arXiv subjects

Cosimo Bambi

Publications and source records attributed to Cosimo Bambi.

At least 127 records · Page 7Linked to original sources

arXiv2021

Testing General Relativity with X-ray reflection spectroscopy: The Konoplya-Rezzolla-Zhidenko parametrization

X-ray reflection spectroscopy is a promising technique for testing general relativity in the strong field regime, as it can be used to test the Kerr black hole hypothesis. In this context, the parametrically deformed black hole metrics proposed by Konoplya, Rezzolla \& Zhidenko (Phys. Rev. D93, 064015, 2016) form an important class of non-Kerr black holes. We implement this class of black hole metrics in \textsc{relxill\_nk}, which is a framework we have developed for testing for non-Kerr black holes using X-ray reflection spectroscopy. We perform a qualitative analysis of the effect of the leading order strong-field deformation parameters on typical observables like the innermost stable circular orbits and the reflection spectra. We also present the first X-ray constraints on some of the deformation parameters of this metric, using \textit{Suzaku} data from the supermassive black hole in Ark~564, and compare them with those obtained (or expected) from other observational techniques like gravitational waves and black hole imaging.

gr-qc↗

arXiv2021

Quasi-Periodic Oscillations in the long term radio light curves of the blazar AO 0235+164

We present time series analyses of three-decade long radio observations of the BL Lacertae object AO 0235+164 made at the University of Michigan Radio Astronomical Observatory operating at three central frequencies of 4.8 GHz, 8.0 GHz and 14.5 GHz. We detected a quasi-periodic oscillation of $\sim$965 days in all three frequency bands in the light curve of the effectively simultaneous observations, along with strong signals at $\sim$1950 d, $\sim$1350 d, and $\sim$660 d. The periodicity is analyzed with three methods: Data Compensated Discrete Fourier Transform, Generalized Lomb-Scargle Periodogram and Weighted Wavelet Z-transform. These methods are chosen as they have different analysis approaches toward robust measurement of claimed periodicities. The QPO at $965\pm 50$ days is found to be significant (at least $3.5σ$) and is persistent throughout the observation for all three radio frequencies, and the others, which may be harmonics, are comparably significant in at least the 8.0 GHz and 14.5 GHz bands. We briefly discuss plausible explanations for the origin of such long and persistent periodicity.

astro-ph.HE↗

arXiv2020

Charged particle motion around non-singular black holes in conformal gravity in the presence of external magnetic field

We consider electromagnetic fields and charged particle dynamics around non-singular black holes in conformal gravity immersed in an external, asymptotically uniform magnetic field. First, we obtain analytic solutions of the electromagnetic field equation around rotating non-singular black holes in conformal gravity. We show that the radial components of the electric and magnetic fields increase with the increase of the parameters $L$ and $N$ of the black hole solution. Second, we study the dynamics of charged particles. We show that the increase of the values of the parameters $L$ and $N$ and of magnetic field causes a decrease in the radius of the innermost stable circular orbits (ISCO) and the magnetic coupling parameter can mimic the effect of conformal gravity giving the same ISCO radius up to $ω_{\rm B}\leq 0.07$ when $N=3$.

gr-qc↗

arXiv2020

Dynamics of test particles around a Bardeen black hole surrounded by perfect fluid dark matter

We study the dynamics of (i) neutral test particles, (ii) magnetically charged test particles, and (iii) test magnetic dipole around a regular Bardeen black hole surrounded by perfect fluid dark matter (PFDM). It has been shown how the magnetic charge of the black hole and the parameter of the surrounding PFDM can influence the innermost stable circular orbit (ISCO) radius of a test particle. We have found that the ISCO radius is strongly affected as a consequence of the combined effect of the magnetic charge parameter and the perfect fluid dark matter. The black hole magnetic charge parameter $g$ and the combined effect of perfect fluid dark matter can mimic the black hole rotation parameter up to $a/M\approx 0.9$. It has been observed that the ISCO for magnetic dipole disappears at the values exceeding the calculated upper value for the magnetic interaction parameter $β$. The upper limit decreases with the increase of both the dark matter and magnetic charge parameters. Finally, as an astrophysical application, we have analyzed degeneracy effects of spin of Kerr black holes and magnetic charge of regular Bardeen black holes for the different values of the dark matter parameter providing exactly the same value for ISCO radius of a magnetic dipole with the same value of the parameter $β=10.2$ of the magnetar called PSR J1745-2900 orbiting around supermassive black hole Sagittarius A*.

gr-qc↗

arXiv2020

Testing the Keplerian disk hypothesis using X-ray reflection spectroscopy

The Novikov-Thorne model is the standard framework for the description of geometrically thin and optically thick accretion disks around black holes and is widely used to study the electromagnetic spectra of accreting black holes. One of the assumptions of the model is that the particles of the gas move on nearly-geodesic circular orbits on the equatorial plane. In this work, we propose to test the Keplerian velocity of the particles in the accretion disk using X-ray reflection spectroscopy. We present a modified version of RELXILL in which we introduce a phenomenological parameter, $α$, to quantify possible deviations from Keplerian motion. We use our model to fit a Suzaku observation of the black hole binary GRS 1915+105. We find that the estimate of $α$ is correlated to that of the inclination angle of the disk, $i$, and that we could test the Keplerian disk hypothesis in the presence of a robust and independent measurement of $i$.

