arXiv subjects

Cosimo Bambi

Publications and source records attributed to Cosimo Bambi.

At least 235 records · Page 13Linked to original sources

arXiv2015

Testing the Kerr Paradigm with X-ray Observations

Astrophysical black hole candidates are thought to be the Kerr black holes of general relativity, but the actual nature of these objects has still to be confirmed. The continuum-fitting and the iron line methods are currently the only available techniques to probe the spacetime geometry around these bodies and test the Kerr black hole paradigm. The continuum-fitting method is a robust approach, but the shape of the disk's thermal spectrum is in general too simple to measure the spin and to constrain possible deviations from the Kerr solution at the same time. The iron line analysis is potentially a powerful technique, but at the moment we do not have high quality data and a robust astrophysical model.

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arXiv2015

Shadows of CPR black holes and tests of the Kerr metric

We study the shadow of the Cardoso-Pani-Rico (CPR) black hole for different values of the black hole spin $a_*$, the deformation parameters $ε_3^t$ and $ε_3^r$, and the viewing angle $i$. We find that the main impact of the deformation parameter $ε_3^t$ is the change of the size of the shadow, while the deformation parameter $ε_3^r$ affects the shape of its boundary. In general, it is impossible to test the Kerr metric, because the shadow of a Kerr black hole can be reproduced quite well by a black hole with non-vanishing $ε_3^t$ or $ε_3^r$. Deviations from the Kerr geometry could be constrained in the presence of high quality data and in the favorable case of a black hole with high values of $a_*$ and $i$. However, the shadows of some black holes with non-vanishing $ε_3^r$ present peculiar features and the possible detection of these shadows could unambiguously distinguish these objects from the standard Kerr black holes of general relativity.

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arXiv2015

Collisional Penrose Process in Rotating Wormhole Spacetime

In a collisional Penrose process, two particles coming from the asymptotically flat region collide in the ergosphere of a compact object. The collision produces two new particles, one with positive energy and one with negative energy. When the particle with positive energy escapes to infinity, the process extracts energy from the compact object. In this paper, we study the collisional Penrose process in a rotating wormhole spacetime. We consider the simple case of a head-on collision at the throat of a Teo wormhole. We find that the process of energy extraction from a Teo wormhole can be substantially more efficient than the collisional Penrose process in the Kerr black hole spacetime.

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arXiv2015

Using iron line reverberation and spectroscopy to distinguish Kerr and non-Kerr black holes

The iron K$α$ line commonly observed in the X-ray spectrum of both stellar-mass and supermassive black hole candidates is produced by the illumination of a cold accretion disk by a hot corona. In this framework, the activation of a new flaring region in the hot corona imprints a time variation on the iron line spectrum. Future X-ray facilities with high time resolution and large effective areas may be able to measure the so-called 2-dimensional transfer function; that is, the iron line profile detected by a distant observer as a function of time in response to an instantaneous flare from the X-ray primary source. This work is a preliminary study to determine if and how such a technique can provide more information about the spacetime geometry around the compact object than the already possible measurements of the time-integrated iron line profile. Within our simplified model, we find that a measurement of iron line reverberation can improve constraints appreciably given a sufficiently strong signal, though that most of the information is present in the time-integrated spectrum. Our aim is to test the Kerr metric. We find that current X-ray facilities and data are unable to provide strong tests of the Kerr nature of supermassive black hole candidates. We consider an optimistic case of $10^5$ iron line photons from a next-generation data set. With such data, the reverberation model improves upon the spectral constraint by an order of magnitude.

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arXiv2015

Melvin Universe in Born-Infeld gravity

We consider a magnetic flux pointing in the $z$ direction of an axially symmetric space-time (Melvin Universe) in a Born-Infeld-type extension of General Relativity (GR) formulated in the Palatini approach. Large magnetic fields could have been produced in the early Universe, and given rise to interesting phenomenology regarding wormholes and black hole remnants. We find a formal analytic solution to this problem that recovers the GR result in the appropriate limits. Our results set the basis for further extensions that could allow the embedding of pairs of black hole remnants in geometries with intense magnetic fields.

