After the first two years of observations of the satellite Ariel V in 1974–1976, it had been clear that X-ray emission is a common characteristic of most AGN (Elvis et al. 1978) and that a large part of their radiation power is emitted in this energy band. In addition, the fast variations of the AGN flux in the X-rays (down to hour time-scales; Grandi et al. 1992) indicate that they are associated with the innermost regions of the nucleus, close to the central black hole. All this assigns to the X-rays a crucial role for the understanding of the AGN phenomenon.
The properties of the X-ray emission are different for radio quiet and radio loud objects, as the strong jet present in the latter establishes the main emission process also at high energies. Within the radio quiet population, type 1 and type 2 objects present spectra whose differences can in general be explained by the unified model, i.e. as determined by the presence of dusty absorbing gas along the line of sight.
2.3.1 X-ray spectra of Seyfert 1
The X-ray spectra of Seyfert 1 galaxies extend from 0.1 to a few hundred keV and are characterised by the following components (Fig. 2.3).
Primary emission. To the first order, the intrinsic continuum of Seyfert galaxies is a power law, extending from 1–2 to a few hundred keV, and with typical photon index ranging between 1.8 and 2.
At high energy, an exponential cut-off is often observed between 60 and 300 keV. This emission is believed to be produced in a two-phase accretion disc, where soft photons from a cold (kT < 50 eV) thick disc are Comptonised by a hot (kT ∼ 100 keV) thermal electron gas in a thin corona located above the disc (Haardt & Maraschi 1993). The soft photons with initial energy Ei are upscattered to an energy that can be approximated by Ef =eyEi, where y ∼(4kT/mec2)max(τ, τ2) is the Compton parameter,τis the optical depth to Compton scattering and T is the electron temperature. Forτ >0.01 and y << 10 (i.e. non-relativistic electrons), this actually results in a power law spectrum up to a thermal cut-off at E∼kT−3kT , determined by the cut-off in the thermal distribution of the electrons (Pozdniakov et al. 1983; Mushotzky et al. 1993).
Reflection components. An important result of the Japanese mission Ginga was the discovery of a hump above 5 keV superposed to the continuum X-ray emission in Seyfert galaxies (Piro et al. 1990).
This feature is interpreted to be due to the primary X-rays that are reflected (i.e. Compton scattered) by optically-thick cold material subtending a large solid angle to the X-ray source. The exact shape of the reflection component varies with the geometry and chemical composition of the reflector, but in general it has a peak around 30 keV, where the reflection efficiency reaches a maximum. The nature of the reflecting medium is still uncertain, possibly being the accretion disc itself (Zdziarski et al. 1990) or the inner edge of the absorbing gas (Ghisellini et al. 1994) or a wind (Elvis 2000). A reflection
2For MBH=109M⊙and Ldisc/LEdd=3×10−3, the radius of the BLR is found to be RBLR=0.02 pc, following Ghisellini
& Tavecchio (2008).
Figure 2.3: Typical total X-ray spectrum (upper black line) with its main components of a type 1 AGN.The primary continuum is a power law with a high-energy cut-off and absorbed at low energies by warm gas. A soft-excess, a cold reflection component and the iron Kαemission line at 6.4 keV are also shown.
component from a warm reflector is also observed in some X-ray spectra of Seyfert galaxies and it has the same spectral shape of the incident emission.
The iron line. In most Seyfert galaxies an iron emission line at 6.4 keV is observed, with a typical equivalent width of 100–200 eV. This emission corresponds to the Fe Kαtransition (n = 2 to n = 1) of cold iron and is attributed to fluorescence of some cold material illuminated by an X-ray source.
The reflection hump and the Fe line are consistent with the idea that the primary X-ray emission is reprocessed and both these features are usually observed in AGN spectra and thought to be connected.
The line often shows two components, a broad and a narrow one. The broad line is thought to originate in the inner part of the accretion disc, and it varies following the continuum variations with almost zero delays. Instead, the narrow line (with widths ≤ 1000 km s−1; Risaliti & Elvis 2004) does not follow the variations of the continuum, suggesting that it origins on a farther reflector, maybe the absorbing circumnuclear gas (Matt et al. 1996). There is evidence in several objects of a broad, red wing on the Fe line (left panel in Fig. 2.4) that is commonly interpreted as due to relativistic broadening and gravitational redshifting produced by a spinning black hole in the core of the AGN (e.g. Fabian & Miniutti 2005). An alternative model, involving complex absorbers with different ionisation states, has been shown to represent well the Fe-line shape with its red wing without need of a relativistically-blurred component, at least in some objects (e.g. MCG−6−30−15, Miller et al.
2008, and NGC 3783, Reeves et al. 2004).
The soft-excess. Many AGN show a prominent excess of emission below 2 keV with respect to the extrapolation of the power law continuum (e.g. Porquet et al. 2004). This so-called soft-excess can be well fitted with a black body with temperatures of 0.1–0.2 keV for a large range of black hole masses, 106−9M⊙(Walter et al. 1994). These temperatures are too high to be explained by the standard accretion-disc model (Shakura & Sunyaev 1973) as thermal emission from the disc, unless one assumes for example super-Eddington luminosities. The soft-excess could instead be due to
The X-ray emission from AGN 15
Figure 2.4: Left: the 3–8 keV data fromSuzaku/XIS divided by a power law model show the presence of a broad Fe emission line in the Seyfert 1 MCG−6−30−15 (Miniutti et al. 2007). Right: 0.8–2.5 keV spectrum fromChandra/HETGS of the Seyfert 1 NGC 3783 (black curve). The numerous absorption features on the source continuum emission are well represented by a two-phase absorber (red curve; Krongold et al. 2003).
