In this chapter we discuss the X-ray radiation from relativistic accretion disks around supermassive black holes, supposed to exist in the centers of Active Galactic Nuclei (AGN). Our focus is on the X-ray radiation, especially in the Fe K$\alpha$ line which originates in the innermost parts of an accretion disk. Moreover, here we discuss some effects which can disturb the Fe K$\alpha$ profile and cause its rapid and irregular variability, observed in the X-ray spectra of some AGN. We will pay attention to three such effects: perturbations in the disk emissivity, absorbtion by warm absorbers and gravitational microlensing. The X-ray emission from accretion disks around non-rotating (Schwarzschild metric), as well as rotating (Kerr metric) supermassive black holes, is discussed. The X-ray radiation of AGN is probably produced in a compact region near their central supermassive black holes, and can provide us some essential information about the plasma conditions and the space-time geometry in these regions. The goal of this chapter is mainly to present a short overview of some important and recent investigations in this field.
Deep Dive into X-ray Emission From Accretion Disks of AGN: Signatures of Supermassive Black Holes.
In this chapter we discuss the X-ray radiation from relativistic accretion disks around supermassive black holes, supposed to exist in the centers of Active Galactic Nuclei (AGN). Our focus is on the X-ray radiation, especially in the Fe K$\alpha$ line which originates in the innermost parts of an accretion disk. Moreover, here we discuss some effects which can disturb the Fe K$\alpha$ profile and cause its rapid and irregular variability, observed in the X-ray spectra of some AGN. We will pay attention to three such effects: perturbations in the disk emissivity, absorbtion by warm absorbers and gravitational microlensing. The X-ray emission from accretion disks around non-rotating (Schwarzschild metric), as well as rotating (Kerr metric) supermassive black holes, is discussed. The X-ray radiation of AGN is probably produced in a compact region near their central supermassive black holes, and can provide us some essential information about the plasma conditions and the space-time geome
Active Galactic Nuclei are powerful sources of radiation in a wide spectral range: from γ rays to radio waves. They derive their extraordinary luminosities (sometimes more than 10.000 times higher than luminosities of "ordinary" galaxies) from energy release by matter accreting towards, and falling into, a central supermassive black hole (see e.g. Peterson, 2004).
According to the unification model of AGN (Antonucci, 1993), their central engine consists of a supermassive black hole (SMBH) with mass ranging from 10 5 to 10 9 solar masses M ⊙ (Kaspi et al., 2000;Peterson et al., 2004), which is surrounded by an accretion disk that radiates in the X-ray band (Fabian et al., 1989). The vast majority of the X-ray sources in the Universe are AGN. The integral emission of AGN reflects the history of accretion onto SMBH over cosmic time. Emission lines are usually seen in the X-ray spectra of AGN. The broad emission Fe Kα spectral line (6.4-6.9 keV, depending on ionization state) with asymmetric profile (narrow bright blue peak and a wide faint red wing) has been observed in a number of type 1 AGN (see e.g. Nandra et al., 2007). Early results from the ASCA (Advanced Satellite for Cosmology and Astrophysics) era suggested that broad relativistic lines might be common in type 1 AGN. However, surprisingly, they have been detected in significant number and their features described in only a small fraction of those sources (Reynolds & Nowak, 2003). The first and the best studied one is MCG-6-30-15 (Tanaka et al., 1995;Fabian & Vaughan, 2003).
In some cases the line width corresponds to one third of speed of light, indicating that its emitters rotate with relativistic velocities. Therefore, the line is probably produced in a very compact region near the central black hole of AGN and can provide us some essential information about the plasma conditions and the space-time geometry in vicinity of the black hole. Consequently, if the line is emitted close enough to the SMBH, it shows a broad relativistic profile affected by SMBH spin (Reynolds & Fabian, 2008) and gravitational redshift (as well as other general relativistic effects) (Fabian et al., 1989;Laor, 1991).
Several studies have been performed over samples of local AGN (see e.g. Nandra et al., 1997;Yaqoob et al., 2005;Nandra et al., 2007;Bianchi et al., 2008;Markowitz et al., 2008, etc), as well as from distant quasars (Corral et al., 2008) in order to characterize the Fe Kα emission. Nandra et al. (2007) performed a spectral analysis of a sample of 26 type 1 to 1.9 Seyferts galaxies (z < 0.05) observed by XMM-Newton. They found that a relativistic line is significantly detected in a half of their sample (54±10 percent) with a mean equivalent width (EW) of ∼ 80 eV, but around 30% of selected AGN showed a relativistic broad line that can be explained by the emission of an accretion disk.
Accretion disks could have different forms, dimensions, and emission, depending on the type of central black hole, whether it is rotating (Kerr metric) or non-rotating (Schwarzschild metric). They represent an efficient mechanism for extracting gravitational potential energy and converting it into radiation, giving us the most probable explanation for the main characteristics of AGN (high luminosity, compactness, jet formation, rapid time variations in radiation and the profile of the Fe Kα spectral line).
Here we discuss the X-ray radiation from relativistic accretion disks around supermassive black holes, supposed to exist in the centers of AGN. Especially, we discuss the Fe Kα line profile which originates from the accretion disk. Moreover, we also present some effects which can disturb the Fe Kα profile, such as: perturbations in the disk emissivity, absorbtion by warm absorbers and gravitational microlensing.
The aim of this chapter is to present a short overview of results of some recent investigations in this field, and it is divided into the following six sections: Active Galactic Nuclei as Hosts of Supermassive Black Holeswhere the main features, classification and unified model of active galaxies are briefly presented, Space-Time Geometry in Vicinity of Supermassive Black Holes -where the basic definitions of Schwarzschild and Kerr metrics are given, Accretion Disk Around a Supermassive Black Hole -where we explain the standard model of an accretion disk, including its emission, accretion rate, luminosity, structure and spectral distribution, Supermassive Black Holes and X-ray Emission -where the focus is on the modeling of the observed X-ray radiation from relativistic accretion disk around a supermassive black hole of AGN in both the Fe Kα spectral line and X-ray continuum, Variability of X-ray Emission Around Supermassive Black Hole -where we present the main causes of rapid and irregular variability of the X-ray emission which can be due to disk instability, reflecting in perturbations of its emissivity, or it could be caused by some external effects, such as gravi
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