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2.4. X-rays

The X-ray properties of bright, optically selected quasars have been intensively studied in the last 25 years (Elvis et al. 1978; Zamorani et al. 1981), mostly with broadband but low resolution spectra. The X-ray emission from quasars extends from the Galactic absorption cut-off at ~ 0.1 keV up to ~ 300 keV. Laor et al. (1997) analyzed the ROSAT soft X-ray (0.5 - 2 keV) observations of the sample of local (z < 0.4) PG quasars, and a subsample of these objects has been studied with ASCA in the 2 - 10 keV band (George et al. 2000) and with BeppoSAX in the 1 - 100 keV band (Mineo et al. 2000). Recent studies of samples of bright Seyfert 1 galaxies are reported in George et al. (1998; ASCA observations) and in Perola et al. (2002; BeppoSAX observations). The main properties of the X-ray spectra of type I AGN are briefly summarized below and are shown in Figure 3.

Figure 3

Figure 3. Average total spectrum (thick black line) and main components (thin grey lines) in the X-ray spectrum of a type I AGN. The main primary continuum component is a power law with an high energy cut-off at E ~ 100 - 300 keV, absorbed at soft energies by warm gas with NH ~ 1021 -1023 cm-2. A cold reflection component is also shown. The most relevant narrow feature is the iron Kalpha emission line at 6.4 keV. Finally, a "soft excess" is shown, due to thermal emission of a Compton thin plasma with temperature kT ~ 0.1 - 1 keV.

bullet Primary emission. The intrinsic continuum X-ray emission of quasars is to first order a power law, extending from about 1 keV to over 100 keV. However, as higher resolution and better signal-to-noise spectra have become available, emission and absorption features have been found that mask a direct view of this "power law" over virtually the whole X-ray band (see Fig. 3). Hence, slight curvatures may be present but unseen. The typical spectral index (3) is between alpha = -0.8 and alpha = -1, both for low luminosity Seyfert galaxies and high luminosity quasars. Radio-loud AGN have a somewhat flatter spectrum (alpha between -0.5 and -0.7). This is thought to be due to the additional hard component emitted by inverse-Compton scattering of the electron in the jet on the radio-synchrotron photons, but this is not fully established.

There is now increasing evidence, mostly from BeppoSAX, for a roughly exponential cut-off to the power law at energies ~ 80 - 300 keV. This is presumably due to the cut-off in the energy distribution of the electrons responsible for the X-ray emission. It is still debated whether the spectral index is redshift or luminosity dependent (Zamorani et al. 1981; Avni & Tananbaum 1982, 1986; Bechtold et al. 2003; Vignali et al. 2003).

In addition to the main power law continuum component, a soft emission component is often observed in AGN, with characteristic temperature kT ~ 0.2 - 1 keV. The physical origin of this component is not clear: warm emitting gas could be located in the accretion disk, or in the broad line region (it could be the confining medium of the broad emission line clouds), or in a region farther from the center. Alternatively, this "soft excess" could be an extension of the big blue bump to higher energies, e.g., via Compton scattering in a hot accretion disk corona (Czerny & Elvis 1987).

bullet Reflection components. The primary emission of AGN can be "reflected", i.e., Thomson scattered by ionized gas. If the reflector has a column density NH > 1.5 × 1024 cm-2 (i.e., ~ 1 / sigmaT) and is not fully ionized, the reflected component has a spectrum like the one shown in Figure 3 (the actual shape slightly varies, depending on the geometry and chemical composition of the reflector). The main features of this reflection component are a continuum due to electron scattering with a peak at ~ 30 keV, and a cut-off at 4 - 5 keV due to photoelectric absorption of the lower energy incident radiation. The reflection efficiency is typically a few percent of the direct emission in the 2 - 10 keV range because of photoelectric absorption, rising to ~ 30% at the 30 keV peak for a Compton-thick reflector covering a significant fraction of the solid angle (Ghisellini et al. 1994). The efficiency drops if the reflecting medium is Compton thin (in this case part of the incident radiation escapes without interaction).

