We study in detail the GR models of the X-ray spectral variability for various geometries of the X-ray source and with various relativistic effects being the dominant cause of spectral variability. The predicted properties are compared with the Suzaku observational data of the Seyfert 1 galaxy MCG--6-30-15. The data disfavor models with the X-ray source (1) moving vertically on the symmetry axis or (2) corotating with the disc and changing height not far above the disc surface. The most likely explanation for the observed variability is given by the model involving the X-ray source located at a very small, varying distance from a rapidly rotating black hole. This model predicts some enhanced variations in the red wing of the Fe line, which are not seen in the Suzaku observations. However, the enhanced variability of the red wing, while ruled out by the Suzaku data, is consistent with an excess RMS variability, between 5 and 6 keV, reported for some previous ASCA and XMM observations. We speculate that the presence or lack of such a feature is related to the change of the ionization state of the innermost part of the disc, however, investigation of such effects is currently not possible in our model (where a neutral disc is assumed). If the model, completed by description of ionization effects, proves to be fully consistent with the observational data, it will provide a strong indication that the central black hole in MCG--6-30-15 rotates rapidly, supporting similar conclusions derived from the Fe line profile.
Deep Dive into GR models of the X-ray spectral variability of MCG--6-30-15.
We study in detail the GR models of the X-ray spectral variability for various geometries of the X-ray source and with various relativistic effects being the dominant cause of spectral variability. The predicted properties are compared with the Suzaku observational data of the Seyfert 1 galaxy MCG–6-30-15. The data disfavor models with the X-ray source (1) moving vertically on the symmetry axis or (2) corotating with the disc and changing height not far above the disc surface. The most likely explanation for the observed variability is given by the model involving the X-ray source located at a very small, varying distance from a rapidly rotating black hole. This model predicts some enhanced variations in the red wing of the Fe line, which are not seen in the Suzaku observations. However, the enhanced variability of the red wing, while ruled out by the Suzaku data, is consistent with an excess RMS variability, between 5 and 6 keV, reported for some previous ASCA and XMM observations. W
AGNs are powerful sources of strongly variable X-ray emission, most likely originating close to a central black hole. The origin of the X-ray variability is poorly understood, however, some of the observed variations are supposed to be directly related to strong gravity effects and their investigation may give insight into properties of the space-time metric in the vicinity of the event horizon.
The bright Seyfert 1 galaxy MCG-6-30-15 (z = 0.00775) is the key object for such studies. Its X-ray spectrum shows an extremely distorted Fe Kα line, indicating that X-ray reprocessing takes place very close to the central black hole (e.g. Fabian et al. 2002;Miniutti et al. 2007). Furthermore, crucially for investigation of strong gravity in MCG-6-30-15, radiation reflected from the inner disc is not contaminated by radiation reflected from distant matter (see Sect. 4.2). The disc-line model for the detected line profile requires that roughly half of the observed Fe photons come from the innermost parts of the accretion disc, within 6 gravitational radii (these photons form the red wing of the line, below 6 keV) and the remaining from a slightly more distant region, at a few tens of gravitational radii (forming the blue peak, observed between 6 and 7 keV).
The time-resolved X-ray spectra in MCG-6-30-15 indicate that the best explanation of its spectral variability pattern is given by a phenomenological, two-component model consisting of (i) a highly variable power-law continuum (referred to as primary emission) and (ii) a much less variable, and uncorrelated, component with a spectrum characteristic of radiation reflected from the inner disc (e.g. Fabian & Vaughan 2003, Vaughan & Fabian 2004, Larsson et al. 2007; an alternative explanation, proposed by Miller et al. 2008, is briefly discussed in Sect. 5.6). The latter component includes both the Fe Kα line and the associated Compton reflection hump, peaking around 30 keV, produced by down-scattering of higher energy photons. The varying relative contributions of these spectral components can explain the observed hardening of the X-ray spectrum at lower fluxes as well as reduced variations of the blue peak of the Fe Kα line.
The apparent lack of connection between the two components indicates that some complex physical mechanism should be involved, as the components are supposed to originate together in the central region and a strict correlation between them would be expected in a simple reflection picture. Fabian & Vaughan (2003) first argued that this disconnectedness may be explained by relativistic effects, in particular by light bending and focusing of the primary emission towards the accretion disc. In this scenario, elaborated under some specific as-sumptions by Miniutti & Fabian (2004, MF04), the change of the height of the primary X-ray source is the cause of spectral variations.
However, a systematic investigation of this class of models by Niedźwiecki & Życki (2008, NZ08) questioned their ability to explain the observed effects. In particular, MF04 assess the reduced variability of the reflected component based on the behavior of the total flux of fluorescent photons received by a distant observer. NZ08 point out that changes of the height of the source induce substantial changes in the line profile, with significant variations of the fluxes in the blue peak and in the red wing. These balance each other for some range of parameters, reducing variations of the total flux, however, this property is not sufficient to explain the observed data. Then, the model fails to reproduce the key property which motivated its development, i.e. reduced variation of the blue peak of the Fe line.
Interestingly, NZ08 find an alternate model, which can explain reduced variations of the blue peak. The model proposed in NZ08 follows the generic idea of Fabian & Vaughan (2003), involving a scenario where the change of position of the primary source leads to large changes of directly observed emission and smaller changes of the amount received by the disc. However, the model assumes changes of the radial distance rather than the height and its properties rely on a qualitatively distinct effect, namely bending to the equatorial plane of the Kerr space-time. The model proposed in NZ08 predicts, however, some enhanced variations of the red wing of the Fe line, which may challenge its applicability.
At present, the time-resolved spectra for sufficiently short (∼ 10 ksec) time bins have poor quality, which does not allow us to study such fine details of spectral variability. Thus, less direct tools are used. In particular, the RMS spectra -showing the fractional variability as a function of energy (see e.g. Edelson et al. 2002;Markowitz et al. 2003;Vaughan et al. 2003b) -allow us to investigate spectral variability in a model-independent manner.
In MCG-6-30-15, the general trend is that the RMS spectrum decreases with the increase of energy, moreover, it has a pronounced, broa
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