Testing Regular Black Holes with X-ray data of GX~339--4

Regular black holes are singularity-free black hole spacetimes proposed to solve the problem of the presence of spacetime singularities that plagues the black holes of general relativity and most theories of gravity. In this work, we consider the regular black holes recently proposed by Mazza, Franzin \& Liberati and we extend previous studies to get a more stringent observational constraint on the regularization parameter $l$. We study simultaneous observations of \textit{NuSTAR} and \textit{Swift} of the Galactic black hole in GX~339--4 during its outburst in 2015. The quality of the \textit{NuSTAR} data is exceptionally good and the spectrum of the source presents both a strong thermal component and prominent relativistically blurred reflection features. This permits us to measure the regularization parameter $l$ from the simultaneous analysis of the thermal spectrum and the reflection features. From our analysis, we find the constraint $l/M < 0.39$ (90\% CL), which is stronger than previous constraints inferred with X-ray and gravitational wave data

Impact of the returning radiation in current tests of the Kerr black hole hypothesis using X-ray reflection spectroscopy

The past 10 years have seen remarkable progress in our capability of analyzing reflection features in the X-ray spectra of accreting black holes. Today X-ray reflection spectroscopy is a mature technique and a powerful tool for studying the accretion process around black holes, measuring black hole spins, and testing Einstein's theory of General Relativity in the strong field regime. However, current reflection models still rely on a number of simplifications and caution is necessary when we derive very precise measurements. In this paper, we study the impact of the returning radiation on our capability of measuring the properties of black holes using X-ray reflection spectroscopy, and in particular on our capability of testing the Kerr black hole hypothesis. While the returning radiation alters the reflection spectrum of the disk, from the analysis of our simulations we find that models without returning radiation can normally recover well the correct black hole spin parameters and can test the Kerr metric. Our study thus confirms that current tests of the Kerr hypothesis using X-ray reflection spectroscopy can be robust.Comment: 12 pages, 10 figures. v2: refereed versio

Public Release of RELXILL_NK: A Relativistic Reflection Model for Testing Einstein's Gravity

We present the public release version of relxill_nk, an X-ray reflection model for testing the Kerr hypothesis and general relativity. This model extends the relxill model that assumes the black hole spacetime is described by the Kerr metric. We also present relxilllp_nk, the first non-Kerr X-ray reflection model with a lamppost corona configuration, as well as all other models available in the full relxill_nk package. In all models the relevant relativistic effects are calculated through a general relativistic ray-tracing code that can be applied to any well-behaved, stationary, axisymmetric, and asymptotically flat black hole spacetime. We show that the numerical error introduced by using a ray-tracing code is not significant as compared with the observational error present in current X-ray reflection spectrum observations. In addition, we present the reflection spectrum for the Johannsen metric as calculated by relxill_nk.Comment: 15 pages, 8 figures. v2: refereed version. Code and documentation available at http://www.physics.fudan.edu.cn/tps/people/bambi/Site/RELXILL_NK.html and at http://www.tat.physik.uni-tuebingen.de/~nampalliwar/relxill_nk

Testing the Kerr metric with X-ray Reflection Spectroscopy of Mrk 335 Suzaku data

Einstein's gravity has undergone extensive tests in the weak field gravitational limit, with results in agreement with theoretical predictions. There exist theories beyond general relativity (GR) which modify gravity in the strong field regime but agree with GR in the weak field. Astrophysical black holes are believed to be described by the Kerr metric and serve as suitable candidates to test strong gravity with electromagnetic radiation. We perform such a test by fitting one Suzaku dataset of the narrow-line Seyfert 1 (NLS1) galaxy Mrk 335 with X-ray reflection spectroscopy, using the Johannsen metric to model the black hole spacetime and test for deviations from Kerr. We find the data is best modeled with a hybrid model that includes both partial covering absorption and a reflection component. This is the first time such a model has been proposed for a high-flux (low reflection) Mrk 335 dataset. We constrain the Johannsen deformation parameter $\alpha_{13}$ to $-1.50.8$, and the $\alpha_{22}$ parameter to $-0.40.7$, both at the 99% confidence level. Although additional solutions at large deviations from the Kerr metric show statistical similarity with the ones above, further analysis suggests these solutions may be manifestations of uncertainties beyond our control and do not represent the data. Hence, our results are in agreement with the idea that the supermassive compact object at the center of Mrk 335 is described by the Kerr metric.Comment: 13 pages, 9 figures. v2: refereed versio