A warm, ionized reflector must be present in the central region of many AGN (since we see a "warm absorber" in ~ 50% of Seyfert 1 galaxies). The reflected emission has the same spectral shape as the incident continuum.

bullet Iron line. The most prominent narrow feature in the 2 - 10 keV X-ray spectra of AGN is an iron emission line at energy 6.4 keV, corresponding to the Fe-K n = 2 - 1 transition of "cold" (i.e., leq FeXVII) iron. The line is usually ascribed to emission due to fluorescence in the inner part of the accretion disk. The typical EW of the line is 100 - 200 eV. There is also evidence for a broad "red wing" extending to lower energies (Tanaka et al. 1995; Nandra et al. 1997). Once thought to be widespread, XMM-Newton spectra now show signs of this red wing only in a few objects (MCG-6-30-15 being the clearest example, Vaughan & Fabian 2004; see Fig. 4). This red wing has caused great excitement as a likely physical cause is the gravitational redshift and relativistic Doppler shift of an Fe-K line originating from an accretion disk at only a few Schwarzschild radii (RS) from the central black hole. Such a broad line would be one of the best tools to look for general relativistic effects in strong gravity. Asymmetric profiles have been calculated for lines emitted at a few Schwarzschild radii from non-rotating (Fabian et al. 1989) and rotating (Laor 1991) black holes.

Figure 4

Figure 4. Iron line profile in an XMM-Newton observation of the Seyfert 1 galaxy MCG-6-30-15. Model is a power law fitted in the 3 - 5 keV and 8 - 10 keV energy bands. Figure is from Vaughan & Fabian (2004; their Fig. 8).

A second, narrow component of the Fe-K line is very clearly present in most AGN. The width of this "narrow" line (which is unresolved in CCD spectra from ASCA, Chandra ACIS, or XMM-Newton EPIC) is a few 1000 km s-1 or smaller when measured with the Chandra HETG spectrograph. This is similar to the width of optical and UV broad emission lines. This narrow component does not vary when the continuum varies, even for delay times of days. Coupled with the line width, this suggests an origin well beyond a few RS, although a small radius is not fully ruled out (Fabian et al. 2002). In the next few years, the ASTRO-E2 satellite, with 6 eV resolution (R = 1000) and an effective area of ~ 150 cm2, is expected to do much better in understanding this issue. If the reflector is highly ionized, the peak energy of this line can be shifted toward high energies (6.7 keV for helium-like iron and 6.96 keV for hydrogen-like iron). It is also possible that two narrow components are present in the spectrum, one emitted by a cold reflector and the other by an ionized reflector. CCD detectors like the ASCA SIS or XMM-Newton EPIC (with energy resolution of ~ 120 - 150 eV at 6 keV) are unable to separate these two lines, while the Chandra HETG spectrograph has limited effective area (~ 40 cm2) at 6 keV.

bullet Warm absorbers. Warm absorber features are present in the soft X-ray spectra of half of the bright Seyfert 1 galaxies observed with ASCA (Reynolds et al. 2000). Recently, the availability of high resolution soft X-ray spectra, obtained with the grating instruments onboard Chandra and XMM-Newton, show that this component is formed by an outflowing gas. We show in Figure 5 the highest signal-to-noise high resolution spectrum of an AGN, obtained with a long observation of the Seyfert 1 galaxy NGC 3783. Recently, Krongold et al. (2003) were able to reproduce all of the observed lines with a two-phase absorber, with the two phases in pressure equilibrium.

Figure 5

Figure 5. Chandra grating spectrum of NGC 3873, superimposed with a simple two-component model fitting most of the absorption features. Figure is from Krongold et al. (2003; their Fig. 3).

3 X-ray astronomers tend to use the "photon index" Gamma, where N(E) propto E-Gamma and alpha = - (Gamma - 1). Back.

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