1,687 research outputs found
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 and by its spin angular momentum . 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 . If we can estimate , , and 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 ,
the bound is .Comment: 4 pages, 2 figures. v2: refereed versio
Unattainable extended spacetime regions in conformal gravity
The Janis-Newman-Winicour metric is a solution of Einstein's gravity
minimally coupled to a real massless scalar field. The -metric is
instead a vacuum solution of Einstein's gravity. These spacetimes have no
horizon and possess a naked singularity at a finite value of the radial
coordinate, where curvature invariants diverge and the spacetimes are
geodetically incomplete. In this paper, we reconsider these solutions in the
framework of conformal gravity and we show that it is possible to solve the
spacetime singularities with a suitable choice of the conformal factor. Now
curvature invariants remain finite over the whole spacetime. Massive particles
never reach the previous singular surface and massless particles can never do
it with a finite value of their affine parameter. Our results support the
conjecture according to which conformal gravity can fix the singularity problem
that plagues Einstein's gravity.Comment: 1+10 pages, 2 figures. v2: refereed versio
Testing the performance and accuracy of the RELXILL model for the relativistic X-ray reflection from accretion disks
The reflection spectroscopic model RELXILL is commonly implemented in
studying relativistic X-ray reflection from accretion disks around black holes.
We present a systematic study of the model's capability to constrain the
dimensionless spin and ionization parameters from 6,000 NuSTAR
simulations of a bright X-ray source employing the lamppost geometry. We employ
high count spectra to show the limitations in the model without being confused
with limitations in signal-to-noise. We find that both parameters are
well-recovered at 90% confidence with improving constraints at higher
reflection fraction, high spin, and low source height. We test spectra across a
broad range - first at 1010 and then 10 total source counts
across the effective 3-79 keV band of NuSTAR, and discover a strong dependence
of the results on how fits are performed around the starting parameters, owing
to the complexity of the model itself. A blind fit chosen over an approach that
carries some estimates of the actual parameter values can lead to significantly
worse recovery of model parameters. We further stress on the importance to span
the space of nonlinear-behaving parameters like carefully and
thoroughly for the model to avoid misleading results. In light of selecting
fitting procedures, we recall the necessity to pay attention to the choice of
data binning and fit statistics used to test the goodness of fit by
demonstrating the effect on the photon index . We re-emphasize and
implore the need to account for the detector resolution while binning X-ray
data and using Poisson fit statistics instead while analyzing Poissonian data.Comment: 9 pages, 6 figures, accepted for publication in Ap
Testing general relativity with the reflection spectrum of the supermassive black hole in 1H0707495
Recently, we have extended the X-ray reflection model relxill to test the
spacetime metric in the strong gravitational field of astrophysical black
holes. In the present Letter, we employ this extended model to analyze
XMM-Newton, NuSTAR, and Swift data of the supermassive black hole in 1H0707-495
and test deviations from a Kerr metric parametrized by the Johannsen
deformation parameter . Our results are consistent with the
hypothesis that the spacetime metric around the black hole in 1H0707-495 is
described by the Kerr solution.Comment: 10 pages, 7 figures. v2: refereed versio
Public Release of RELXILL_NK: A Relativistic Reflection Model for Testing Einstein's Gravity
We present the public release version of relxill_nk, an X-ray reflection
model for testing the Kerr hypothesis and general relativity. This model
extends the relxill model that assumes the black hole spacetime is described by
the Kerr metric. We also present relxilllp_nk, the first non-Kerr X-ray
reflection model with a lamppost corona configuration, as well as all other
models available in the full relxill_nk package. In all models the relevant
relativistic effects are calculated through a general relativistic ray-tracing
code that can be applied to any well-behaved, stationary, axisymmetric, and
asymptotically flat black hole spacetime. We show that the numerical error
introduced by using a ray-tracing code is not significant as compared with the
observational error present in current X-ray reflection spectrum observations.
In addition, we present the reflection spectrum for the Johannsen metric as
calculated by relxill_nk.Comment: 15 pages, 8 figures. v2: refereed version. Code and documentation
available at
http://www.physics.fudan.edu.cn/tps/people/bambi/Site/RELXILL_NK.html and at
http://www.tat.physik.uni-tuebingen.de/~nampalliwar/relxill_nk
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 () seem to have a high radiative efficiency () and a
moderate mass accretion rate (). 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 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.Comment: 12 pages, 6 figures. v2: some typos correcte
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