arXiv subjects

Ashutosh Tripathi

Publications and source records attributed to Ashutosh Tripathi.

At least 55 records · Page 3Linked to original sources

arXiv2020

Impact of the reflection model on the estimate of the properties of accreting black holes

Relativistic reflection features in the X-ray spectra of black hole binaries and AGNs originate from illumination of the inner part of the accretion disk by a hot corona. In the presence of high quality data and with the correct astrophysical model, X-ray reflection spectroscopy can be quite a powerful tool to probe the strong gravity region, study the morphology of the accreting matter, measure black hole spins, and even test Einstein's theory of general relativity in the strong field regime. There are a few relativistic reflection models available today and developed by different groups. All these models present some differences and have a number of simplifications introducing systematic uncertainties. The question is whether different models provide different measurements of the properties of black holes and how to arrive at a common model for the whole X-ray astronomy community. In this paper, we start exploring this issue by analyzing a Suzaku observation of the stellar-mass black hole in GRS 1915+105 and simultaneous XMM-Newton and NuSTAR observations of the supermassive black hole in MCG-6-30-15. The relativistic reflection component of these sources is fitted with RELCONV$\times$REFLIONX, RELCONV$\times$XILLVER, and RELXILL. We discuss the differences and the impact on the study of accreting black holes.

astro-ph.HE↗

arXiv2020

Testing general relativity with the stellar-mass black hole in LMC X-1 using the continuum-fitting method

The iron line and the continuum-fitting methods are currently the two leading techniques for measuring black hole spins with electromagnetic radiation. They can be naturally extended for probing the spacetime geometry around black holes and testing general relativity in the strong field regime. In the past couple of years, there has been significant work to use the iron line method to test the nature of black holes. In this Letter, we use the continuum-fitting method and we show its capability of constraining the spacetime geometry around black holes by analyzing 17 RXTE data of the X-ray binary LMC X-1.

gr-qc↗

arXiv2020

X-ray reflection spectroscopy with Kaluza-Klein black holes

Kaluza-Klein theory is a popular alternative theory of gravity, with both non-rotating and rotating black hole solutions known. This allows for the possibility that the theory could be observationally tested. We present a model which calculates the reflection spectrum of a black hole accretion disk system, where the black hole is described by a rotating solution of the Kaluza-Klein theory. We also use this model to analyze X-ray data from the stella-mass black hole in GRS 1915+105 and provide constraints on the free parameters of the Kaluza-Klein black holes.

gr-qc↗

arXiv2020

Thermal spectra of thin accretion disks of finite thickness around Kerr black holes

The analysis of the thermal spectrum of geometrically thin and optically thick accretion disks of black holes, the so-called continuum-fitting method, is one of the leading techniques for measuring black hole spins. Current models normally approximate the disk as infinitesimally thin, while in reality the disk thickness is finite and increases as the black hole mass accretion rate increases. Here we present an XSPEC model to calculate the multi-temperature blackbody spectrum of a thin accretion disk of finite thickness around a Kerr black hole. We test our new model with an RXTE observation of the black hole binary GRS 1915+105. We find that the spin value inferred with the new model is slightly higher than the spin value obtained with a model with an infinitesimally thin disk, but the difference is small and the effect is currently subdominant with respect to other sources of uncertainties in the final spin measurement.

astro-ph.HE↗

arXiv2020

Relativistic reflection spectra of super-spinning black holes

We construct a relativistic reflection model in a non-Kerr spacetime in which, depending on the value of the deformation parameter of the metric, there are black hole solutions with spin parameter $|a_*| > 1$. We apply our model to fit Suzaku data of four Seyfert galaxies (Ton S180, Ark 120, 1H0419-577, and Swift J0501.9-3239). These galaxies host at the center supermassive black holes that were previously interpreted as near-extremal Kerr black holes. For Ton S180 and 1H0419-577, our measurements are still consistent with the Kerr hypothesis. For Ark 120 and Swift J0501.9-3239, the Kerr solution is not recovered at 3-$σ$. We discuss our results and possible systematic uncertainties in the model.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

arXiv2019

Intra-day variability of three Seyfert Galaxies measured with XMM-Newton

We present and analyze the variability of three Seyfert galaxies on intra-day timescales. We have analyzed in a uniform manner the 38 longest ( $>30$ ks) observations made for NGC 4051, MCG$-06-30-15$ and NGC 4151 by XMM-Newton between 2000 and 2015. The nuclei were quite active during essentially all of these observations and the overall X-ray fluxes (0.3-10 keV) varied by an order of magnitude. Most of the observations do appear to show characteristic timescales, estimated through their auto-correlation functions, ranging between $\sim 2.9$ ks and $\sim 45.3$ ks. The hard (2-10 keV) and soft (0.3-02 keV) bands are very well correlated but consideration of their hardness ratios show that the sources typically soften during flares. We also provide new estimates of the central black hole masses for these three AGN that support the hypothesis that Narrow Line Seyfert 1 galaxies have relatively small central black holes.

astro-ph.HE↗

arXiv2018

Detection of a quasi-periodic oscillation in gamma-ray light curve of the high redshift blazar B2 1520+31

