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Astron. Astrophys. 326, 51-58

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A spectral study of gamma-ray emitting active galactic nuclei

M. Pohl1, R.C. Hartman2, B.B. Jones3, and P. Sreekumar2,4

1Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
2NASA/Goddard Space Flight Center, Code 660, Greenbelt, MD 20771, USA
3W.W. Hansen Expt. Physics Lab., Stanford Univ., Stanford, CA 94305-4085, USA
4Universities Space Research Association, USA

Received 4 November 1996 / Accepted 12 March 1997


In this paper we present a statistical analysis of the FORMULA -ray spectra of flat-spectrum radio quasars (FSRQ) compared to those of BL Lacs. The average spectra and possible systematic deviations from power-law behaviour are investigated by summing up the intensity and the power-law fit statistic for both classes of objects. We also compare the time-averaged spectrum to that at the time of FORMULA -ray outbursts.

The spectrum of the average AGN is softer than that of the extragalactic FORMULA -ray background. It may be that BL Lacs, which on average have a harder spectrum than FSRQs, make up the bulk of the extragalactic background.

We also find apparent cut-offs at both low and high energies in the spectra of FSRQs at the time of FORMULA -ray outbursts. While the cut-off at high energies may have something to do with opacity, the cut-off at low energies may be taken as indication that the FORMULA -ray emission of FSRQs is not a one component spectrum.

Key words: methods: statistical - BL Lacertae objects: general - quasars: general - Gamma-rays: observations

Send offprint requests to: http://www.dsri.dk/ FORMULA mkp/

© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 20, 1998