B eppo SAX Average Spectra of Seyfert Galaxies

Abstract

We have studied the average 3-200 keV spectra of Seyfert galaxies of type 1 and 2, using data obtained with BeppoSAX. The average Seyfert 1 spectrum is well fitted by a power-law continuum with photon spectral index Γ ~ 1.9, a Compton reflection component R ~ 0.6-1 (depending on the inclination angle between the line of sight and the reflecting material), and a high-energy cutoff at around 200 keV; there is also an iron line at 6.4 keV characterized by an equivalent width of 120 eV. Seyfert 2 galaxies, on the other hand, show stronger neutral absorption [NH = × 1022 atoms cm-2], as expected, but are also characterized by an X-ray power law that is substantially harder (Γ ~ 1.75) and with a cutoff at lower energies (Ec ~ 130 keV); the iron line parameters are instead substantially similar to those measured in type 1 objects. There are only two possible solutions to this problem: to assume more reflection in Seyfert 2 galaxies than observed in Seyfert 1 galaxies or more complex absorption than estimated in the first instance. The first possibility is ruled out by the Seyfert 2 to Seyfert 1 ratio, while the second provides an average Seyfert 2 intrinsic spectrum very similar to that of the Seyfert 1. The extra absorber is likely an artifact due to summing spectra with different amounts of absorption, although we cannot exclude its presence in at least some individual sources. Our result argues strongly for a very similar central engine in both types of galaxies, as expected under the unified theory.