We detected a possible quasi-periodic oscillation (QPO) of ~ 71 days in the 0.1 -- 300 GeV gamma-ray Fermi-LAT light curve of the high redshift flat spectrum radio quasar B2 1520+31. We identify and confirm that quasi-period by Lomb Scargle periodogram (LSP), and weighted wavelet z-transform (WWZ) analyses. Using this QPO period, and assuming it originates from accretion-disc fluctuations at the innermost stable circular orbit, we estimate the central supermassive black hole mass to range between ~ 5.4 * 10$^{9}$ M$_{\odot}$ for a non-rotating black hole and ~ 6.0 * 10$^{10}$ M$_{\odot}$ for a maximally rotating black hole. We briefly discuss other possible radio-loud active galactic nuclei emission models capable of producing a gamma-ray QPO of such a period in a blazar.

astro-ph.HE↗

arXiv2018

Possible ~ 1 hour quasi-periodic oscillation in narrow-line Seyfert 1 galaxy MCG--06--30--15

We found a possible ~ 1 hour quasi-periodic oscillation (QPO) in a ~ 55 ks X-ray observation of the narrow-line Seyfert 1 galaxy MCG--06--30--15 made with the XMM-Newton EPIC/pn detector in the energy range 0.3 -- 10 keV. We identify a total modulation of ~ 16% in the light curve and find a $\simeq$ 3670~s quasi-period using Lomb-Scargle periodogram (LSP) and weighted wavelet Z-transform (WWZ) techniques. Our analyses of eight light curves of MCG--06--30--15, indicated the possible presence of an oscillation during one of them. The LSP indicates a statistically significant ($\simeq$ 3$σ$) QPO detection. A WWZ analysis shows that the signal at this possible roughly 3670s period is present, and rather persistent, throughout the observation; however, a signal around 8735s is more persistent. We briefly discuss models that can produce X-ray QPOs with such periods in narrow line Seyfert 1 galaxies, as both other claimed QPO detections in this class of AGN had very similar periods.

astro-ph.HE↗

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.

gr-qc↗

arXiv2018

Testing General Relativity with X-ray Reflection Spectroscopy of a 'Bare' Active Galactic Nucleus

Einstein's gravity has been extensively tested in the weak field regime, primarily with experiments in the Solar System and observations of binary pulsars, and current data agree well with theoretical predictions. On the other hand, strong gravity is largely unexplored and there are a number of theories beyond Einstein's gravity that make the same predictions for weak fields and present deviations only when gravity becomes strong. The best laboratory for testing strong gravity is the spacetime around astrophysical black holes. X-ray reflection spectroscopy can be a powerful tool to probe the strong gravity region around astrophysical black holes and test the nature of these objects. In this paper, we will introduce RELXILL_NK, which is the first XSPEC reflection model to test Einstein's gravity in the strong field regime, and we will present the constraints on possible deviations from Einstein's gravity that we have obtained by analyzing Suzaku data of Ark 564.

gr-qc↗

arXiv2018

Observational constraints on quintessence models of dark energy

Scalar fields aptly describe equation of state of dark energy. The scalar field models were initially proposed to circumvent the fine tuning problem of cosmological constant. However, the model parameters also need a fine tuning of their own and it is important to use different observations to determine these parameters. In this paper, we use a combination of low redshift data to constrain the low redshift evolution of canonical scalar field parameters. For this analysis, we use the Supernova Type Ia observations, the Baryon Acoustic Observations and the Hubble parameter measurement data. We consider scalar field models of the thawing type of two different functional forms of potentials. The constraints on the model parameters are more stringent than those from earlier observations although these datasets do not rule out the models entirely. The parameters which let dark energy dynamics closely emulate that of a cosmological constant are preferred. The constraints on the parameters are suitable priors for further quintessence dark energy studies.

astro-ph.CO↗

arXiv2018

RELXILL_NK: a relativistic reflection model for testing Einstein's gravity

Einstein's theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in the weak field regime. However, the strong field regime is largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein's gravity for weak fields and present deviations when gravity becomes strong. RELXILL_NK is the first relativistic reflection model for probing the spacetime metric in the vicinity of astrophysical black holes and testing Einstein's gravity in the strong field regime. Here we present our current constraints on possible deviations from Einstein's gravity obtained from the black holes in 1H0707-495, Ark 564, GX 339-4, and GS 1354-645.

gr-qc↗

arXiv2017

Testing general relativity with X-ray reflection spectroscopy of MCG-06-30-15

The spacetime geometry around astrophysical black holes is well approximated by the Kerr metric, but deviations from standard predictions are possible in a number of scenarios beyond Einstein's gravity and in the presence of exotic matter. In this paper, we present the constraints on possible deviations from the Kerr solution using X-ray reflection spectroscopy from the analysis of real data. We use the relativistic X-ray reflection code RELXILL modified to a generic stationary, axisymmetric and asymptotically flat black hole metric. We analyze 350 ks long XMM-Newton observations of the AGN in MCG-06-30-15 taken during July-August 2001 and constrain the Johannsen deformation parameter $α_{13}$.

astro-ph.HE↗