arXiv subjects

Cosimo Bambi

Publications and source records attributed to Cosimo Bambi.

At least 271 records · Page 15Linked to original sources

arXiv2013

Testing the space-time geometry around black hole candidates with the available radio and X-ray data

Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, but the actual nature of these objects has still to be proven. The Kerr black hole hypothesis can be tested by observing strong gravity features and check if they are in agreement with the predictions of General Relativity. In particular, the study of the properties of the electromagnetic radiation emitted by the gas of the accretion disk can provide information on the geometry of the space-time around these objects and constrain possible deviations from the Kerr background.

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arXiv2013

Testing the space-time geometry around black hole candidates with the analysis of the broad K$α$ iron line

Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, but there is not yet clear evidence that the geometry of the space-time around these objects is really described by the Kerr metric. In order to confirm the Kerr black hole hypothesis, we have to observe strong gravity features and check that they are in agreement with the ones predicted by General Relativity. In this paper, I study the broad K$α$ iron line, which is often seen in the X-ray spectrum of both stellar-mass and super-massive black hole candidates and whose shape is supposed to be strongly affected by the space-time geometry. As found in previous studies in the literature, there is a strong correlation between the spin parameter and the deformation parameter; that is, the line emitted around a Kerr black hole with a certain spin can be very similar to the one coming from the space-time around a non-Kerr object with a quite different spin. Despite that, the analysis of the broad K$α$ iron line is potentially more powerful than the continuum-fitting method, as it can put an interesting bound on possible deviations from the Kerr geometry independently of the value of the spin parameter and without additional measurements.

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arXiv2012

Attempt to find a correlation between the spin of stellar-mass black hole candidates and the power of steady jets: relaxing the Kerr black hole hypothesis

The rotational energy of a black hole can be extracted via the Blandford-Znajek mechanism and numerical simulations suggest a strong dependence of the power of the produced jet on the black hole spin. A recent study has found no evidence for a correlation between the spin and the power of steady jets. If the measurements of the spin and of the jet power are correct, it leads one to conclude that steady jets are not powered by the black hole spin. In this paper, I explore another possibility: I assume that steady jets are powered by the spin and I check if current observations can be explained if astrophysical black hole candidates are not the Kerr black hole predicted by General Relativity. It turns out that this scenario might indeed be possible. While such a possibility is surely quite speculative, it is definitively intriguing and can be seriously tested when future more accurate measurements will be available.

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arXiv2012

A code to compute the emission of thin accretion disks in non-Kerr space-times and test the nature of black hole candidates

Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, but the actual nature of these objects has still to be proven. The analysis of the electromagnetic radiation emitted by a geometrically thin and optically thick accretion disk around a black hole candidate can provide information about the geometry of the space-time around the compact object and it can thus test the Kerr black hole hypothesis. In this paper, I present a code based on a ray-tracing approach and capable of computing some basic properties of thin accretion disks in space-times with deviations from the Kerr background. The code can be used to fit current and future X-ray data of stellar-mass black hole candidates and constrain possible deviations from the Kerr geometry in the spin parameter-deformation parameter plane.

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arXiv2012

Probing the space-time geometry around black hole candidates with the resonance models for high-frequency QPOs and comparison with the continuum-fitting method

