333 research outputs found
Testing the Kerr Paradigm with the Black Hole Shadow
Within 5-10 years, submillimeter VLBI facilities will be hopefully able to
image the "shadow" of SgrA. When a black hole is surrounded by an optically
thin emitting medium, the boundary of the shadow corresponds to the apparent
photon capture sphere and only depends on the background metric. An accurate
determination of the shape of the shadow of SgrA could constrain possible
deviations from the Kerr solution. In combination with other measurements,
these observations could test the Kerr black hole paradigm.Comment: 6 pages, 3 figures. Talk given at the "14th Marcel Grossmann Meeting"
(12-18 July 2015, Rome, Italy). To appear in the conference proceeding
Constraining possible variations of the fine structure constant in strong gravitational fields with the K iron line
In extensions of general relativity and in theories aiming at unifying
gravity with the forces of the Standard Model, the value of the "fundamental
constants" is often determined by the vacuum expectation value of new fields,
which may thus change in different backgrounds. Variations of fundamental
constants with respect to the values measured today in laboratories on Earth
are expected to be more evident on cosmological timescales and/or in strong
gravitational fields. In this paper, I show that the analysis of the K
iron line observed in the X-ray spectrum of black holes can potentially be used
to probe the fine structure constant in gravitational potentials
relative to Earth of . At present, systematic effects
not fully under control prevent to get robust and stringent bounds on possible
variations of the value of with this technique, but the fact that
current data can be fitted with models based on standard physics already rules
out variations of the fine structure constant larger than some percent.Comment: 1+15 pages, 6 figures. v2: refereed versio
Testing the space-time geometry around black hole candidates with the analysis of the broad K 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
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 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.Comment: 14 pages, 9 figures. v2: refereed version covering a wider range of
spins and inclination
Testing the Kerr black hole hypothesis with RELXILL_NK
Astrophysical black hole candidates are thought to be the Kerr black holes of
general relativity. However, macroscopic deviations from the Kerr background
are predicted by a number of scenarios beyond Einstein's gravity. X-ray
reflection spectroscopy can be a powerful tool to probe the strong gravity
region of these objects and test the Kerr black hole hypothesis. Here I briefly
review the state of the art of this line of research and I present some
constraints on possible deviations from the Kerr metric obtained with the new
X-ray reflection model relxill_nk and XMM-Newton, NuSTAR, and Swift data of the
supermassive black hole in 1H0707-495.Comment: 6 pages, 5 figures. Talk given at the "3rd Karl Schwarzschild
Meeting" (Frankfurt am Main, Germany, 24-28 July 2017). v2: refereed versio
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.Comment: 8 pages, 6 figures. v2: some typos correcte
Can the supermassive objects at the centers of galaxies be traversable wormholes? The first test of strong gravity for mm/sub-mm VLBI facilities
The near future mm/sub-mm VLBI experiments are ambitious projects aiming at
imaging the "shadow" of the supermassive black hole candidate at the center of
the Milky Way and of the ones in nearby galaxies. An accurate observation of
the shape of the shadow can potentially test the nature of these objects and
verify if they are Kerr black holes, as predicted by general relativity.
However, previous work on the subject has shown that the shadows produced in
other spacetimes are very similar to the one of the Kerr background, suggesting
that tests of strong gravity are not really possible with these facilities in
the near future. In this work, I instead point out that it will be relatively
easy to distinguish black holes from wormholes, topologically non-trivial
structures of the spacetime that might have been formed in the early Universe
and might connect our Universe with other universes.Comment: 5 pages, 2 figures. v2: some typos correcte
Evolution of the spin parameter of accreting compact objects with non-Kerr quadrupole moment
There is robust observational evidence supporting the existence of
compact bodies in X-ray binary systems and of
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 , where 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 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 . 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 .Comment: 20 pages, 16 figures. v2: published version with a few typos
correcte
Broad K iron line from accretion disks around traversable wormholes
It has been proposed that the supermassive black hole candidates at the
centers of galaxies might be wormholes formed in the early Universe and
connecting our Universe with other sister Universes. The analysis of the
profile of the relativistic K iron line is currently the only available
approach to probe the spacetime geometry around these objects. In this paper, I
compute the expected K iron line in some wormhole spacetimes and I
compare the results with the line produced around Kerr black holes. The line
produced in accretion disks around non-rotating or very slow-rotating wormholes
is relatively similar to the one expected around Kerr black holes with mid or
high value of spin parameter and current observations are still marginally
compatible with the possibility that the supermassive black hole candidates in
galactic nuclei are these objects. For wormholes with spin parameter , the associated K iron line is instead quite different
from the one produced around Kerr black holes, and their existence may already
be excluded.Comment: 9 pages, 8 figures. v2: refereed versio
Violation of the Carter-Israel conjecture and its astrophysical implications
On the basis of the Carter-Israel conjecture, today we believe that some
compact and massive objects in the Galaxy and in the Universe are Kerr black
holes. However, this idea cannot yet be confirmed by observations. We can
currently obtain reliable estimates of the masses of these objects, but we do
not know if the space-time around them is described by the Kerr metric and if
they have an event horizon. A fundamental limit for a Kerr black hole is the
Kerr bound . Here I discuss some astrophysical implications
associated with the violation of this bound, which can thus be used to test the
Carter-Israel conjecture.Comment: 8 pages, 6 figures. To appear in the proceedings of "NEB 14: Recent
Developments in Gravity" (Ioannina, Greece, 8 - 11 June 2010
Measuring the Kerr spin parameter of a non-Kerr compact object with the continuum-fitting and the iron line methods
Under the assumption that astrophysical black hole candidates are the Kerr
black holes of general relativity, the continuum-fitting method and the
analysis of the K iron line are today the only available techniques
capable of providing a relatively reliable estimate of the spin parameter of
these objects. If we relax the Kerr black hole hypothesis and we try to test
the nature of black hole candidates, we find that there is a strong correlation
between the measurement of the spin and possible deviations from the Kerr
solution. The properties of the radiation emitted in a Kerr spacetime with spin
parameter are indeed very similar, and practically indistinguishable,
from the ones of the radiation emitted around a non-Kerr object with different
spin. In this paper, I address the question whether measuring the Kerr spin
with both the continuum-fitting method and the K iron line analysis of
the same object can be used to claim the Kerr nature of the black hole
candidate in the case of consistent results. In this work, I consider two
non-Kerr metrics and it seems that the answer does depend on the specific
background. The two techniques may either provide a very similar result (the
case of the Bardeen metric) or show a discrepancy (Johannsen-Psaltis
background).Comment: 1+14 pages, 5 figures. v2: refereed versio
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