arXiv subjects

Askar B. Abdikamalov

Publications and source records attributed to Askar B. Abdikamalov.

At least 55 records · Page 3Linked to original sources

arXiv2020

Testing the Kerr metric using X-ray reflection spectroscopy: spectral analysis of GX 339-4

Signatures of X-ray reprocessing (reflection) out of an accretion disk are commonly observed in the high-energy spectrum of accreting black holes, and can be used to probe the strong gravity region around these objects. In this paper, we extend previous work in the literature and we employ a full emission model for relativistic reflection in non-Kerr spacetime to demonstrate an approach that tests the Kerr black hole hypothesis. We analyze a composite spectrum obtained with the Proportional Counter Array in the Rossi X-ray Timing Explorer (RXTE), of the stellar-mass black hole GX 339-4 in its brightest hard state. With a remarkable sensitivity of ~0.1% and 40 million counts in the 3-45 keV band to capture the faint features in the reflection spectrum, we demonstrate that it is possible with existing data and an adequate model to place constraints on the black hole spin $a_*$ and the deformation parameter that quantifies the departure from the Kerr metric. Our measurement obtained with the best fit model, which should be regarded as principally a proof of concept, is $a_*=0.92^{+0.07}_{-0.12}$ and $α_{13}=-0.76^{+0.78}_{-0.60}$ with a 90% confidence level and is consistent with the hypothesis that the compact object in GX 339-4 is a Kerr black hole. We also discuss how the physical model choice and the emissivity profile adopted could make an impact on the constraints of $α_{13}$ and spin. To enable Kerr metric test using X-ray reflection spectroscopy, it is essential to improve our astrophysical understanding of accreting black holes, e.g., the natures of accretion flow and corona.

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arXiv2020

Reflection features in the X-ray spectrum of Fairall 9 and implications for tests of general relativity

X-ray reflection spectroscopy is potentially a powerful tool to probe the spacetime geometry around astrophysical black holes and test general relativity in the strong field regime. However, precision tests of general relativity are only possible if we employ the correct astrophysical model and we can limit the systematic uncertainties. It is thus crucial to select the sources and the observations most suitable for these tests. In this work, we analyze simultaneous observations of XMM-Newton and NuSTAR of the supermassive black hole in Fairall 9. This source has a number of properties that make it a promising candidate for tests of general relativity using X-ray reflection spectroscopy. Nevertheless, we find that with the available data there is not a unique interpretation of the spectrum of Fairall 9, which prevents, for the moment, to use this source for robust tests of general relativity. This issue may be solved by future X-ray missions with a higher energy resolution near the iron line.

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arXiv2020

Search for traversable wormholes in active galactic nuclei using X-ray data

In a previous paper, one of us calculated iron K$α$ line profiles emitted from possible accretion disks around traversable wormholes as a first step to use X-ray reflection spectroscopy to search for astrophysical wormholes in active galactic nuclei. In the present paper, we extend that work and construct an XSPEC model for the whole relativistic reflection spectrum. We apply our model to XMM-Newton and NuSTAR observations of the supermassive object in MCG-6-30-15 and we check whether these observations prefer the hypothesis that the central body is a Kerr black hole or a traversable wormhole. We find that the two models provide equally good fits, so with the available data we cannot distinguish the black hole and wormhole scenarios.

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arXiv2020

Singularity-free black holes in conformal gravity: new observational constraints

We consider the family of singularity-free rotating black hole solutions in Einstein's conformal gravity found in Bambi, Modesto & Rachwal (2017) and we constrain the value of the conformal parameter $L$ from the analysis of a 30 ks NuSTAR observation of the stellar-mass black hole in GS 1354-645 during its outburst in 2015. Our new constraint is much stronger than that found in previous work. Here we obtain $L/M < 0.12$ (99% confidence level, statistical uncertainty only).

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arXiv2019

Tests of the Kerr hypothesis with GRS 1915+105 using different RELXILL flavors

In a previous paper, we tried to test the Kerr nature of the stellar-mass black hole in GRS 1915+105 by analyzing NuSTAR data of 2012 with our reflection model RELXILL_NK. We found that the choice of the intensity profile of the reflection component is crucial and eventually we were not able to get any constraint on the spacetime metric around the black hole in GRS 1915+105. In the present paper, we study the same source with Suzaku data of 2007. We confirm that the intensity profile plays an important role, but now we find quite stringent constraints consistent with the Kerr hypothesis. The key differences with respect to our previous study are likely the lower disk temperature in the Suzaku observation and the higher energy resolution near the iron line of the Suzaku data. We also apply different RELXILL flavors (different descriptions of the coronal spectrum and variable disk electron density) obtaining essentially the same results. We thus conclude that this choice is not very important for our tests of the Kerr hypothesis while the intensity profile does play an important role, and that with high quality data it is possible to measure both the spacetime metric and the intensity profile.