Astrophysical black hole candidates are thought to be the Kerr black hole predicted by General Relativity. In order to confirm the Kerr-nature of these objects, we need to probe the geometry of the space-time around them and see if the observations are consistent with the predictions of the Kerr metric. That can be achieved, for instance, by studying the properties of the electromagnetic radiation emitted by the gas in the accretion disk. The high-frequency quasi-periodic oscillations observed in the X-ray flux of some stellar-mass black hole candidates might do the job. As the frequencies of these oscillations depend only very weakly on the observed X-ray flux, it is thought they are mainly determined by the metric of the space-time. In this paper, I consider the resonance models proposed by Abramowicz and Kluzniak and I extend previous results to the case of non-Kerr space-times. The emerging picture is more complicated than the one around a Kerr black hole and there is a larger number of possible combinations between different modes. I then compare the bounds inferred from the twin peak high-frequency quasi-periodic oscillations observed in three micro-quasars (GRO J1655-40, XTE J1550-564, and GRS 1915+105) with the measurements from the continuum-fitting method of the same objects. For Kerr black holes, the two approaches do not provide consistent results. In a non-Kerr geometry, this conflict may be solved if the observed quasi-periodic oscillations are produced by the resonance $ν_θ: ν_r = 3:1$, where $ν_θ$ and $ν_r$ are the two epicyclic frequencies. It is at least worth mentioning that the deformation from the Kerr solution required by observations would be consistent with the one suggested in another recent work discussing the possibility that steady jets are powered by the spin of these compact objects.

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arXiv2012

Direct imaging rapidly-rotating non-Kerr black holes

Recently, two of us have argued that non-Kerr black holes in gravity theories different from General Relativity may have a topologically non-trivial event horizon. More precisely, the spatial topology of the horizon of non-rotating and slow-rotating objects would be a 2-sphere, like in Kerr space-time, while it would change above a critical value of the spin parameter. When the topology of the horizon changes, the black hole central singularity shows up. The accretion process from a thin disk can potentially overspin these black holes and induce the topology transition, violating the Weak Cosmic Censorship Conjecture. If the astrophysical black hole candidates are not the black holes predicted by General Relativity, we might have the quite unique opportunity to see their central region, where classical physics breaks down and quantum gravity effects should appear. Even if the quantum gravity region turned out to be extremely small, at the level of the Planck scale, the size of its apparent image would be finite and potentially observable with future facilities.

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arXiv2012

Testing the Kerr-nature of stellar-mass black hole candidates by combining the continuum-fitting method and the power estimate of transient ballistic jets

Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, as these objects cannot be explained otherwise without introducing new physics. However, there is no observational evidence that the space-time around them is really described by the Kerr solution. The Kerr black hole hypothesis can be tested with the already available X-ray data by extending the continuum-fitting method, a technique currently used by astronomers to estimate the spins of stellar-mass black hole candidates. In general, we cannot put a constraint on possible deviations from the Kerr geometry, but only on some combination between these deviations and the spin. The measurement of the radio power of transient jets in black hole binaries can potentially break this degeneracy, thus allowing for testing the Kerr-nature of these objects.

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arXiv2012

Towards the use of the most massive black hole candidates in AGN to test the Kerr paradigm

The super-massive objects in galactic nuclei are thought to be the Kerr black holes predicted by General Relativity, although a definite proof of their actual nature is still lacking. The most massive objects in AGN ($M \sim 10^9 M_\odot$) seem to have a high radiative efficiency ($η\sim 0.4$) and a moderate mass accretion rate ($L_{\rm bol}/L_{\rm Edd} \sim 0.3$). The high radiative efficiency could suggest they are very rapidly-rotating black holes. The moderate luminosity could indicate that their accretion disk is geometrically thin. If so, these objects could be excellent candidates to test the Kerr black hole hypothesis. An accurate measurement of the radiative efficiency of an individual AGN may probe the geometry of the space-time around the black hole candidate with a precision comparable to the one achievable with future space-based gravitational-wave detectors like LISA. A robust evidence of the existence of a black hole candidate with $η> 0.32$ and accreting from a thin disk may be interpreted as an indication of new physics. For the time being, there are several issues to address before using AGN to test the Kerr paradigm, but the approach seems to be promising and capable of providing interesting results before the advent of gravitational wave astronomy.

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arXiv2011

Can an astrophysical black hole have a topologically non-trivial event horizon?

