A model for the origin of relativistic particles and gamma rays in active galactic nuclei and quasars, together with recent HEAO-1 observations of the spectra of active galaxies from 2 to 165 keV, provide the basis for a reexamination of the nature of the extragalactic gamma ray background. Active galaxies account for the observed background if their X-ray spectra steepen to E.021 above 100 keV, as observed in Cen-A, together with a further steepening to E.021 as a result of absorption of gamma rays by photon-photon pair production interactions with X-ray photons. The compactness of active galaxies required to give this steepening is consistent with estimates of their typical luminosity and radius

Kazanas, D.

Protheroe, R. J.

English

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https://ntrs.nasa.gov/search.jsp?R=19830013817 2020-03-21T03:47:55+00:00Z
-MwF - - qPI 	 ' I
(NASit-TM-84974)
	 ACTIVE GALAXIES AND THE
DIFFUSE GAMMA-FAY 6ACK6HUUND (NASA)
	 14 p
HL AJ2/Mlr AJ 1
	 LSLL 03b
N83-22086
Unclas
G3/93 09544
ROM
Technical Memorandum 84974
I	 Active Galaxies and the Diffuse
Gamma-Ray Background
D. Kazanas and R. J. Protheroe
JANUARY 1983
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771
IORONAL PAGE a
OF POOR QUALr y
ACTIVE GALAXIES AND THE DIFFUSE
GAMMA-RAY BACKGROUND
D. Kazanas* and R. J. Protheroe*+
Laboratory for High Energy Astrophysics
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771, U.S.A.
A new model for the origin of relativistic particles and gamma-rays
in active galactic nuclei and quasars, together with recent HEA0-1
observationsz
 of the spectra of active galaxies from 2 to 165 keV, provide
the basis for a re-examination of the nature of the extragalactic gamma-ray
3A
background	 We find that active galaxies could account for the observed
-at
background if their X-rey spectra steepen to E	 above - 100 keV, as has
been observed in Cen- A , together with a further steepening to E 	 as a
result of absorption of gamma-rays by photon-photon pair production
interactions with X-ray photons. The compactness of active galaxies required
to give this steepening is consistent with current estimates of their typical
luminosity and radius.
*Also at Department of Physics and Astronomy, University of Maryland, U.S.A.
+Present Address: Department of Physics, University of Adelaide, Adelaide,
South Australia 5001, Australia.
r -
2ORIGINAL PAGE IS
OF POOR QUALM
C
The observed spectrum, as reviewed by Ramaty and Lingenfelter , is
shown in Fig,. 1. These recent observations are consistent with earlier
results 7,819 . In particular the feature at MeV energies observed in
balloon experiments (e.g. Shonfelder et. al. 9 ) is also apparent in the more
receat data. The X-ray spectrum from 3 keV to 50 keV is consistent with
thermal bremsstrahlung from a plasma at -40 keV and may result from a hot
fo
intergalactic medium	 while at the highest energies (30 - 150 MeV) the
-ray photon spectrum is consistent with a power law of index 2.7. Several
other possibilities have been proposed to account for the observed diffuse
1^ a s
gamma-radiation (see reviews by Silk 	 and Horstman at al. ^ ). Stecker
for example, has suggested that it may result from matter
antimatter-annihilation in a baryon symmetric universe, the annihilation
uccuring at the boundaries between domains of matter and antimatter.
Emission from unresolved active galaxies has also been proposed to account
for the diffuse gamma-ray f 	
14, Is, 16, 17
lux	 . To date, only one extragalactic
18
object, the quasar 3C273, has been detected at 100 MeV energies 	 The
contribution of quasars to the gamma-ray ba_kground is however quite
uncertain since the volurr.e density of gamma-ray emitting quasars is unknown.
1
Seyfert 1 galaxies show flat power law energ y spectra at X-ray energies 
9 
and
could contribute significantly to the diffuse gamma-ray flux. We shall
consider the latter possibility in this letter.
2
Recent HEA0-1 observations by Rothschild et al. 	 of 12 active galaxies
t!
i	 (the sample is dominated by Seyfert 1 galaxies) show that the X-ray photon
spectra of all these objects from 2 keV to 165 keV are consistent with a
single power law with exponent - 1.6. Upper limits exist above 30 MeV from
_L
3ORIGINAL PAa 18
OF POOR QV"
SAS-2 observations of 7 of these objects if and indicate that their spectra
must. Stppnpn hp tween hard X—ray and gamma—ray energies.
Rothschild et al. 2 also showed, using the luminosity function of
20
active galaxies from the HEA0-1 flux limited X—ray survey , that active
galaxies could account for all of the diffuse X—ray emission above — 150 keV
with no evolution. It has also been suggested	 that active galaxies might
account for the observed flattening in the hard X—ray background from 500 keV
to 2 MeV. We propose that the gamma—ray background may also be due to these
unresolved active galaxies and will show how the observed spectral shape
could arise naturally in the model for relativistic particles and gamma—rays
t.
in active galaxies and quasars described in our earlier work
The model i we use for the X—ray and gamma —ray spectra of active
galaxies has already been applied to the obs ,^rved spectrum of 3C273. The
salient features of this model are: a) protons are accelerated by the first
order Fermi mechanism at a shock in a spherical accretion flow onto a massive
black hole; b) relativistic protons have a spectrum which is a power law in
-(2+6)
energy E	 , where (;<<l  for strong shocks; c) nuclear interactions result
in production of gamma—rays from r decay and electrons from 7r-^--e decay,
lz•E)
both being produced with	 spectra E	 above several hundred MeV; d)
synchrotron and inverse Compton losses steepen the ambient electron spectrum
-^3tE)
to E	 ; e) gamma—ray photons from inverse Compton interactions then have
