arXiv subjects

Cosimo Bambi

Publications and source records attributed to Cosimo Bambi.

At least 73 records · Page 4Linked to original sources

arXiv2024

About the ability of agnostic X-ray tests of the Kerr Hypothesis to discover new physics

In the past decade, we have seen an unprecedented progress in our ability of testing general relativity in the strong field regime with black hole observations. Most studies have focused on the so-called tests of the Kerr hypothesis: they have tried to verify whether the spacetime geometry around black holes is described by the Kerr solution as expected in general relativity. One can follow either a theory-specific analysis or an agnostic approach. Each strategy has its advantages and disadvantages. In this work, we study the ability of agnostic X-ray tests of the Kerr hypothesis to discover new physics. We simulate X-ray observations of bright Galactic black holes of specific theories of gravity and we analyze the simulated data with a reflection model employing the correct theory of gravity and another reflection model for agnostic tests of the Kerr hypothesis. Our results suggest that agnostic X-ray tests are valid tools to discover new physics, but their constraining power may be lower than a theory-specific analysis.

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arXiv2024

Measuring Black Hole Spins through X-ray Reflection Spectroscopy and the Relativistic Precession Model: the case of XTE J1859+226

The development of techniques to measure accurately black hole spins is crucial to study the physics and astrophysics of these objects. X-ray reflection spectroscopy is currently the most popular method to estimate the spins of accreting black holes; so far it has provided a spin measurement of about 40 stellar-mass black holes in X-ray binaries and 40 supermassive black holes in active galactic nuclei. The relativistic precession model (RPM) is another method to measure the spins of stellar-mass black holes: it requires the measurement of the frequencies of three simultaneous quasi-periodic oscillations and can potentially provide precise estimates of the black hole mass and spin. However, the two methods do not seem to provide consistent results when applied to the same sources, which questions the reliability and accuracy of these measurements. Recently, the RPM has been applied to infer the spin of the black hole in XTE J1859+226. The authors found $a_* = 0.149 \pm 0.005$ (68% CL). There are no other spin measurements of this source. We looked for archived RXTE observations of XTE J1859+226 with blurred reflection features and found 23 spectra suitable for measuring the spin. We employed two different models with relxill and relxillD and obtained a higher spin value from all these fits. From simultaneous fitting of 7 spectra of higher quality, we found $a_* = 0.986^{+0.001}_{-0.004}$ and $a_* =0.987 \pm 0.003$ (90% CL, statistical) with relxill and relxillD, respectively. Our results confirm the discrepancy between the spin measurements inferred from the two techniques.

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arXiv2023

Testing General Relativity with black hole X-ray data

The theory of General Relativity has successfully passed a large number of observational tests without requiring any adjustment from its original version proposed by Einstein in 1915. The past 8 years have seen significant advancements in the study of the strong-field regime, which can now be tested with gravitational waves, X-ray data, and black hole imaging. This is a compact and pedagogical review on the state-of-the-art of the tests of General Relativity with black hole X-ray data.

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arXiv2023

Kerr-Taub-NUT spacetime to explain the jet power and the radiative efficiency of astrophysical black holes

In this work, we investigate the electromagnetic energy released by astrophysical black holes within the Kerr-Taub-NUT solution, which describes rotating black holes with a nonvanishing gravitomagnetic charge. In our study, we consider the black holes in the X-ray binary systems GRS 1915+105, GRO J1655-40, XTE J1550-564, A0620-00, H1743-322, and GRS 1124-683. We show that the Kerr-Taub-NUT spacetime can explain the radiative efficiency of these sources inferred from the continuum fitting method (CFM). We also show that, in the framework of the Blandford-Znajeck mechanism, it is possible to reproduce the observed jet power. We unify the results of the two analyses for the selected objects to get more stringent constraints on the spacetime parameters. We show that, as in the case of the Kerr spacetime, the Kerr-Taub-NUT solution cannot simultaneously explain the observed jet power and radiative efficiency of GRS 1915+105.