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arXiv2015

High energy collision of two particles in wormhole spacetimes

We study the collision of two particles in the Teo wormhole spacetime, in which the wormhole is stationary and axisymmetric. We show that a non-rotating Teo wormhole cannot be a particle accelerator, while a rotating Teo wormhole can be used to accelerate particles and create high energy collisions because of the deep effective potential of the colliding particles. The process is different from that in the vicinity of a near-extremal black hole, since here there is no event horizon. This is the first example of particle collision with high center-of-mass energy in a spacetime with no event horizon, no naked singularity, and not being extremal in a clear sense. The process can unlikely have direct implications for astrophysical observations, but it is interesting as a tool to investigate wormhole instabilities.

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arXiv2015

Can static regular black holes form from gravitational collapse?

Starting from the Oppenheimer-Snyder model, we know how in classical general relativity the gravitational collapse of matter form a black hole with a central spacetime singularity. It is widely believed that the singularity must be removed by quantum gravity effects. Some static quantum-inspired singularity-free black hole solutions have been proposed in the literature, but when one considers simple examples of gravitational collapse the classical singularity is replaced by a bounce, after which the collapsing matter expands for ever. We may expect 3 possible explanations: $i)$ the static regular black hole solutions are not physical, in the sense that they cannot be realized in Nature, $ii)$ the final product of the collapse is not unique, but it depends on the initial conditions, or $iii)$ boundary effects play an important role and our simple models miss important physics. In the latter case, after proper adjustment, the bouncing solution would approach the static one. We argue that the "correct answer" may be related to the appearance of a ghost state in de Sitter spacetimes with super Planckian mass. Our black holes have indeed a de Sitter core and the ghost would make these configurations unstable. Therefore we believe that these black hole static solutions represent the transient phase of a gravitational collapse, but never survive as asymptotic states.

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arXiv2015

Constraining the Cardoso-Pani-Rico metric with future observations of SgrA$^*$

SgrA$^*$, the supermassive black hole candidate at the center of our Galaxy, seems to be one of the most promising object to test the Kerr black hole hypothesis with near future observations. In a few years, it will hopefully be possible to measure a number of relativistic effects around this body, and the combination of different observations can be used to constrain possible deviations from the Kerr solution. In this paper, I discuss the combination of three promising techniques in the framework of the Cardoso-Pani-Rico parametrization: the observation of blobs of plasma orbiting near the innermost stable circular orbit, the detection of the black hole shadow, and timing observations of a radio pulsar in a compact orbit. The observations of blobs of plasma and of the shadow can probe the strong gravitational field around SgrA$^*$, while the radio pulsar would be sensitive to the weak field region at larger radii. In the case of a fast-rotating object, the combination of the three measurements could provide strong constraints on the actual nature of SgrA$^*$. For a non-rotating or slow-rotating object, the bounds would be weak.

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arXiv2014

Gravitational blueshift from a collapsing object

We discuss a counterintuitive phenomenon of classical general relativity, in which a significant fraction of the radiation emitted by a collapsing object and detected by a distant observer may be blueshifted rather than redshifted. The key-point is that when the radiation propagates inside the collapsing body it is blueshifted, and this time interval may be sufficiently long for the effect to be larger than the later redshift due to the propagation in the vacuum exterior, from the surface of the body to the distant observer. Unfortunately, the phenomenon can unlikely have direct observational implications, but it is interesting by itself as a pure relativistic effect.

