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 well fitted by a power-law X-ray continuum with an energy spectral index of alpha~=0.9 moderately absorbed by an ionized medium and with a Compton reflection component. A high-energy cut-off (or a break) in 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 alpha~=0.9) and the previous results of EXOSAT and HEAO-1 (with alpha~=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 gamma-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 either with emission from an optically thin relativistic thermal plasma in a disc corona, or with a non-thermal plasma with a power-law injection of relativistic electrons.