In 4-dimensional General Relativity, there are several theorems restricting the topology of the event horizon of a black hole. In the stationary case, black holes must have a spherical horizon, while a toroidal spatial topology is allowed only for a short time. In this paper, we consider spinning black holes inspired by Loop Quantum Gravity and by alternative theories of gravity. We show that the spatial topology of the event horizon of these objects changes when the spin parameter exceeds a critical value and we argue that the phenomenon may be quite common for non-Kerr black holes. Such a possibility may be relevant in astrophysics, as in some models the accretion process can induce the topology transition of the horizon.

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arXiv2011

Testing the Kerr black hole hypothesis

It is thought that the final product of the gravitational collapse is a Kerr black hole and astronomers have discovered several good astrophysical candidates. While there is some indirect evidence suggesting that the latter have an event horizon, and therefore that they are black holes, a proof that the space-time around these objects is described by the Kerr geometry is still lacking. Recently, there has been an increasing interest in the possibility of testing the Kerr black hole hypothesis with present and future experiments. In this paper, I briefly review the state of the art of the field, focussing on some recent results and work in progress.

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arXiv2011

Can we constrain the maximum value for the spin parameter of the super-massive objects in galactic nuclei without knowing their actual nature?

In 4-dimensional General Relativity, black holes are described by the Kerr solution and are subject to the bound $|a_*| \le 1$, where $a_*$ is the black hole spin parameter. If current black hole candidates are not the black holes predicted in General Relativity, this bound does not hold and $a_*$ might exceed 1. In this letter, I relax the Kerr black hole hypothesis and I find that the value of the spin parameter of the super-massive black hole candidates in galactic nuclei cannot be higher than about 1.2. A higher spin parameter would not be consistent with a radiative efficiency $η> 0.15$, as observed at least for the most luminous AGN. While a rigorous proof is lacking, I conjecture that the bound $|a_*| \lesssim 1.2$ is independent of the exact nature of these objects.

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arXiv2011

The final stages of accretion onto non-Kerr compact objects

The $5 - 20 M_\odot$ dark objects in X-ray binary systems and the $10^5 - 10^9 M_\odot$ dark objects in galactic nuclei are currently thought to be the Kerr black holes predicted by General Relativity. However, direct observational evidence for this identification is still elusive, and the only viable approach to confirm the Kerr black hole hypothesis is to explore and rule out any other possibility. Here we investigate the final stages of the accretion process onto generic compact objects. While for Kerr black holes and for more oblate bodies the accreting gas reaches the innermost stable circular orbit (ISCO) and plunges into the compact object, we find that for more prolate bodies several scenarios are possible, depending on the spacetime geometry. In particular, we find examples in which the gas reaches the ISCO, but then gets trapped between the ISCO and the compact object. In this situation, accretion onto the compact object is possible only if the gas loses additional angular momentum, forming torus-like structures inside the ISCO.

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arXiv2011

Spinning super-massive objects in galactic nuclei up to $a_* > 1$

Nowadays we believe that a typical galaxy contains about $10^7$ stellar-mass black holes and a single super-massive black hole at its center. According to general relativity, these objects are characterized solely by their mass $M$ and by their spin parameter $a_*$. A fundamental limit for a black hole in general relativity is the Kerr bound $|a_*| \le 1$, but the accretion process can spin it up to $a_* \approx 0.998$. If a compact object is not a black hole, the Kerr bound does not hold and in this letter I provide some evidences suggesting that the accretion process could spin the body up to $a_* > 1$. While this fact should be negligible for stellar-mass objects, some of the super-massive objects at the center of galaxies may actually be super-spinning bodies exceeding the Kerr bound. Such a possibility can be tested by gravitational wave detectors like LISA or by sub-millimeter very long baseline interferometry facilities.