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arXiv2023

Testing the $δ$-Kerr metric with black hole X-ray data

The spacetime around astrophysical black holes is thought to be described by the Kerr solution. However, even within general relativity, there is not yet a proof that the final product of the complete collapse of an uncharged body can only be a Kerr black hole. We can thus speculate on the possibility that the spacetime around astrophysical black holes may be described by other solutions of the Einstein Equations and we can test such a hypothesis with observations. In this work, we consider the $δ$-Kerr metric, which is an exact solution of the field equations in vacuum and can be obtained from a non-linear superposition of the Kerr metric with a static axially symmetric solution, often referred to as the $δ$-metric. The parameter $δ=1+q$ quantifies the departure of the source from the Kerr metric and for $q=0$ we recover the Kerr solution. From the analysis of the reflection features in the X-ray spectrum of the Galactic black hole in EXO 1846-031, we find $-0.1 < q < 0.7$ (90% CL), which is consistent with the hypothesis that the spacetime around the compact object in EXO 1846-031 is a Kerr black hole but does not entirely rule out the $δ$-Kerr metric.

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arXiv2023

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.

astro-ph.HE↗

arXiv2023

Observational appearance of Kaluza-Klein black holes

The optical properties of rotating black holes in Kaluza-Klein theory described by the total mass, spin, and electric and magnetic charges are investigated in detail. Using a developed general relativistic ray-tracing code to calculate the motion of photons, the numerous shadows of Kaluza-Klein black holes are generated. The properties of the black hole shadow and the light deflection angle around these black holes are also studied in order to put constraints on the parameters of Kaluza-Klein black holes using M87* shadow observations. The possibility of imposing constraints on Kaluza-Klein black holes using shadow observations is investigated. Moreover, we find that small charges (electric and magnetic) of the black hole can meet these constraints. We conclude that with the current precision of the M87* black hole shadow image observation by the EHT collaboration, the shadow observations of Kaluza-Klein black holes are indistinguishable from that of the Kerr black hole. Much better observational accuracy than the current capabilities of the EHT collaboration are required in order to place verified constraints on the parameters of modified theories of gravity in the strong field regime.

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arXiv2023

Regular Black Holes: Towards a New Paradigm of Gravitational Collapse

Black holes are one of the most fascinating predictions of general relativity. They are the natural product of the complete gravitational collapse of matter and today we have a body of observational evidence supporting the existence of black holes in the Universe. However, general relativity predicts that at the center of black holes there are spacetime singularities, where predictability is lost and standard physics breaks down. It is widely believed that spacetime singularities are a symptom of the limitations of general relativity and must be solved within a theory of quantum gravity. Since we do not have yet any mature and reliable candidate for a quantum gravity theory, researchers have studied toy-models of singularity-free black holes and of singularity-free gravitational collapses in order to explore possible implications of the yet unknown theory of quantum gravity. This book reviews all main models of regular black holes and non-singular gravitational collapses proposed in the literature, and discuss the theoretical and observational implications of these scenarios.

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arXiv2023

High-density reflection spectroscopy of black hole X-ray binaries in the hard state

We present a high-density relativistic reflection analysis of 21 spectra of six black hole X-ray binaries in the hard state with data from \textit{NuSTAR} and \textit{Swift}. We find that 76\% of the observations in our sample require a disk density higher than the 10$^{15}$~cm$^{-3}$ assumed in the previous reflection analysis. Compared with the measurements from active galactic nuclei, stellar mass black holes have higher disk densities. Our fits indicate that the inner disk radius is close to the innermost stable circular orbit in the hard state. The coronal temperatures are significantly lower than the prediction of a purely thermal plasma, which can be explained with a hybrid plasma model. If the disk density is fixed at 10$^{15}$~cm$^{-3}$, the disk ionization parameter would be overestimated while the inner disk radius is unaffected.

astro-ph.HE↗

arXiv2023

X-Ray Tests of General Relativity with Black Holes

General relativity is one of the pillars of modern physics. For decades, the theory has been mainly tested in the weak field regime with experiments in the Solar System and radio observations of binary pulsars. Until 2015, the strong field regime was almost completely unexplored. Thanks to new observational facilities, the situation has dramatically changed in the last few years. Today we have gravitational wave data of the coalesce of stellar-mass compact objects from the LIGO-Virgo-KAGRA Collaboration, images at mm wavelengths of the supermassive black holes in M87$^*$ and SgrA$^*$ from the Event Horizon Telescope Collaboration, and X-ray data of accreting compact objects from a number of X-ray missions. Gravitational wave tests and black hole imaging tests are certainly more popular and are discussed in other articles of this Special Issue. The aim of the present manuscript is to provide a pedagogical review on X-ray tests of general relativity with black holes and to compare this kind of tests with those possible with gravitational wave data and black hole imaging.

