Vl.~asin Axtronomy, Vol. 31, pp. 579-583 . 1988 0083-6656188 $0.00+ .50

Copyright © 1988 Science Press & Pergamon Journals Ltd.

COSMIC GAMMA RAYS FROM ACTIVE GALACTIC NUCLEI

E. C. M. Young Dept. of Applied Science, City Polytechnic of Hong Kong

K. N. Yu Physics Dept., University of Hong Kong, Hong Kong

ABSTRACT

We have used the SAS-Z y-ray data to search for y-rays of energy >I00 MeV from extragalactic active galactic nuclei (AGNs). The correlation function devised by the COS-B group to search for galactic y-ray sources has been used. We have identified 18 such AGNs, including 13 quasars, 3 BL Lac objects, 1Seyfert galaxy and 1 radiogalaxy. We have further investigated the relationship between the y-ray fluxes in the energy regions > I00 MeV and 35-100 MeV and their redshifts, and we have also derived their y-ray spectral indices. The results give further support to our claim that the sources detected are genuine extragalactic objects.

I. INTRODUCTION

In searching for y-rays from AGNs, we attempt to look for statis-

tically significant excesses above ~he background at the AGN

positions. The main limitation of the SAS-2 data is the small number

of photon~ detected, and as a result it is necessary to use the

"Cross-correlation function" ~Hermsen, 1980) in our analysis.

The cross-correlation function was originally devised by

Hermsen (1980) to look for y-ray point sources from the COS-B

satellite fluxes around a source are correlated with the detector

point spread function (PSF) to give the correlated count or the

correlated flux of the source. A y-ray source is defined as a

significant excess of the correlated count or the correlated flux

over the underlying background. From the amount of correlation, we

can also derive the source strength, or in the case of lack of

correlation we can obtain an upper limit to the source strength.

The PSF of a detector is expressed as (Hermsen, 1980)

f(Q) = N exp (-(¢I¢)2c), ( I ) 0

where N is a normalisation constant,0 is the angular distance from

579

580 E.C.M. Young and K. N. Yu

from the source, 0 ° and c are parameters that are different for

different detectors and received photon energies. We adopt the

values(Houston, 1985) of 0 =I752 and c=0.5 for E >I00 MeV, and o y

0 = 5 ° and c=0.8 for 35 MeV < E <I00 MeV for the SAS-2 detector. o y

It should be remarked here that it does not appear possible to Use

this method to search for extragalactic y-ray sources with the

COS-B data in view of the low y-ray fluxes of these sources and

the relatively high and uncertain instrumental background of the

SAS-2 detector.

2. THE METHOD AND THE RESULTS

We exclude the region of the Galactic plane (Ibl % 22 ° ) to avoid

the influence of Galactic sources which will affect the values of

correlations. Outside this region, the cosmic background is assumed

to be a non-structured, flat, diffuse extragakactic one, and we hay.

therefore used the equations fo a flat background En searching for

correlations. We also exclude all the bins with tbI~73.4 ° since the

SAS-2 bins are too large here to give good positional accuracies.

The correlation functrions are determined only at the centre

of the bins. The correlated fluxes C of the range >I00 MeV are

calculated from a matrix with a size of 12.5 ° (q) x -12.5 ° (b).

Those bins with no y-ray data and those with SAS-2 sensitivity

values less than 15 units are omitted in calculating the correla-

tions. The SAS-2 sensitivity values reflect the exposure of the

SAS-2 detector to a particular region of the sky and the bins with

too low sensitivity values are those near the edge of the SAS-2

field of view where the sensitivity normalisation is less centain

(Fichtel et al,, 1978).

In the region 22°<~b~ % 73.4 ° , there are all together 7037

useful bins in the energy region >I00 MeV. We consider that a source

occurs when C/a o ~ 3.63 so that statistically there is less than one

spurious source in our list of sources, where a ° is the parent

standard deviation of a flat background caused by a random distribu-

tion of fluxes. There are all together 59 such bins. Identification

is made with the AGNs that fall within 3.8 ° of the SAS-2 peak (I~

width of the PSF for >I00 MeV ). Similarly, we have calculated the

correlations for the energy range 35-100 MeV. The correlated fluxes

Cosmic Ga aa Rays 581

C are calculated from a matrix with size of ~20 ° (I) x ~20 ° (b).

The identified AGNs are considered to e genuine Y-ray sources if

they also fall within 6.4 ° (1o width of lhe PSF for 35-]00 MeV)

of a C/o o ~ 2.5 peak for the 35-100 MeV i gion (so that th~

statistically expected number of spurious ~urces within the 1o

width for 35-100 MeV of a particular source s less than one).