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arXiv2019

RELXILL_NK: A Black Hole Relativistic Reflection Model for Testing General Relativity

In this paper, we briefly present RELXILL_NK, the first and currently only readily available model of the relativistic reflection spectrum of black hole accretion disks that includes non-Kerr solutions for the black hole spacetime, thus allowing for tests of the Kerr hypothesis and GR. RELXILL_NK makes use of a general relativistic ray-tracing code to calculate the relativistic effects of any well-behaved, stationary, axisymmetric, and asymptotically flat black hole spacetime, while the disk physics is handled through the non-relativistic X-ray reflection code XILLVER. A number of different flavors are available within RELXILL_NK; we summarize and compare these flavors using the Johannsen metric for the black hole spacetime.

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arXiv2019

Black hole mimicker hiding in the shadow: Optical properties of the $γ$-metric

Can the observation of the "shadow" allow us to distinguish a black hole from a more exotic compact object? We study the motion of photons in a class of vacuum static axially-symmetric space-times that is continuously linked to the Schwarzschild metric through the value of one parameter that can be interpreted as a measure of the deformation of the source. We investigate the lensing effect and shadow produced by the source with the aim of comparing the expected image with the shadow of a Schwarzschild black hole. In the context of astrophysical black holes we found that it may not be possible to distinguish an exotic source with small deformation parameter from a black hole. However, as the deformation increases noticeable effects arise. Therefore, the future more precise measurement of the shadow of astrophysical black hole candidates would in principle allow to put constraints on the deviation of the object from spherical symmetry.

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arXiv2019

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 $α_{13}$ to $-1.5<α_{13}<0.6$ with spin parameter $a_{*}>0.8$, and the $α_{22}$ parameter to $-0.4<α_{22}<2.1$ with $a_{*}>0.7$, 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.

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arXiv2019

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.

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arXiv2019

Testing the Kerr hypothesis using X-ray reflection spectroscopy with NuSTAR data of Cygnus X-1 in the soft state

We continue exploring the constraining capabilities of X-ray reflection spectroscopy to test the Kerr-nature of astrophysical black holes and we present the results of our analysis of two NuSTAR observations of Cygnus X-1 in the soft state. We find that the final measurement can strongly depend on the assumption of the intensity profile. We conclude that Cygnus X-1 is not suitable for tests of general relativity using X-ray reflection spectroscopy and we discuss the desired properties of a source to be a good candidate for our studies.

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arXiv2019

Testing general relativity with supermassive black holes using X-ray reflection spectroscopy

In this paper, we review our current efforts to test General Relativity in the strong field regime by studying the reflection spectrum of supermassive black holes. So far we have analyzed 11 sources with observations of NuSTAR, Suzaku, Swift, and XMM-Newton. Our results are consistent with general relativity, according to which the spacetime metric around astrophysical black holes should be well approximated by the Kerr solution. We discuss the systematic uncertainties in our model and we present a preliminary study on the impact of some of them on the measurement of the spacetime metric.

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arXiv2019

XSPEC model for testing the Kerr black hole hypothesis using the continuum-fitting method

We present NKBB, an XSPEC model for the thermal spectrum of thin accretion disks in parametric black hole spacetimes. We employ the Novikov-Thorne model for the description of the accretion disk and the formalism of the transfer function proposed by Cunningham for storing all the relativistic effects of the spacetime metric. The current version of the model assumes the Johannsen metric, but it can be easily extended to any stationary, axisymmetric, and asymptotically-flat spacetime without pathological properties. The model can be used within the XSPEC package to test the Kerr nature of astrophysical black holes with the continuum-fitting method.

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arXiv2019

Towards precision tests of general relativity with black hole X-ray reflection spectroscopy

Astrophysical black hole systems are the ideal laboratories for testing Einstein's theory of gravity in the strong field regime. We have recently developed a framework which uses the reflection spectrum of black hole systems to perform precision tests of general relativity by testing the Kerr black hole hypothesis. In this paper, we analyze XMM-Newton and NuSTAR observations of the supermassive black hole in the Seyfert 1 galaxy MCG-06-30-15 with our disk reflection model. We consider the Johannsen metric with the deformation parameters $α_{13}$ and $α_{22}$, which quantify deviations from the Kerr metric. For $α_{22} = 0$, we obtain the black hole spin $0.928 < a_* < 0.983$ and $-0.44 < α_{13} < 0.15$. For $α_{13} = 0$, we obtain $0.885 < a_* < 0.987$ and $-0.12 < α_{22} < 1.05$. The Kerr solution is recovered for $α_{13} = α_{22} = 0$. Thus, our results include the Kerr solution within statistical uncertainties. Systematic uncertainties are difficult to account for, and we discuss some issues in this regard.