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arXiv2023

Multi-wavelength temporal variability of the blazar PKS 1510-089

We perform correlation and periodicity search analyses on long-term multi-band light curves of the FSRQ 1510-089 observed by the space-based Fermi--Large Area Telescope in gamma-rays, the SMARTS and Steward Observatory telescopes in optical and near-infrared (NIR) and the 13.7 m radio telescope in Metsahovi Radio Observatory between 2008 and 2018. The z-transform discrete correlation function method is applied to study the correlation and possible time lags among these multi band light curves. Among all pairs of wavelengths, the gamma-ray vs. optical/NIR and optical vs. NIR correlations show zero time lags; however, both the gamma-ray and optical/NIR emissions precede the radio radiation. The Generalized Lomb-Scargle periodogram, Weighted Wavelet Z-transform, and REDFIT techniques are employed to investigate the unresolved-core-emission dominated 37 GHz light curve and yield evidence for a quasi-period around 1540 days, although given the length of the whole data set it cannot be claimed to be significant. We also investigate the optical/NIR color variability and find that this source shows a simple redder-when-brighter behavior over time, even in the low flux state.

astro-ph.HE↗

arXiv2023

The hard to soft transition of GX 339-4 as seen by Insight-HXMT

We present an analysis of the relativistic reflection spectra of GX 339-4 during the hard-to-soft transition of its 2021 outburst observed by Insight-HXMT. The strong relativistic reflection signatures in the data suggest a high black hole spin ($a_*>0.86$) and an intermediate disk inclination angle (35-43 deg) of the system. The transition is accompanied by an increasing temperature of the disk and a softening of the corona emission while the inner disk radius remains stable. Assuming a lamppost geometry, the corona height is also found to stay close to the black hole across the state transition. If we include the Comptonization of the reflection spectrum, the scattering fraction parameter is found to decrease during the state transition. We also perform an analysis with a reflection model designed for hot accretion disks of stellar mass black holes where the surface of the innermost accretion disk is illuminated by emission from the corona and the thermal disk below. Our results support the scenario in which the state transition is associated with variations in the corona properties.

astro-ph.HE↗

arXiv2023

Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A$^*$

Horizon-scale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests of gravity and fundamental physics in the strong-field regime. We consider a wide range of well-motivated deviations from classical General Relativity (GR) BH solutions, and constrain them using the Event Horizon Telescope (EHT) observations of Sagittarius A$^*$ (Sgr A$^*$), connecting the size of the bright ring of emission to that of the underlying BH shadow and exploiting high-precision measurements of Sgr A$^*$'s mass-to-distance ratio. The scenarios we consider, and whose fundamental parameters we constrain, include various regular BHs, string-inspired space-times, violations of the no-hair theorem driven by additional fields, alternative theories of gravity, novel fundamental physics frameworks, and BH mimickers including well-motivated wormhole and naked singularity space-times. We demonstrate that the EHT image of Sgr A$^*$ places particularly stringent constraints on models predicting a shadow size larger than that of a Schwarzschild BH of a given mass, with the resulting limits in some cases surpassing cosmological ones. Our results are among the first tests of fundamental physics from the shadow of Sgr A$^*$ and, while the latter appears to be in excellent agreement with the predictions of GR, we have shown that a number of well motivated alternative scenarios, including BH mimickers, are far from being ruled out at present.

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arXiv2023

The Low Temperature Corona in ESO 511$-$G030 Revealed by NuSTAR and XMM-Newton

We present the results from a coordinated XMM-Newton $+$ NuSTAR observation of the Seyfert 1 Galaxy ESO 511$-$G030. With this joint monitoring programme, we conduct a detailed variability and spectral analysis. The source remained in a low flux and very stable state throughout the observation period, although there are slight fluctuations of flux over long timescales. The broadband (0.3-78~keV) spectrum shows the presence of a power-law continuum with a soft excess below 2~keV, a relatively narrow iron K$α$ emission ($\sim$6.4~keV), and an obvious cutoff at high energies. We find that the soft excess can be modeled by two different possible scenarios: a warm ($kT_{\rm e} \sim$ 0.19~keV) and optically thick ($τ- 18\sim25$) Comptonizing corona or a relativistic reflection from a high-density ($\log [n_{\rm e}/{\rm cm}^{-3}]=17.1 \sim 18.5$) inner disc. All models require a low temperature ($kT_{\rm e} \sim$ 13~keV) for the hot corona.