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arXiv2014

Testing a class of non-Kerr metrics with hot spots orbiting SgrA$^*$

SgrA$^*$, the supermassive black hole candidate at the Galactic Center, exhibits flares in the X-ray, NIR, and sub-mm bands that may be interpreted within a hot spot model. Light curves and images of hot spots orbiting a black hole are affected by a number of special and general relativistic effects, and they can be potentially used to check whether the object is a Kerr black hole of general relativity. However, in a previous study we have shown that the relativistic features are usually subdominant with respect to the background noise and the model-dependent properties of the hot spot, and eventually it is at most possible to estimate the frequency of the innermost stable circular orbit. In this case, tests of the Kerr metric are only possible in combination with other measurements. In the present work, we consider a class of non-Kerr spacetimes in which the hot spot orbit may be outside the equatorial plane. These metrics are difficult to constrain from the study of accretion disks and indeed current X-ray observations of stellar-mass and supermassive black hole candidates cannot put interesting bounds. Here we show that near future observations of SgrA$^*$ may do it. If the hot spot is sufficiently close to the massive object, the image affected by Doppler blueshift is brighter than the other one and this provides a specific observational signature in the hot spot's centroid track. We conclude that accurate astrometric observations of SgrA$^*$ with an instrument like GRAVITY should be able to test this class of metrics, except in the more unlikely case of a small viewing angle.

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arXiv2014

Attempt to explain black hole spin in X-ray binaries with new physics

It is widely believed that the spin of black holes in X-ray binaries is mainly natal. A significant spin-up from accretion is not possible. If the secondary has a low mass, the black hole spin cannot change too much even if the black hole swallows the whole stellar companion. If the secondary has a high mass, its lifetime is too short to transfer the necessary amount of matter and spin the black hole up. However, while black holes formed from the collapse of a massive star with Solar metallicity are expected to have low birth spin, current spin measurements show that some black holes in X-ray binaries are rotating very rapidly. Here I show that, if these objects are not the Kerr black holes of general relativity, the accretion of a small amount of matter ($\sim 2$~$M_\odot$) can make them look like very fast-rotating Kerr black holes. Such a possibility is not in contradiction with any observation and it can explain current spin measurements in a very simple way.

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arXiv2014

Fermi-bounce Cosmology and scale invariant power-spectrum

We develop a non-singular bouncing cosmology using a non-trivial coupling of general relativity to fermionic fields. The usual Big Bang singularity is avoided thanks to a negative energy density contribution from the fermions. Our theory is ghost-free since the fermionic operator that generates the bounce is equivalent to torsion, which has no kinetic terms. The physical system consists of standard general relativity plus a topological sector for gravity, and fermionic matter described by Dirac fields with a non-minimal coupling. We show that a scale invariant power-spectrum generated in the contracting phase can be recovered by suitable choices of fermion number density and bare mass, thus providing a possible alternative to the inflationary scenario.

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arXiv2014

Constraints on the spacetime geometry around 10 stellar-mass black hole candidates from the disk's thermal spectrum

In a previous paper, one of us has described a code to compute the thermal spectrum of geometrically thin and optically thick accretion disks around generic stationary and axisymmetric black holes, which are not necessarily of the Kerr type. As the structure of the accretion disk and the propagation of electromagnetic radiation from the disk to the distant observer depend on the background metric, the analysis of the thermal spectrum of thin disks can be used to test the actual nature of black hole candidates. In this paper, we consider the 10 stellar-mass black hole candidates for which the spin parameter has been already estimated from the analysis of the disk's thermal spectrum and under the assumption of the Kerr background, and we translate the measurements reported in the literature into constraints on the spin parameter--deformation parameter plane. The analysis of the disk's thermal spectrum can be used to estimate only one parameter of the geometry close to the compact object, and therefore it is not possible to get independent measurements of both the spin and the deformation parameters. The constraints obtained here will be used in combination with other measurements in future work, with the final goal to break the degeneracy between the spin and possible deviations from the Kerr solution and thus test the Kerr black hole hypothesis.

