Seyfert galaxies as X-ray sources

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Abstract
The Ariel V Sky Survey Instrument has detected X-ray emission from 13 Seyfert galaxies (11 of which have not been previously detected at X-ray wavelengths), bringing the total number of Seyfert galaxies detected in this waveband to 15. Two of these galaxies were not known to be Seyferts prior to these observations and have been discovered by obtaining optical spectra of galaxies inside error boxes of previous unidentified Ariel V sources. In this paper we give a detailed description of the identifications, both from statistical and individual points of view, and a discussion of the implications of the results. The galaxies detected are the brightest, at optical wavelengths, of the Type 1 Seyferts, with the exception of NGC 1275 whose classification is uncertain. The power radiated in the 2–10 keV band is typically in the range 1042.5–1044.5 erg/s. At least three of the galaxies exhibit variable X-ray flux density. Using a compilation of published data together with new observations from the Anglo-Australian Telescope, we find the X-ray power to be correlated with the infrared and optical continuum powers, and with the luminosity in the Hα emission line. The X-ray power is not significantly correlated with the radio power or the luminosity in forbidden optical emission lines. These results indicate that the X-ray emission originates in the dense ‘core’ region of extent < 0.1 pc which is peculiar to Type 1 Seyferts. The X-ray emission mechanism is discussed in the light of this conclusion. The X-ray luminosity function of the Seyfert galaxies is derived and shown to drop rapidly outside the observed range of X-ray luminosities. This luminosity function is used to show that Seyfert galaxies contribute 6 (±3) per cent of the isotropic background radiation in the energy range 2–10 keV, on the assumption that there is no cosmological evolution of their density or luminosity. If the X-ray spectrum of NGC 4151 is typical of Seyfert galaxies, this fraction becomes ≃ 15 per cent at 50 keV. By demanding that the contribution to the background intensity from Seyferts be less than the observed value, an upper limit to any cosmological evolution of their X-ray volume emissivity is obtained.

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