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2.2.2. The X-ray properties of Seyfert 1 galaxies

The X-ray spectra of Seyfert 1s are dominated in the 2-10 keV (HX) range by a power-law component with a photon spectral index Gamma =1.95 ± 0.05 ([141]). In most Seyfert 1s, a flattening of the spectrum is observed above 10 keV ([312]). It is satisfactorily explained by Compton reflection from cold (T < 106K), optically thick matter ([272]), presumably an accretion disk ([264]; [286]).

Seyfert 1s and QSOs generally do not contain significant column densities of neutral gas in excess of the Galactic value (NH < 1020 cm-2) (Gondhalekar et al. 1997; [246]) (interstellar photoelectric absorption cross sections in the range 0.03-10 keV, computed assuming solar abundances, have been published by [304]). However Mark6, a Seyfert 1.5, has an X-ray spectrum with complex absorption implying column densities of neutral hydrogen of ~ (3-20) 1022 cm-2 typical of a Seyfert 2 ([118]).

In most broad emission line objects, the Fe Kalpha line at 6.4 keV is observed with a mean EW of 100-150 keV ([312]). Very often, the iron line is resolved with FWHM up to ~ 50 000 km s-1; it is redshifted, asymmetric and variable. It is thought to arise from fluorescence from the innermost regions of a X-ray illuminated relativistic accretion disk close to the central BH ([406]; [315]; [176]; [204]; [462]). In other objects, such as BLRGs, the line is narrow and not variable and may be formed in the molecular torus if its hydrogen column density is larger than NH = 1023 cm-2 ([481]).

Most Seyfert 1s show evidence for an excess of soft X-rays above the hard X-ray power-law extrapolation, dominant below ~ 1 keV ([422]; [457]). The nature of this soft X-ray excess is still an open issue. A multi-temperature black body (with temperatures in the range 40-140 eV) gives a satisfactory fit for most of the sources ([91]; [338]). Objects with a large UV (around lambda1375 Å) excess do also show a strong soft X-ray excess suggesting that the big blue bump in Seyfert 1s is an UV to soft X-ray bump; the relative strength of the big blue bump to the HX component varies by a factor of up to one hundred from object to object ([457]; [458]).

X-ray variability is very common among Seyfert 1s; in many of these objects, the variability amplitude below 2 keV is greater than that in the hard X-ray band ([313]).

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