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 =1.95 ± 0.05 (). In most Seyfert 1s, a flattening of the spectrum is observed above 10 keV (). It is satisfactorily explained by Compton reflection from cold (T < 106K), optically thick matter (), presumably an accretion disk (; ).
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; ) (interstellar photoelectric absorption cross sections in the range 0.03-10 keV, computed assuming solar abundances, have been published by ). 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 ().
In most broad emission line objects, the Fe K line at 6.4 keV is observed with a mean EW of 100-150 keV (). 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 (; ; ; ; ). 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 ().
Most Seyfert 1s show evidence for an excess of soft X-rays above the hard X-ray power-law extrapolation, dominant below ~ 1 keV (; ). 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 (; ). Objects with a large UV (around 1375 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 (; ).
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 ().