a photon spectrum E 
= (2t Erg, 
very similar to that of 7f *gamma—rays; f) the
high energy gaiama —ray spectrum is steepened by photon—photon pair production
interactions with X--rays.
4
ORIGINAL PAGE 18
OF POOR QUALITY
- (2 Ir
In this model, one might initially expect the E 	 gamma-ray
photon spectrum to continue without a break to X-ray energies, contrary to
observation. A break in the electron spectrum from E 
(30E)
at high energies
to -E 2 at low energies, may however manifest itself as a break in the photon
-(.2+ ^)	 -1.5
spectrum from E	 at 0 -ray energies to -E	 at X-ray energies. Two
possibilities may account for this. The first possibility is that
relativistic particles might leak out of the central object on some timescale
t(leak), perhaps producing the beams or jets associated with many active
galaxies 
22	
At energies below which the energy loss time is equal to the
leakage time, i.e. E/(dE/dt) > t(leak), the ambient electron spectrum would
have the same power law index as that at producton, i.e. 2 +E. The power law
index of the photon spectrun of inverse Compton X-rays would then be (3 +E) /2;
the obse rved spectral index of 1.6 implying E -0.2, consistent with shock
acceleration. The second possibility is that the observed X-ray spectral
index may result from the kinematical cut-off of secondary electrons, i.e.
those resulting from lr-µ-e decay. The production spectrum of secondary
electrons 1 due to pion production cuts off steeply below - 100 MeV below
which knock-on electrons become important. There is thus a change in the
production spectrum which may manifest itself as a change in slope of the
inverse Compton radiation. In the case of no leakar.e (energy losses balance
production) and if the knock-on component were small, the ambient spectrum
would be inversely proportional to the rate of energy loss below the cut-off
energy; i.e. an E 
-2
spectrum if synchrotron and inverse Compton losses
dominate. The resulting X-ray photon spectral index would be 1.5, just
slightly flatter than the observed spectrum. The inclusion of a knock-on
iS	 ^
'' aor POOR gym►
component will however steepen the spectrum slightly.
In active galaxies, we expect a further steepening in their gamma-ray
spectra due to photon-photon pair production interactions with X-ray photons.
For the photon spectrum which is observed at X-ray energies in most active
galaxies, E 1.6 , the gamma-ray spectral index would be steepened by 0.6,
relative to that at production, above the energy at which the optical depth
to this process is unity. This break energy depends on the X-ray source
compactness, the ratio of X-ray luminosity to the dimensions of the emitting
region. The expected steepening in photon spectral index from (2+E/2)-2.1 to
2.7 is just what is required if the observed diffuse gamma-ray background is
due to unresolved active galaxies.
The radio galaxy Cen-A has a very similar X-ray spectrum 5 to the
2
active galaxies observed by Rothschild et al. ; indeed, the compact object
responsible for the X-ray emission from Cen-A may be of the same class of
object as the X-ray sources in Seyfert nuclei (the nucleus of Cen-A is
obscured at optical to infra-red wavelengths by dust). In Cen A, the
spectrum is seen to break from E 	 below 140 keV to -E 2 above 140 keV 3.
Such an E-2 ,  or slightly steeper, spectrum is expected if the relativistic
particles result from shock acceleration and the emission is due to ivIerse
Compton scattering of soft photons. A further steepening must then occur at
some energy above a few MeV to be consistent with the SAS-2 upper limit^g.
This could then result from absorption of high energy gamma-rays by
photon-photon pair production interactions with the X-ray photons. (The
23
observation of a flux of gamma-rays above 100 GeV from Cen-A which is above
0
,. R .GINAL PAGE IS	 6
OF POOR QUALITY
- 2,7
an E	 extrapolation from the SAS-2 upper limit merely implies, in this
model, that the photon density at -2.5 eV in the region of the compact
gamma-ray source is low.)
In Figure 1, we show (solid line) the expected contribution of active
galaxies to the X-ray background calculated, assuming no evolution, by
Rothschild et al. 2 from the X-ray luminosity function of active galaxies
obtained by Piccinotti et al ? o and taking a low luminosity cut-off at
42
3.5x10	 erg/s. We have extrapolated this spectrum to 140 keV where we have
assumed that the X-ray spectrum of average active galaxies smoothly breaks
- 1.6	 -Z.i
from E	 to E	 , in a way similar to that observed in Cen-A. A further
-2.7
break is then required at -5 MeV (to E 	 ) to account for the observed
gamma-ray background from 30 - 100 MeV (see Fig. 1).
We shall now calculate the X-ray source compactness required to account
for the gamma-iay background with unresolved active galaxies. We do not
present here results of a full cosmological integration which would take into
account the red shifting of energy spectra of distant active galaxies which
I
contribute to the background. We have found, however, that the effect on the
	 i
spectral shape is small in the present case; the greatest contribution to the
i
gamma-ray flux being due to relatively nearby active galaxies. The optical
depth of gamma-rays of energy E 6 to photon-photon pair production is given
by:
V(E 9 ) . s J nx (Ex ) Q'(E d ,E,) dEx
04offt PAW is
OF 
POOR QUALITY
where: s - 1.33 R is the average distance through the source, of radius R;
uX(EX ) is the average density of X-ray photons at energy EX and is given by
nx (EX ) - LA(EX) 
EX2 / (
T R2 c)
where LX (EX ) is the X-ray luminosity at energy Ex (energy /unit time
/ln[energy]);Q(E d ,Ex ) is the cross section 
4 
for a gamma-ray photon of
energy E producing a pair on interacting with an X-ray photon of energy E .
	
