Testing the Kerr Black Hole Hypothesis Using X-Ray Reflection Spectroscopy and a Thin Disk Model with Finite Thickness

Abstract

X-ray reflection spectroscopy is a powerful tool for probing the strong gravity region of black holes and can be used for testing general relativity in the strong field regime. Simplifications of the available relativistic reflection models limit the capability of performing accurate measurements of the properties of black holes. In this paper, we present an extension of the model relxill nk in which the accretion disk has a finite thickness rather than being infinitesimally thin. We employ the accretion disk geometry proposed by Taylor & Reynolds (2018) and we construct relativistic reflection models for different values of the mass accretion rate of the black hole. We apply the new model to high quality Suzaku data of the X-ray binary GRS 1915+105 to explore the impact of the thickness of the disk on tests of the Kerr metric.

Testing general relativity with x-ray reflection spectroscopy: The Konoplya-Rezzolla-Zhidenko parametrization

“…These include, but are not limited to, studies involving more parameters [e.g., estimating ϵ 0 parameter, see Eq. 1], estimating multiple deformation parameters simultaneously (e.g., as done in [92]), employing more sophisticated disk geometry (e.g., as done in [62]), studying more astrophysical sources (e.g., MCG-06-30-15 [24,70,93], GRS 1915 þ 105 [68,78,94], GX 339-4 [19,95,96]), and exploring potential synergy with other observational techniques [82,92,97].…”

“…The inset shows conversion of incident radiation in to reflected radiation. Studies of their effect on the reflection spectrum in the presence of non-Kerr metrics is an ongoing effort[61,62].More details can be found on the public version webpage at Refs. [63,64].…”

Towards Precision Measurements of Accreting Black Holes Using X-Ray Reflection Spectroscopy

“…Taylor & Reynolds (2018) find that the values of the BH spin and of the coronal height are underestimated in simulations of a BH with a * = 0.9 illuminated by a lamppost corona when the theoretical model employs an infinitesimally thin disk. On the contrary, Abdikamalov et al (2020) and Tripathi et al (2021d) do not find much difference in the best-fit values of high-quality data of a few sources (GRS 1915+105, EXO 1846-031, andMCG-6-30-15) between the infinitesimally thin disk model and the model with disk of finite thickness, assuming a broken power-law for the disk's emissivity profile.…”

“…If the disk has a finite thickness, a fraction of the inner part of the accretion disk may not be visible to the observer; for the values of r e in which this happens, the transfer function is an open curve (Taylor & Reynolds 2018, Abdikamalov et al 2020. In non-Kerr spacetimes, it is not guaranteed that the transfer function can be constructed as in the Kerr case, because we may not be able to use g * to parametrize the two curves connecting g * = 0 with g * = 1 (we may have more than two points with the same value of g * at some emission radii r e ).…”

“…In practice, it is common to employ relativistic reflection models that assume infinitesimally thin Novikov-Thorne disks to fit any BH spectrum showing relativistic reflection features. Taylor & Reynolds (2018) have recently proposed a simple framework to include the disk thickness in a Novikov-Thorne disk with R in = R ISCO , and the model has been further explored in Abdikamalov et al (2020) and Tripathi et al (2021d). The thickness of the disk alters the relativistic effects, as the emission is at a different point of the spacetime off the equatorial plane.…”