99,755 research outputs found
Black hole tidal charge constrained by strong gravitational lensing
Spherically symmetric brane black holes have tidal charge, which modifies
both weak and strong lensing characteristics. Even if lensing measurements are
in agreement with a Schwarzschild lens, the margin of error of the detecting
instrument allows for a certain tidal charge. In this paper we derive the
respective constraint on the tidal charge of the supermassive black hole (SMBH)
in the center of our galaxy, from the radius of the first relativistic Einstein
ring, emerging in strong lensing. We find that even if general relativistic
predictions are confirmed by high precision strong lensing measurements, SMBHs
could have a much larger tidal charge, than the Sun or neutron stars
Improved Algorithms for Recognizing Perfect Graphs and Finding Shortest Odd and Even Holes
Various classes of induced subgraphs are involved in the deepest results of
graph theory and graph algorithms. A prominent example concerns the {\em
perfection} of that the chromatic number of each induced subgraph of
equals the clique number of . The seminal Strong Perfect Graph Theorem
confirms that the perfection of can be determined by detecting odd holes in
and its complement. Chudnovsky et al. show in 2005 an algorithm
for recognizing perfect graphs, which can be implemented to run in
time for the exponent of square-matrix
multiplication. We show the following improved algorithms.
1. The tractability of detecting odd holes was open for decades until the
major breakthrough of Chudnovsky et al. in 2020. Their algorithm is
later implemented by Lai et al. to run in time, leading to the best
formerly known algorithm for recognizing perfect graphs. Our first result is an
algorithm for detecting odd holes, implying an algorithm for
recognizing perfect graphs.
2. Chudnovsky et al. extend in 2021 the algorithms for detecting odd
holes (2020) and recognizing perfect graphs (2005) into the first polynomial
algorithm for obtaining a shortest odd hole, which runs in time. We
reduce the time for finding a shortest odd hole to .
3. Conforti et al. show in 1997 the first polynomial algorithm for detecting
even holes, running in about time. It then takes a line of
intensive efforts in the literature to bring down the complexity to
, , , and finally . On the other hand,
the tractability of finding a shortest even hole has been open for 16 years
until the very recent algorithm of Cheong and Lu in 2022. We
improve the time of finding a shortest even hole to .Comment: 29 pages, 5 figure
Sparse Reconstruction-based Detection of Spatial Dimension Holes in Cognitive Radio Networks
In this paper, we investigate a spectrum sensing algorithm for detecting
spatial dimension holes in Multiple Inputs Multiple Outputs (MIMO)
transmissions for OFDM systems using Compressive Sensing (CS) tools. This
extends the energy detector to allow for detecting transmission opportunities
even if the band is already energy filled. We show that the task described
above is not performed efficiently by regular MIMO decoders (such as MMSE
decoder) due to possible sparsity in the transmit signal. Since CS
reconstruction tools take into account the sparsity order of the signal, they
are more efficient in detecting the activity of the users. Building on
successful activity detection by the CS detector, we show that the use of a
CS-aided MMSE decoders yields better performance rather than using either
CS-based or MMSE decoders separately. Simulations are conducted to verify the
gains from using CS detector for Primary user activity detection and the
performance gain in using CS-aided MMSE decoders for decoding the PU
information for future relaying.Comment: accepted for PIMRC 201
Orbital motion effects in astrometric microlensing
We investigate lens orbital motion in astrometric microlensing and its
detectability. In microlensing events, the light centroid shift in the source
trajectory (the astrometric trajectory) falls off much more slowly than the
light amplification as the source distance from the lens position increases. As
a result, perturbations developed with time such as lens orbital motion can
make considerable deviations in astrometric trajectories. The rotation of the
source trajectory due to lens orbital motion produces a more detectable
astrometric deviation because the astrometric cross-section is much larger than
the photometric one. Among binary microlensing events with detectable
astrometric trajectories, those with stellar-mass black holes have most likely
detectable astrometric signatures of orbital motion. Detecting lens orbital
motion in their astrometric trajectories helps to discover further secondary
components around the primary even without any photometric binarity signature
as well as resolve close/wide degeneracy. For these binary microlensing events,
we evaluate the efficiency of detecting orbital motion in astrometric
trajectories and photometric light curves by performing Monte Carlo simulation.
We conclude that astrometric efficiency is 87.3 per cent whereas the
photometric efficiency is 48.2 per cent.Comment: 9 pages, 8 figures, accepted for publication in MNRA
Binary sdB Stars with Massive Compact Companions
Original paper can be found at: http://astrosociety.org/pubs/cs/381.html Copyright ASPThe masses of compact objects like white dwarfs, neutron stars and black holes are fundamental to astrophysics, but very difficult to measure. We present the results of an analysis of subluminous B (sdB) stars in close binary systems with unseen compact companions to derive their masses and clarify their nature. Radial velocity curves were obtained from time resolved spectroscopy. The atmospheric parameters were determined in a quantitative spectral analysis. Based on high resolution spectra we were able to measure the projected rotational velocity of the stars with high accuracy. In the distribution of projected rotational velocities signs of tidal locking with the companions are visible. By detecting ellipsoidal variations in the lightcurve of an sdB binary we were able to show that subdwarf binaries with orbital periods up to 0.6 d are most likely synchronized. In this case, the inclination angles and companion masses of the binaries can be tightly constrained. Five invisible companions have masses that are compatible with that of normal white dwarfs or late type main sequence stars. However, four sdBs have compact companions massive enough to be heavy white dwarfs (> 1M⊙), neutron stars or even black holes. Such a high fraction of massive compact companions is not expected from current models of binary evolution
Where does the physics of extreme gravitational collapse reside?
The gravitational collapse of massive stars serves to manifest the most
severe deviations of general relativity with respect to Newtonian gravity: the
formation of horizons and spacetime singularities. Both features have proven to
be catalysts of deep physical developments, especially when combined with the
principles of quantum mechanics. Nonetheless, it is seldom remarked that it is
hardly possible to combine all these developments into a unified theoretical
model, while maintaining reasonable prospects for the independent experimental
corroboration of its different parts. In this paper we review the current
theoretical understanding of the physics of gravitational collapse in order to
highlight this tension, stating the position that the standard view on
evaporating black holes stands for. This serves as the motivation for the
discussion of a recent proposal that offers the opposite perspective,
represented by a set of geometries that regularize the classical singular
behavior and present modifications of the near-horizon Schwarzschild geometry
as the result of the propagation of non-perturbative ultraviolet effects
originated in regions of high curvature. We present an extensive exploration of
the necessary steps on the explicit construction of these geometries, and
discuss how this proposal could change our present understanding of
astrophysical black holes and even offer the possibility of detecting genuine
ultraviolet effects on future gravitational wave experiments.Comment: 43 pages, 1 figure. Review article with new results on the black to
white hole transition. Prepared for the special issue "Open Questions in
Black Hole Physics" edited by Gonzalo J. Olm
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