-1.6	 X
For nX ( EX )OC E	 , and an optical depth of unity at the required break
energy, i.e.'C(5 MeV)-1, we find for the X-ray source compactness,
27
LX(2-10 keV)/R - 6x10
	 erg/s/cm. For a luminosity function 2 0 proportional
-
to (luminosity 
T2,75
 , the mean luminosity is 2.4 times the low luminosity
cut-off, L 1. Then, for L1 (2-10 keV) - 3.5x1042
 erg/s (solid line in Fig. 1),
42
the mean luminosity is 8.4x10 	 erg/s and we obtain a typical source radius
IS
of 1.4x10	 cm. The low luminosity cut-off in the X-ray luminosity function
43
of active galaxies is however somewhat uncertain. If L i
 were 10 erg/s,
then the contrib:ition to the X-ray background z would be as shown by the
dashed line in Fig. 1. The gamma -ray
 observations would then permit a single
-1.6
	
-2.7
spectral break from E 	 to E	 at about 2.5 MeV (i.e. energy at break
from spectral index of 1.6 to 2.1 coincident with that from 2.1 to 2.7),
producing the observed flattening in the energy spectrum at MeV energies. On
27
this assumption, the X-ray compactness would be 9x10	 erg/s/cm for a typical
active galaxy, corresponding to a source radius of 2.7x10 15 cm. Values of
the radii of Seyfert 1 galaxies obtained from their X-ray time variability ZS
	
