85 research outputs found
Constraints on the Size of Extra Dimensions from the Orbital Evolution of the Black-Hole X-Ray Binary XTE J1118+480
In a universe of the Randall-Sundrum type, black holes are unstable and emit
gravitational modes in the extra dimension. This leads to dramatically
shortened lifetimes of astrophysical black holes and to an observable change of
the orbital period of black-hole binaries. I obtain an upper limit on the rate
of change of the orbital period of the binary XTE J1118+480 and constrain the
asymptotic curvature radius of the extra dimension to a value that is of the
same order as the constraints from other astrophysical sources. A unique
property of XTE J1118+480 is that the expected rate of change of the orbital
period due to magnetic braking alone is so large that only one additional
measurement of the orbital period would lead to the first detection of orbital
evolution of a black-hole binary and impose the tightest constraint to date on
the size of one extra dimension of the order of 35 microns.Comment: accepted for publication in A&
Testing the No-Hair Theorem with Sgr A*
The no-hair theorem characterizes the fundamental nature of black holes in
general relativity. This theorem can be tested observationally by measuring the
mass and spin of a black hole as well as its quadrupole moment, which may
deviate from the expected Kerr value. Sgr A*, the supermassive black hole at
the center of the Milky Way, is a prime candidate for such tests thanks to its
large angular size, high brightness, and rich population of nearby stars. In
this review I discuss a new theoretical framework for a test of the no-hair
theorem that is ideal for imaging observations of Sgr A* with very-long
baseline interferometry (VLBI). The approach is formulated in terms of a
Kerr-like spacetime that depends on a free parameter and is regular everywhere
outside of the event horizon. Together with the results from astrometric and
timing observations, VLBI imaging of Sgr A* may lead to a secure test of the
no-hair theorem.Comment: 8 pages, 7 figures, invited review for Advances in Astronomy, Special
Issue: "Seeking for the Leading Actor on the Cosmic Stage: Galaxies versus
Supermassive Black Holes
A Ray-Tracing Algorithm for Spinning Compact Object Spacetimes with Arbitrary Quadrupole Moments. I. Quasi-Kerr Black Holes
We describe a new numerical algorithm for ray tracing in the external
spacetimes of spinning compact objects characterized by arbitrary quadrupole
moments. Such spacetimes describe non-Kerr vacuum solutions that can be used to
test the no-hair theorem in conjunction with observations of accreting black
holes. They are also appropriate for neutron stars with spin frequencies in the
300-600 Hz range, which are typical of the bursting sources in low-mass X-ray
binaries. We use our algorithm to show that allowing for the quadrupole moment
of the spacetime to take arbitrary values leads to observable effects in the
profiles of relativistic broadened fluorescent iron lines from geometrically
thin accretion disks.Comment: submitted to the Astrophysical Journa
Magnetised Accretion Discs in Kerr Spacetimes II: Hot Spots
Context. Quasi-periodic variability has been observed in a number of X-ray
binaries harboring black hole candidates. In general relativity, black holes
are uniquely described by the Kerr metric and, according to the cosmic
censorship conjecture, curvature singularities always have to be clothed by an
event horizon. Aims. In this paper, we study the effect of an external magnetic
field on the observed light curves of orbiting hot spots in thin accretion
discs around Kerr black holes and naked singularities. Methods. We employ a
ray-tracing algorithm to calculate the light curves and power spectra of such
hot spots as seen by a distant observer for uniform and dipolar magnetic field
configurations assuming a weak coupling between the magnetic field and the disc
matter. Results. We show that the presence of an external dipolar magnetic
field leads to potentially observable modifications of these signals for both
Kerr black holes and naked singularities, while an external uniform magnetic
field has practically no effect. In particular, we demonstrate that the
emission from a hot spot orbiting near the innermost stable circular orbit of a
naked singularity in a dipolar magnetic field can be significantly harder than
the emission of the same hot spot in the absence of such a magnetic field.
Conclusions. The comparison of our model with observational data may allow us
study the geometry of magnetic fields around compact objects and to test the
cosmic censorship conjecture in conjunction with other observables such as
thermal continuum spectra and iron line profiles.Comment: 7 pages, 3 figures, submitted to Astronomy and Astrophysic
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