astro-ph.HE↗

arXiv2020

Testing General Relativity with Black Hole X-ray Data

Einstein's theory of General Relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in weak gravitational fields. On the contrary, the strong field regime is still largely unexplored. Astrophysical black holes are ideal laboratories for testing gravity in the strong field regime. Here I will briefly review the current efforts of my group to test the strong gravitational field around black holes using X-ray data from NuSTAR, RXTE, Suzaku, and XMM-Newton.

gr-qc↗

arXiv2020

On the properties of a deformed extension of the NUT space-time

We consider a class of space-times given by a stationary extension of the Zipoy-Voorhees metric that was found by Halilsoy. We show that the solutions do not describe rotating sources but must be interpreted, similarly to the NUT case, as deformed sources endowed with a gravitomagnetic charge. We show that the Halilsoy family is directly linked to the NUT space-time, which can be obtained in the limit of vanishing deformations. We investigate the motion of test particles and photons in this class of space-times, in particular the innermost stable circular orbits and photon capture radius. Finally we show that this class of solutions possesses a sub-manifold where closed time-like curves are allowed.

gr-qc↗

arXiv2020

Testing the Kerr black hole hypothesis using X-ray reflection spectroscopy and a thin disk model with finite thickness

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.

astro-ph.HE↗

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

Can the dynamics of test particles around charged stringy black holes mimic the spin of Kerr black holes?

We study the motion of electrically charged particles, magnetic monopoles, and magnetic dipoles around electrically and magnetically charged stringy black holes. From the analysis of the radius of the innermost stable circular orbit (ISCO) of electrically charged particles, we show that the electric charge $Q$ of stringy black holes can mimic well the spin of Kerr black holes; the black hole magnetic charge $Q_m$ can mimic spins up to $a_* \simeq 0.85$ for magnetic dipoles; the magnetic charge parameter $g$ of a magnetic monopole can mimic spins up to $a_* \simeq 0.8$. This is due to the destructive character of the magnetic field and such a result excludes the existence of an appreciable black hole magnetic charge in astrophysical black holes from the observation of rapidly rotating objects with dimensionless spin up to $a_* \simeq 0.99$. We then consider the magnetar SGR (PSR) J1745-2900 as a magnetic dipole orbiting the supermassive black hole Sagittarius A* (Sgr A*). We show that Sgr A* may be interpreted as a stringy black hole with magnetic charge $Q_{\rm m}/M \leq 0.4118$.

gr-qc↗

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

Modeling uncertainties in X-ray reflection spectroscopy measurements II: Impact of the radiation from the plunging region

X-ray reflection spectroscopy is a powerful tool to probe the strong gravity region around black holes, but the available relativistic reflection models have a number of simplifications that lead to systematic uncertainties (not fully under control) in the measurement of the properties of a source. In Paper I, we considered the case of an optically thin plunging region and we studied the impact of the radiation produced by the other side of the disk or circling the black hole one or more times. In the present paper, we discuss the case of an optically thick plunging region and we study the impact of the reflection spectrum of the plunging gas. We show that the contribution of such radiation is more important for low values of the black hole spin parameter and large values of the viewing angle, and it decreases significantly as the spin parameter increases and the inclination angle decreases. While the estimate of some parameters may be affected by the reflection spectrum of the plunging gas if this is not included in the theoretical model, we find that such radiation does not appreciably limit our capability of testing the Kerr black hole hypothesis.

astro-ph.HE↗

arXiv2020

Quasi-harmonic oscillations of charged particles in static axially symmetric space-times immersed in a uniform magnetic field

In continuation of our previous study [Phys. Rev. D 99 (2019) 4, 044012], we investigate the motion of charged particles in the $γ$-metric. We provide some examples of curled trajectories in the equatorial plane and escape trajectories outside the equatorial plane. Finally, we consider harmonic oscillations due to small displacements from stable circular orbits on the equatorial plane and compute the epicyclic frequencies for different values of deformation parameter and magnetic field.

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

Modeling bias in supermassive black hole spin measurements

X-ray reflection spectroscopy (or iron line method) is a powerful tool to probe the strong gravity region of black holes, and currently is the only technique for measuring the spin of the supermassive ones. While all the available relativistic reflection models assume thin accretion disks, we know that several sources accrete near or above the Eddington limit and therefore must have thick accretion disks. In this work, we employ the Polish donut model for the description of thick disks. We thus estimate the systematic error on the spin measurement when a source with a thick accretion disk is fitted with a thin disk model. Our results clearly show that spin measurements can be significantly affected by the morphology of the accretion disk. Current spin measurements of sources with high mass accretion rate are therefore not reliable.

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

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.

gr-qc↗