K	 14
range from 6x10	 to 8x10	 cm and bracket the values we obtain here.
7
ORItifM PPM^.	 q
r . T' V	 OF POOR QUALA'Y
8
If the bulk of the diffuse t-ray background is due to active galaxies,
one might expect considerable fluctuations in the diffuse flux to arise from
the few nearest sources. Unfortunately , the magnitude of the fluctuations we
expect, after appropriately scaling the corresponding fraction of the X-ra y
fluctuations due to active galaxies 26 , is well below the upper limits of the
SAS-2 data 4 . Even the Gamma Ray Observatory (GRO) R7 , with its large area
and long exposure time, may still be limited by photon statistics.
If, however, the model proposed (ref. 1) for the I -ray spectra of active
galaxies is correct, GRO should be able to detect several of these objects.
It will then be possible to infer the active galaxy contribution to the
diffuse gamma-ray background and thus provide a test for the present
hypothesis.
In conclusion, we have shown that the observed spectrum of the diffuse
gamma-ray background could be accounted for by unresolved active galaxies,
even with no evolution, if their spectra are described by a spherical
accretion/shock acceleration model (ref. 1) and are steepened by
photon-photon pair production interactions at high energies.
We are indebted to R. Mushot=ky, R. Shafer and D. Thomson
for their helpful discussions on the fluctuations.
9
ORIGINAL PAGE IS
OF POOR QUALMY
REFERENCES
1. Protheroe R.J. and Kazanas D., Ap. J., in press (1983), February 15 issue.
2. Rothschild R.E. at al., Ap. J., in press.
3. Trombka J.I. at al., Ap. J., 212, 925 (1977)
4. Fichtel. C.E. et al., Ap. J., 222, 833 (1978)
5. Baity W.A. at al., Ap. J., 244, 429 (1981)
6. Ramat y R. and LinRenfelter R.E., Ann. Rev. of Nuclear and Particle
Science, 32, in press.
7. Horstman, H. M. et al., Rev. Nuovo Cimento, 5,255 (1975)
8. SchonfeUer, V. et al., Ap. J., 217, 306 (1977)
9. Pinkau K., Nature, 277, 17 (1979)
10. Marshall F.E. et al., Ap. J., 235, 4 (1980)
11. Silk J., Space Sci. Rev., 11, 671 (1970)
12. Silk J., An. Rev. As. Ap., 11, 696 (1973)
13. Stecker F.W., Ann. N.Y. Aced. Sci., 375, 69 (1981)
14. Strong A.W., Wolfendale A.W. and Worrell D.M. , J. Phys. 	 A. 9, 1553
(1976)
15. Grindley J.E., Nature, 273, 211 (1978)
16. Schonfelder V., Nature, 274, 344 (1978)
17. Bignami G.F. et al., Ap. J., 232, 64Q (1979)
18. Bignar.i C.F. et al., Aatron. Astrophys., 93, 71 (1981)
1 Q . Mushotskv R.F. et al., Ap. J., 235, 377 (1980)
20. Piccinotti G. et al., Ap. J., 253, 485 (1982)
21. Boldt E., Comments on Astrophys., 9, 97 (1981)
22. Rees M.J., IAU Symp. 94 'origin of Cosmic Rays', ed. C. Setti et al.,
Dordrecht: Reidel (1981)
10
OF POOR QUALITY
23. Grindlay J.E. et al., Ap. J. Lett., 197, L9 (1975)
2.4. Heitler W., 'The Quantum Theory of Radiation', Oxford: Oxford Univ. Press
(1954)
25. Bassani L. and Dean A.J., Nature, 294, 332 (1981)
26. Shafer, R. A., Ph.D Thesis, University of Maryland (1982)
27. Hughes, E. B., et al., Proc. I.E.E.E. Science Symposium,
San Francisco (1979)
. .o
11C R!GINAL PAGE 19
OF PO-OR QUALRY
FIGURE CAPTION
Fig. I.
Energy spectrum of the diffuse X-ray and V -ray background
(solid curve from 3 to 50 keV — ref. 7; filled circles -- preliminary
HEA0-1 data reported in ref. 2; open circles -- ref 3; 30 to 150 MeV
data -- ref 4). Solid and dashed lines show possible active galaxy
	 E
I
contributions to the diffuse background (see text for discussion); a
	 j
power law approximation to the photon spectrum is indicated for three
energy regions.
a i
m i
ORIGINAL
OF ^OR PAQ
QUA ,
10i
10°
a^
10
N
102a
^ -
^ 10 3
^n
z 104
z
105
C Os L
163
r
10 ,	 IG°	 10^
	
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ENERGY (MeV)


Active galaxies and the diffuse Gamma-Ray background

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