There are 18 such AGNs. In Table I, we have I sted the redshifts,

the y-ray flu=~es for the two energy ranges F ">I00 MeV) and

Fy(35-]O0 MeV), and the derived Y -ray spectral ndices ~y for the

18 AGNs.

3. DISCUSSION

Further analysis has been made in order to make sure ~at the

detected sources are genuine. In Fig. l, we have plotteo the distri- --] -9

bution of Fy (>I00 MeV) (in units of 10 -6 photons s cm ') with the

redshift z. It can be seen that in general F decreases with z and Y

this is expected for extragalactic y-ray sources. In Fig.2, we

show the relationship between F (>I00 MeV) and F (35-100 MeV). Y Y

Good,correlation and a positive slope are apparent as would be

expected for genuine extragalactic sources. These two results have

demonstrated that the 18 sources detected in y-rays are very likely

to be genuine. In Fig.3, we have shown the distribution of ~ As Y

these are close to 2.7, which is characteristic of the extragalactic

background, in contrast to the value of about 2.0 for galactic

sources and the galactic background, we conclude that these sources

should be of extragalactic origin.

ACKNOWLEDGEMENTS

The authors are grateful to Professor A. W. Wolfendale, F.R.S. for

helpful discussions. YKN acknowledges the financial support from

the Croucher Foundation for joining this symposium.

582 E.C.M. Young and K. N. Yu

0 I

I l

+t + Z

I

2 3

I 1 I 1

2

Fig. I. The distribution

of Oy_~ h -~-2 F >I00 MeV) i un'ts of ] p otons s cm with the redshift z.

Fig. 2. The distribution of Fy (<I00 MeV) with F~ (35-100 M~V), both i~l units of I0- photons s cm The uncertainties in F y (35-100 MeV) are omitted for the uncertainty is shown in the figure).

4

21 0uAsA s l

Fig. 3. The values of the y-ray spectral indices. • :quasars; A : BL Lac objects; • : Seyfert galaxy; ~: radio- galaxy. Dotted line: average value of ~ for detected sources which is 2.67.

Cosmic Gamma Rays 583

Table 1

a Object z

Fy (> ]OOMeV) F (35- 100MeV) - -I -2

(x;0 ;6photons s cm ) C~ -(

1144+115 2.438 0.35+0.12 1121+423 0.234 0.43+0.21 1049+302 0.97 0.34+0.19 1427+480 0.221 0.60+0.33 1238+006 0.31 0.45+0.22 1328-034 1.352 0.47+0.22 1011+250 1.631 0.60+0.27 1352-104 0.332 0.87+0.26 0923+201 0.190 0.94+0.42 1806+456 0.830 1.13+0.53 0726+431 1.072 1.38+0.68 0251-675 1.002 0.49+0.22 0153-520 0.19 . 0.64+0.34 1133+704 0.046 D 1.02~0.46

C 0317+185 0. 19 0.98+0.38 0306+102 1.07+0.50 " ' ' ' "

MKN 34 0.051 d 1.05+0.63 1130-037 0 . 0 4 8 2 e 0 . 7 1 ~ 0 . 2 4

1 56+0.30 1 88+0.34 ; 89+0.38 2 83+0.78 I 81+0.36 2 41+0.36 4 99+0.65 3 98+0.44 4.88+0.68 5 40+1.16 3 54 +0 ; 94 4 58+0.63 3 55+0.91 3 31+0.82_ 4 80~I .08~ 4 80~I.08 t 5.54+ 1.02 2.73+0.35

2.62(3.10,2.24) 2.60(3.29,2.17) 2.79(3.65,2.28) 2.66(3.53,2.11) 2.53(3.23,2.09) 2.73(3.37,2.31) 3.13(3.75,2.70) 2.63(3.01,2.35) 2.74(3.34,2.34) 2.67(3.36,2.21) 2.21(2.91,1.78) 3.23(3.84,2.79) 2.79(3.63,2.24) 2.38(3.02,1.94) 2.69(3.27,2.26) 2.62(3.31,2.16) 2.75(3.68,2.24) 2.50(2.93,2.19)

aFrom Hewitt and Burbidge (1987) unless otherwise stated.

bFrom Schwartz and Ku (1983).

CFrom Gioia et al (1984).

dFrom Veron-Cetty and Veron (1984).

eFrom Burbidge and Crowne (1979).

fit is assumed that the BL Lac Objects 0317+185 and 0316+102

contribute equally to the same >100 MeV excess.

REFENENCES

Burbidge, G. & Crowne, A.H. (1979) Astrophys. J. Suppl., 40 , 583. Fichtel, C.E. et al. (1978) NASA Tech. Memo 79650. Gioia, I.M. et al. (1984) Astrophys. J., 283 , 495. Hermsen,W. (1980) Ph.D. Thesis, University of Leiden. Hewitt, A. & Burbidge, G. (1987) Astrophys. J. Suppl., 63 , I. Houston, B.P. (1985) Ph.D. Thesis, University of Durham. Schwartz, D.A. & Ku, W.H.-M. (1983) Astrophys. J., 266 , 459. V~ron-Cetty, M.P. & Veron, P. (1984) ESO Scientific Report No.l.