The average X-ray/gamma-ray spectrum of radio-quiet Seyfert 1s

Abstract

We have obtained the average 1-500 keV spectrum of radio-quiet Seyfert 1s using data from EXOSAT, Ginga, HEAO-1, and GRO OSSE. The spectral fit to the combined average EXOSAT and OSSE data is fully consistent with that for Ginga and OSSE, confirming results from an earlier Ginga/OSSE sample. The average spectrum is wellfitted by a power-law X-ray continuum with an energy spectral index of α ≃ 0.9 moderately absorbed by an ionized medium and with a Compton reflection component. A high-energy cutoff (or a break) in the the power-law component at a few hundred keV or more is required by the data. We also show that the corresponding average spectrum from HEAO-1 A1 and A4 is fully compatible with that obtained from EXOSAT, Ginga and OSSE. These results confirm that the apparent discrepancy between the results of Ginga (with α ≃ 0.9) and the previous results of EXOSAT and HEAO-1 (with α ≃ 0.7) is indeed due to ionized absorption and Compton reflection first taken into account for Ginga but not for the previous missions. Also, our results confirm that the Seyfert-1 spectra are on average cut off in γ-rays at energies of at least a few hundred keV, not at ∼ 40 keV (as suggested earlier by OSSE data alone). The average spectrum is compatible with emission from either an optically-thin relativistic thermal plasma in a disk corona, or with a nonthermal plasma with a power-law injection of relativistic electrons.

X-ray spectral parameters for a sample of 95 active galactic nuclei

“…Also there may be an effect of wrong accounting of a reflection or the presence of of cut off E -in the energy range less than~100 keV ( Tables 3 and 4 are typical representatives of this effect. Note that Gondek et al (1996), Zdziarski et al (1995) and Zdziarski (2000) found the flattening of the spectra for galaxies with strong absorption, and our data are consistent with these results.…”

The second INTEGRAL AGN catalogue

“…by Gondek et al (1996) using combined EXOSAT, Ginga, HEAO-1, and CGRO/OSSE spectra. A study of BeppoSAX PDS spectra of 45 Seyfert galaxies came to a similar conclusion, although the spectra of Seyfert 2 appeared steeper when considering a possible cut-off in the spectra of Seyfert 1 galaxies (Deluit & Courvoisier 2003).…”

AChandraObservation of the Circinus Galaxy

“…The shape of the incident spectrum above 10 keV is important, even when considering measurements below this energy, because of the e †ects of Compton downscattering ; the power-law continuum was therefore taken to be exponentially cut o † with an e-folding energy of 150 keV, similar to that observed in other Seyfert galaxies Gondek et al 1996). Finally, we added a thermal bremsstrahlung continuum to represent the soft X-ray emission (a power-law continuum of large photon index could have been used instead).…”