Comptonisation of the UV-disc photons, but its almost constant temperature would be difficult to explain, considering that it should be related to the disc temperature, depending in turn on the black hole mass. A more natural explanation for this constant temperature would therefore come from atomic processes, smeared out by high velocities or relativistic effects. In this frame, different models have been proposed, suggesting that the soft-excess origins as reflected X-ray emission (relativistic-blurred) on a disc partially ionised by the incident flux (Ross & Fabian 2005), or as relativistic-blurred absorption from a disc wind, producing a large absorption feature in the spectrum at 1–2 keV and therefore an apparent soft-excess below 1 keV (Gierli´nski & Done 2004).
Warm absorbers. Thanks to ASCA observations first and more recently to the high resolution spectra obtained by XMM-Newton and Chandra, it has been possible to observe strong absorption features in the X-ray emission of AGN. The spectral characteristics of these features associate them to warm absorbers probably in the form of outflowing gas. The geometry, the location and the physical conditions of the absorbing gas are still under debate. For the best studied case of NGC 3783 (right panel in Fig. 2.4), a two-phase absorbing medium with different temperatures and ionisation states, but in pressure equilibrium, might be required (Krongold et al. 2003), whereas Gonçalves et al. (2006) suggested for the same object a single medium with different temperatures producing a stratification of the ionisation structure. Studying the variations of the ionisation parameters, Nicastro et al. (2007) proposed that in NGC 4051 the warm absorber is associated to outflows from the disc at a distance of .5 light days consistent with the high-ionisation BLR, whereas in NGC 5548 the absorber would be located at larger radii (>0.2 pc), rather consistent with the torus region.
2.3.2 X-ray spectra of Seyfert 2
In the unified models, the differences between the X-ray spectra of type 1 and type 2 AGN can be accounted for by considering the presence of absorbing/scattering material on the line of sight. This is supported by the observations, showing that the X-ray emission from Seyfert 2 can be represented with the same components found in Seyfert 1, but modified by a certain amount of absorbing material.
Figure 2.5: Examples of X-ray spectra for an unabsorbed Seyfert 1 galaxy, IC 4329A (left panel, Zdziarski et al. 1996), and the absorbed (NH∼2×1022cm−2) Seyfert 1.9 MCG–5–23–16 (right panel, Reeves 2007).
In the X-rays obscuration is due to photoelectric absorption and Compton scattering. Depending on the amount of absorbing material in the line of sight (parametrised with the hydrogen column density NH, in atoms cm−2 units), part of the AGN X-ray emission can be absorbed between 1 and 10 keV when NH<1.5×1024cm−2, or even up to a few tens of keV, if 1024< NH<1025cm−2(Fig. 2.5).
The iron line equivalent width is found to depend on the absorbing column density, showing larger values for more obscured sources (Risaliti & Elvis 2004). This is interpreted as due to the fact that the line is, at least in part, emitted by the reflecting material along free lines of sight, whereas the continuum at the line energy is strongly dependent on the NHand is more absorbed than the line.
The primary continuum emission of Seyfert 2 in the X-rays is also well represented by a power law, on average harder than in Seyfert 1 (see Section 5.2.1) and with a high-energy cut-off in the same range of energies as in Seyfert 1. A reflection component is present as well; in particular for Compton-thick objects (i.e. NH>1024cm−2) no direct emission but only the reflected and the scattered continua are observable below 10 keV. Soft-excess emission is present in Seyfert 2 and could be associated to the warm gas confining the BLR rather than to the disc (Risaliti 2002). In fact, in type 2 AGN the emission from the disc, if present, is expected to be absorbed like the power law component.
2.3.3 X-ray spectra of radio loud AGN
The presence of a jet dominates the properties of radio loud AGN over several energy decades. As seen in Sect. 2.2, the spectral energy distribution is characterised by two broad peaks where most of the emission is produced through synchrotron and inverse Compton processes. Differently from radio quiet AGN, the X-ray emission of radio loud AGN is therefore thought to be of non-thermal origin and it is located around the minimum between the two peaks of the SED, where both synchrotron and inverse Compton can contribute. In particular, in FSRQs, the X-ray emission corresponds to the beginning of the Compton peak (Fig. 2.6, top left panel) and in LBL it corresponds to the transition between the synchrotron and the Compton peaks (Fig. 2.6, top right panel) In HBL, the synchrotron emission usually peaks in the soft X-ray band (Fig. 2.6, bottom left panel), but the peak can move up to 100 keV during flaring states, as observed, for example, in Mrk 501 in April 1997 by BeppoSAX (Pian et al. 1998; Tavecchio et al. 2001). This makes the X-ray studies of these objects very valuable to understand what is the relative importance of the synchrotron and inverse Compton processes.
The hard X-ray emission of AGN 17
Figure 2.6:Examples of spectral energy distributions of radio loud AGN.Top left:SED of the FSRQ 3C 454.3 (Pian et al. 2006). Top right: SED of the LBL BL Lac (Guetta et al. 2004). Bottom left: SED of the HBL 1E S1959+650 (Tagliaferri et al. 2003).Bottom right:SED of the radio galaxy Centaurus A (Steinle 2006).
A contribution from thermal Comptonisation to the spectra of radio loud AGN cannot be excluded and might be hidden or diluted by the much stronger non-thermal component. Especially in radio galaxies the results are more controversial, with the detection of high-energy cut-offs (Molina et al.
2007), narrow Fe lines (Rothschild et al. 2006) but no or weak reflection components. There could be a transition leading from blazars to radio galaxies to Seyferts, in which the thermal Comptonisation becomes more and more important over the non-thermal emission and reprocessing features, like the reflection component and the Fe line, slowly appear and become stronger (Grandi et al. 2006).