astro-ph.HE↗

arXiv2023

An XMM-Newton Study of Six Narrow-Line Seyfert 1 Galaxies at z = 0.35--0.92

We report a detailed analysis of the XMM-Newton spectra of six Narrow-Line Seyfert 1 (NLS1) galaxies at redshift z = 0.35--0.92. Compared with the NLS1s at lower redshift in the previously most-studied sample, these NLS1s have larger black hole (BH) masses ($\log\,M_\text{BH}>7.5$) with similar or even lower Eddington ratios. Our extended XMM-Newton sample of NLS1s shows strong soft X-ray excess emission below 2 keV. The quantified soft excess strength does not show an obvious discrepancy from previous studies of the lower-redshift NLS1s. The systematic effect in the measurement of the Eddington ratio mainly lies in the bolometric correction factor. We also tentatively fit the spectra assuming two more physical models for the soft excess: warm Comptonization and relativistic reflection from the inner accretion disk. In the first scenario, we confirm the ubiquity of a warm and optically thick corona. The behavior of a single source can be better explained by relativistic reflection, although we cannot distinguish which model is a more favorable explanation for the soft excess based on the best-fit statistics.

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arXiv2022

Impact of ionization and electron density gradients in X-ray reflection spectroscopy measurements

The models currently used for the analysis of the reflection spectra of black holes usually assume a disk with constant ionization and electron density. However, there is some debate on the impact of these assumptions on the estimate of the properties of the sources, in particular when the fits suggest very steep emissivity profiles in the inner part of the accretion disk. In this work, we re-analyze a selected set of high-quality NuSTAR and Suzaku data of Galactic black holes and we fit the reflection component with three different models: RELXILL_NK, in which the ionization parameter and the electron density are constant, RELXILLION_NK, where the electron density is still constant but the ionization profile is described by a power law, and RELXILLDGRAD_NK, where the electron density profile is described by a power law and the ionization profile is calculated self-consistently from the electron density and the emissivity. While RELXILLION_NK may provide slightly better fits, we do not find any substantial difference in the estimate of the properties of the sources among the three models. Our conclusion is that models with constant electron density and ionization parameter are probably sufficient, in most cases, to fit the currently available X-ray data of accreting black holes.

astro-ph.HE↗

arXiv2022

Testing regular black holes with X-ray and GW data

The presence of spacetime singularities in physically relevant solutions of the Einstein Equations is normally interpreted as a symptom of the breakdown of classical general relativity at very high densities/curvatures. However, despite significant efforts in the past decades, we do not have yet any robust theoretical framework to solve the problem of spacetime singularities. In this context, the past few years have seen an increasing interest in the study of phenomenological scenarios to describe singularity-free black holes, gravitational collapses, and cosmological models. In the present work, we consider the recent proposal by Mazza, Franzin & Liberati for a rotating regular black hole and we measure their regularization parameter $l$ from the available X-ray and gravitational wave black hole data. For $l = 0$, we recover the singular Kerr solution of general relativity, while for $l \neq 0$ we can have a regular black hole or a regular wormhole. Our analysis shows that the available data are consistent with a vanishing regularization parameter $l$ and we can constrain its value. From a NuSTAR spectrum of the Galactic black hole in EXO 1846-031, we find $l/M < 0.49$ (90% CL). From the gravitational wave event GW190707A, we find $l/M < 0.72$ (90% CL).

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arXiv2022

Search for Variations of Fundamental Constants

The possibility of variations of the values of fundamental constants is a phenomenon predicted by a number of scenarios beyond General Relativity. This can happen if ``our'' fundamental constants are not the actual constants of the fundamental theory and their value is instead determined, for example, by the background value of some new field or the size of extra dimensions. So far, most studies have been devoted to constrain possible temporal variations of the electromagnetic fine structure constant $α$, the proton to electron mass ratio $μ= m_p/m_e$, and the gravitational constant $G_{\rm N}$. Apart some claims of the detection of a temporal or spatial variation of $α$ and $μ$, so far there is no clear observational evidence of any variation of our fundamental constants.

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