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arXiv2014

Note on the Cardoso-Pani-Rico parametrization to test the Kerr black hole hypothesis

The construction of a generic parametrization to describe the spacetime geometry around astrophysical black hole candidates is an important step to test the Kerr black hole hypothesis. In the last few years, the Johannsen-Psaltis metric has been the most common framework to study possible deviations from the Kerr solution with present and near future observations. Recently, Cardoso, Pani and Rico have proposed a more general parametrization. The aim of the present paper is to study this new metric in a specific context, namely the thermal spectrum of geometrically thin and optically thick accretion disks. The most relevant finding is that the spacetime geometry around objects that look like very fast-rotating Kerr black holes may still have large deviations from the Kerr solution. This was not the case with the Johannsen-Psaltis metric, which means the latter was missing an important class of non-Kerr spacetimes.

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arXiv2014

Singularity avoidance in quantum-inspired inhomogeneous dust collapse

In a previous paper, some of us studied general relativistic homogeneous gravitational collapses for dust and radiation, in which the density profile was replaced by an effective density justified by some quantum gravity models. It was found that the effective density introduces an effective pressure that becomes negative and dominant in the strong-field regime. With this set-up, the central singularity is replaced by a bounce, after which the cloud starts expanding. Motivated by the fact that in the classical case homogeneous and inhomogeneous collapse models have different properties, here we extend our previous work to the inhomogeneous case. As in the quantum-inspired homogeneous collapse model, the classical central singularity is replaced by a bounce, but the inhomogeneities strongly affect the structure of the bounce curve and of the trapped region.

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arXiv2014

Can we observationally test the weak cosmic censorship conjecture?

In general relativity, gravitational collapse of matter fields ends with the formation of a spacetime singularity, where the matter density becomes infinite and standard physics breaks down. According to the weak cosmic censorship conjecture, singularities produced in the gravitational collapse cannot be seen by distant observers and must be hidden within black holes. The validity of this conjecture is still controversial and at present we cannot exclude that naked singularities can be created in our Universe from regular initial data. In this paper, we study the radiation emitted by a collapsing cloud of dust and check whether it is possible to distinguish the birth of a black hole from the one of a naked singularity. In our simple dust model, we find that the properties of the radiation emitted in the two scenarios is qualitatively similar. That suggests that observational tests of the cosmic censorship conjecture may be very difficult, even in principle.

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arXiv2014

Distinguishing black holes and wormholes with orbiting hot spots

The supermassive black hole candidates at the center of every normal galaxy might be wormholes created in the early Universe and connecting either two different regions of our Universe or two different universes in a Multiverse model. Indeed, the origin of these supermassive objects is not well understood, topological non-trivial structures like wormholes are allowed both in general relativity and in alternative theories of gravity, and current observations cannot rule out such a possibility. In a few years, the VLTI instrument GRAVITY will have the capability to image blobs of plasma orbiting near the innermost stable circular orbit of SgrA$^*$, the supermassive black hole candidate in the Milky Way. The secondary image of a hot spot orbiting around a wormhole is substantially different from that of a hot spot around a black hole, because the photon capture sphere of the wormhole is much smaller. The radius of the photon capture sphere is independent of the hot spot model, and therefore its possible detection, which is observationally challenging but not out of reach, can unambiguously test if the center of our Galaxy harbors a wormhole rather than a black hole.

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arXiv2014

Note on the effect of a massive accretion disk in the measurements of black hole spins

The spin measurement of black holes has important implications in physics and astrophysics. Regardless of the specific technique to estimate the black hole spin, all the current approaches assume that the space-time geometry around the compact object is exactly described by the Kerr solution. This is clearly an approximation, because the Kerr metric is a stationary solution of the vacuum Einstein equations. In this paper, we estimate the effect of a massive accretion disk in the measurement of the black hole spin with a simple analytical model. For typical accretion disks, the mass of the disk is completely negligible, even for future more accurate measurements. However, for systems with very massive disks the effect may not be ignored.

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