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
([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
K
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
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
([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]).