Reflection Spectra of Accretion Disks Illuminated by Disk-like Coronae

Abstract

Relativistic reflection features in the X-ray spectra of black hole binaries and active galactic nuclei are thought to be produced through illumination of a cold accretion disk by a hot corona. In this work, we assume that the corona has the shape of an infinitesimally thin disk with its central axis the same as the rotational axis of the black hole. The corona can either be static or corotate with the accretion disk. We calculate the disk’s emissivity profiles and iron line shapes for a set of coronal radii and heights. We incorporate these emissivity profiles into relxill_nk and we simulate some observations of a black hole binary with the Nuclear Spectroscopic Telescope Array to study the impact of a disk-like coronal geometry on the measurement of the properties of the system, and in particular, on the possibility of testing the Kerr nature of the source. We find that, in general, the astrophysical properties of the accretion disk are recovered well even if we fit the data with a model employing a broken power law or a lamppost emissivity profile, while it is more challenging to constrain the geometric properties of the black hole spacetime.

Impact of ionization and electron density gradients in X-ray reflection spectroscopy measurements

“…We note that the emissivity profile of GRS 1739-278 is steep in the inner part of the accretion disk and that the outer emissivity index is qout ≈ 2-3. This is the typical emissivity profile of a compact corona close to the black hole (Martocchia & Matt 1996;Dauser et al 2013;Riaz et al 2022). For the other two sources, GS 1354-645 and GRS 1915+105, we find instead a steep emissivity profile in the inner part of the accretion disk and an almost flat profile for the outer part.…”

“…However, they still rely on a number of simplifications that may introduce unacceptably large systematic uncertainties in the final measurements of the properties of the sources. It is thus crucial to understand well the systematic uncertainties of the theoretical models, as well as to develop more and more sophisticated theoretical model, in order to obtain precise and accurate measurements (Reynolds & Fabian 2008;Zhou et al 2020;Cárdenas-Avendaño et al 2020;Riaz et al 2020Riaz et al , 2021Riaz et al , 2022Tripathi et al 2020aTripathi et al , 2021b. Otherwise, with the possibility of analyzing higher and higher quality spectra, there is the risk to get very precise but not very accurate measurements of accreting black holes, which would nullify the efforts to design and launch more powerful X-ray observatories.…”

“…The radial profile of the X-ray flux FX is determined by the coronal geometry. Compact coronae very close to their black hole can naturally produce very steep X-ray flux in the inner part of the accretion disk as a result of light bending in the strong gravity region near the compact object (Martocchia & Matt 1996;Dauser et al 2013;Riaz et al 2022).…”

Testing Relativistic Reflection Models with GRMHD Simulations of Accreting Black Holes

“…For instance, there are now models that permit nontrivial radial disk profiles of the ionization parameter and of the electron density (Abdikamalov et al 2021a(Abdikamalov et al , 2021b, but such improvements do not seem to be strictly necessary for current spin measurements (Mall et al 2022). Specific coronal geometries have been investigated in a few studies, but the general conclusion is that phenomenological profiles like a broken power law or a twice broken power law can fit well the current spectra without introducing undesired bias in the parameter estimates (see, e.g., Wilkins & Fabian 2011, 2012Gonzalez et al 2017;Riaz et al 2022a). Even the returning radiation, namely the radiation that is emitted by the disk and returns to the disk because of strong light bending near the compact object (Riaz et al 2021), has been always neglected in the calculations and only very recently implemented in some models (Dauser et al 2022), but it does not seem to affect the estimates of key parameters like the black hole spin.…”

Testing relativistic reflection models with GRMHD simulations of accreting black holes

“…For instance, there are now models that permit a nontrivial radial disk profile of the ionization parameter and of the electron density (Abdikamalov et al 2021b,a), but such improvements do not seem to be strictly necessary for current spin measurements (Mall et al 2022). Specific coronal geometries have been investigated in a few studies, but the general conclusion is that phenomenological profiles like a broken power law or a twice broken power law can fit well the current spectra without introducing undesired bias in the parameter estimates (see, e.g., Wilkins & Fabian 2011, 2012Gonzalez et al 2017;Riaz et al 2022a). Even the returning radiation, namely the radiation emitted by the disk and returning to the disk because of the strong light bending near the black hole (Riaz et al 2021), has been always neglected in the calculations and only very recently implemented in some models (Dauser et al 2022), but it does not seem to affect the estimate of key parameters like the black hole spin.…”