X—ray flux can be explained by superposition of unresolved Seyferts at 50 keV. In Fig. 15 this

68 V. Sch~nfe1der -

contribution is extrapolated into the gamma—ray range using the mean Seyfert spectrum ~.E-l.b4

derived from HEAO—1observations of 10 active galaxies between 21-165 keV (Rothschild [36]).Seyferts could indeed explain the observed cosmic gamma—ray flux if their spectral slopewould remain constant. Their summed contribution would even exceed the observed diffuse emis-sion above a few MeV. Therefore a change in the slope of their spectra is required, which isconsistent with the previous discussion.

A similar discussion was performed by Bassani et al. [37] based on low energy gamma—ray dataof CenA, NGC 4151, and MCG8—11-il. If it is true that special galaxies contribute signifi-cantly to the diffuse flux, then it is not surprising that no single power law dependence isobserved for the diffuse cosmic energy spectrum, since different types of galaxies may domi-nate at different energies. The question, whether there is a remaining really diffuse compo-nent of primordial origin like the one from matter—antimatter annihilation in a baryon symme-tric universe as suggested by Stecker, Morgan and Bredekamp [38] can only be answered, ifmuch more information on gamma—ray emission of active galaxies is available.

CONCLUSION

In summary I conclude that the few new data which have become available in the field of lowand medium energy gamma ray astronomy during the last few years, have already led to very in-teresting discussions. The topics that could be addressed are the Milky Way as a whoje, ga-lactic gamma ray sources like radio pulsars and the unidentified high energy gamma—ray sour-ces, active galaxies and the diffuse cosmic gamma—ray sky. In the GRO-area these topics willbe studied in much more detail: It is expected that many of the open questions of this artic-le will then find their answers. In addition probably quite new questions will come up as wasalways the case, when a new field was explored.

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