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arXiv2011

Evolution of the spin parameter of accreting compact objects with non-Kerr quadrupole moment

There is robust observational evidence supporting the existence of $5 - 20$ $M_\odot$ compact bodies in X-ray binary systems and of $10^5 - 10^9$ $M_\odot$ bodies at the center of many galaxies. All these objects are commonly interpreted as black holes, even is there is no direct evidence that they have an event horizon. A fundamental limit for a black hole in 4-dimensional general relativity is the Kerr bound $|a_*| \le 1$, where $a_*$ is the spin parameter. This is just the condition for the existence of the event horizon. The accretion process can spin a black hole up to $a_* \approx 0.998$ and some super-massive objects in galactic nuclei could be rapidly rotating black holes with spin parameter close to this limit. However, if these super-massive objects are not black holes, the Kerr bound does not hold and the accretion process can spin them up to $a_* > 1$. In this paper, I consider compact bodies with non-Kerr quadrupole moment. I study the evolution of the spin parameter due to accretion and I find its equilibrium value. Future experiments like the gravitational wave detector LISA will be able to test if the super-massive objects at the center of galaxies are the black holes predicted by general relativity. If they are not black holes, some of them may be super-spinning objects with $a_* > 1$.

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arXiv2011

Constraint on the quadrupole moment of super-massive black hole candidates from the estimate of the mean radiative efficiency of AGN

The super-massive objects at the center of many galaxies are commonly thought to be black holes. In 4-dimensional general relativity, a black hole is completely specified by its mass $M$ and by its spin angular momentum $J$. All the higher multipole moments of the gravitational field depend in a very specific way on these two parameters. For instance, the mass quadrupole moment is $Q = - J^2/M$. If we can estimate $M$, $J$, and $Q$ for the super-massive objects in galactic nuclei, we over-constrain the theory and we can test the black hole hypothesis. While there are many works studying how this can be done with future observations, in this paper a constraint on the quadrupole moment of these objects is obtained by using the current estimate of the mean radiative efficiency of AGN. In terms of the anomalous quadrupole moment $q$, the bound is $-2.01 < q < 0.14$.

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arXiv2011

Compact objects with spin parameter $a_* > 1$

In 4-dimensional General Relativity, black holes are described by the Kerr solution and are completely specified by their mass $M$ and by their spin angular momentum $J$. A fundamental limit for a black hole in General Relativity is the Kerr bound $|a_*| \le 1$, where $a_* = J/M^2$ is the spin parameter. Future experiments will be able to probe the geometry around these objects and test the Kerr black hole hypothesis. Interestingly, if these objects are not black holes, the accretion process may spin them up to $a_* > 1$.

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arXiv2010

Thick disk accretion in Kerr space-time with arbitrary spin parameters

In this paper we extend our previous works on spherically symmetric accretion onto black holes and super-spinars to the case in which the fluid has a finite angular momentum initially. We run 2.5D and 3D general relativistic hydrodynamic simulations of the accretion of a fat disk. We study how the accretion process changes by changing the values of the parameters of our model. We show that the value of the fluid angular momentum critically determines turn-on and off the production of powerful equatorial outflows around super-spinars. For corotating disks, equatorial outflows are efficiently generated, even for relatively low spin parameters or relatively large super-spinar radii. For counterrotating disks, equatorial outflows are instead significantly suppressed, and they are possible only in limited cases. We also study accretion around a tilted disk.

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arXiv2010

3D simulations of the accretion process in Kerr space-time with arbitrary value of the spin parameter

We present the results of three-dimensional general relativistic hydrodynamic simulations of adiabatic and spherically symmetric accretion in Kerr space-time. We consider compact objects with spin parameter $|a_*| \le 1$ (black holes) and with $|a_*| > 1$ (super-spinars). Our full three-dimensional simulations confirm the formation of equatorial outflows for high values of $|a_*|$, as found in our previous work in 2.5 dimensions. We show that the critical value of $|a_*|$ determining the onset of powerful outflows depends mainly on the radius of the compact object. The phenomenon of equatorial outflows can hardly occur around a black hole and may thus be used to test the bound $|a_*| \le 1$ for astrophysical black hole candidates.

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