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arXiv2019

About the Kerr nature of the stellar-mass black hole in GRS 1915+105

We employ the accretion disk reflection model RELXILL_NK to test the spacetime geometry around the stellar-mass black hole in GRS 1915+105. We adopt the Johannsen metric with the deformation parameters $α_{13}$ and $α_{22}$, for which the Kerr solution is recovered when $α_{13} = α_{22} = 0$. We analyze a NuSTAR observation of 2012, obtaining vanishing and non-vanishing values of the deformation parameters depending on the astrophysical model adopted. Similar difficulties were not found in our previous tests with other sources. The results of this work can shed light on the choice of sources suitable for testing the Kerr metric using X-ray reflection spectroscopy and on the parts of our reflection models that more urgently require improvement.

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arXiv2019

Constraining the Johannsen deformation parameter $ε_3$ with black hole X-ray data

We extend our previous work on tests of the Kerr black hole hypothesis using X-ray reflection spectroscopy and we infer observational constraints on the Johannsen deformation parameter $ε_3$. We analyze a NuSTAR observation of the stellar-mass black hole in GS 1354-645, Suzaku data of the supermassive black holes in Ark 564, 1H0419-577, and PKS 0558-504, and some simultaneous XMM-Newton and NuSTAR observations of the supermassive black hole in MCG-6-30-15. All our measurements of $ε_3$ are consistent with the Kerr metric and we find quite strong constraints from Ark 564 and MCG-6-30-15. We discuss the implications of our results and the next steps to improve our tests.

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arXiv2019

Constraints on the spacetime metric around seven "bare" AGNs using X-ray reflection spectroscopy

We present the study of a sample of seven "bare" active galactic nuclei (AGN) observed with Suzaku. We interpret the spectrum of these sources with a relativistic reflection component and we employ our model RELXILL_NK to test the Kerr nature of their supermassive black holes. We constrain the Johannsen deformation parameters $α_{13}$ and $α_{22}$, in which the Kerr metric is recovered when $α_{13} = α_{22} = 0$. All our measurements are consistent with the hypothesis that the spacetime geometry around these supermassive objects is described by the Kerr solution. For some sources, we obtain quite strong constraints on $α_{13}$ and $α_{22}$ when compared to those found in our previous studies. We discuss the systematic uncertainties in our tests and the implications of our results.

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arXiv2018

A study of the strong gravity region of the black hole in GS 1354-645

It is thought that the spacetime metric around astrophysical black holes is well described by the Kerr solution of Einstein's gravity. However, a robust observational evidence of the Kerr nature of these objects is still lacking. Here we fit the X-ray spectrum of the stellar-mass black hole in GS 1354-645 with a disk reflection model beyond Einstein's gravity in order test the Kerr black hole hypothesis. We consider the Johannsen metric with the deformation parameters $α_{13}$ and $α_{22}$. The Kerr metric is recovered for $α_{13} = α_{22} = 0$. For $α_{22} = 0$, our measurements of the black hole spin and of the deformation parameter $α_{13}$ are $a_* > 0.975$ and $-0.34 < α_{13} < 0.16$, respectively. For $α_{13} = 0$, we find $a_* > 0.975$ and $-0.09 < α_{22} < 0.42$. All the reported uncertainties are at 99% of confidence level for two relevant parameters.

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arXiv2018

Testing the Kerr nature of the supermassive black hole in Ark 564

Einstein's theory of general relativity has been extensively tested in weak gravitational fields, mainly with experiments in the Solar System and observations of radio pulsars, and current data agree well with the theoretical predictions. Nevertheless, there are a number of scenarios beyond Einstein's gravity that have the same predictions for weak fields and present deviations only when gravity becomes strong. Here we try to test general relativity in the strong field regime. We fit the X-ray spectrum of the supermassive black hole in Ark 564 with a disk reflection model beyond Einstein's gravity, and we are able to constrain the black hole spin $a_*$ and the Johannsen deformation parameters $α_{13}$ and $α_{22}$ separately. For $α_{22} = 0$, we find $a_* > 0.96$ and $-1.0 < α_{13} < 0.2$ with a 99% confidence level. For $α_{13} = 0$, we get $a_* > 0.96$ and $-0.1 < α_{22} < 0.9$ with a 99% confidence level. Our measurements are thus consistent with the hypothesis that the supermassive compact object in Ark 564 can be described by the Kerr metric.

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