211 research outputs found
Multiverse: Mobility pattern understanding improves localization accuracy
Department of Computer Science and EngineeringThis paper presents the design and implementation of Multiverse, a practical indoor localization system that can be deployed on top of already existing WiFi infrastructure. Although the existing WiFi-based positioning techniques achieve acceptable accuracy levels, we find that existing solutions are not practical for use in buildings due to a requirement of installing sophisticated access point (AP) hardware or special application on client devices to aid the system with extra information. Multiverse achieves sub-room precision estimates, while utilizing only received signal strength indication (RSSI) readings available to most of today's buildings through their installed APs, along with the assumption that most users would walk at the normal speed. This level of simplicity would promote ubiquity of indoor localization in the era of smartphones.ope
Relativistic simulations of the phase-transition-induced collapse of neutron stars
An increase in the central density of a neutron star may trigger a phase
transition from hadronic matter to deconfined quark matter in the core, causing
it to collapse to a more compact hybrid-star configuration. We present a study
of this, building on previous work by Lin et al. (2006). We follow them in
considering a supersonic phase transition and using a simplified equation of
state, but our calculations are general relativistic (using 2D simulations in
the conformally flat approximation) as compared with their 3D Newtonian
treatment. We also improved the treatment of the initial phase transformation,
avoiding the introduction of artificial convection. As before, we find that the
emitted gravitational-wave spectrum is dominated by the fundamental
quasi-radial and quadrupolar pulsation modes but the strain amplitudes are much
smaller than suggested previously, which is disappointing for the detection
prospects. However, we see significantly smaller damping and observe a
nonlinear mode resonance which substantially enhances the emission in some
cases. We explain the damping mechanisms operating, giving a different view
from the previous work. Finally, we discuss the detectability of the
gravitational waves, showing that the signal-to-noise ratio for current or
second generation interferometers could be high enough to detect such events in
our Galaxy, although third generation detectors would be needed to observe them
out to the Virgo cluster, which would be necessary for having a reasonable
event rate.Comment: 28 pages, 27 figures. Minor changes to be consistent with published
versio
Measuring the Angular Momentum Distribution in Core-Collapse Supernova Progenitors with Gravitational Waves
The late collapse, core bounce, and the early postbounce phase of rotating
core collapse leads to a characteristic gravitational wave (GW) signal. The
precise shape of the signal is governed by the interplay of gravity, rotation,
nuclear equation of state (EOS), and electron capture during collapse. We
explore the dependence of the signal on total angular momentum and its
distribution in the progenitor core by means of a large set of axisymmetric
general-relativistic core collapse simulations in which we vary the initial
angular momentum distribution in the core. Our simulations include a
microphysical finite-temperature EOS, an approximate electron capture treatment
during collapse, and a neutrino leakage scheme for the postbounce evolution. We
find that the precise distribution of angular momentum is relevant only for
very rapidly rotating cores with T/|W|>~8% at bounce. We construct a numerical
template bank from our baseline set of simulations, and carry out additional
simulations to generate trial waveforms for injection into simulated advanced
LIGO noise at a fiducial galactic distance of 10 kpc. Using matched filtering,
we show that for an optimally-oriented source and Gaussian noise, advanced
Advanced LIGO could measure the total angular momentum to within ~20%, for
rapidly rotating cores. For most waveforms, the nearest known degree of
precollapse differential rotation is correctly inferred by both our matched
filtering analysis and an alternative Bayesian model selection approach. We
test our results for robustness against systematic uncertainties by injecting
waveforms from simulations using a different EOS and and variations in the
electron fraction in the inner core. The results of these tests show that these
uncertainties significantly reduce the accuracy with which the total angular
momentum and its precollapse distribution can be inferred from observations.Comment: 22 pages, 16 figure
The impact of progenitor asymmetries on the neutrino-driven convection in core-collapse supernovae
The explosion of massive stars in core-collapse supernovae may be aided by
the convective instabilities that develop in their innermost nuclear burning
shells. The resulting fluctuations support the explosion by generating
additional turbulence behind the supernova shock. It was suggested that the
buoyant density perturbations arising from the interaction of the pre-collapse
asymmetries with the shock may be the primary contributor to the enhancement of
the neutrino-driven turbulent convection in the post-shock region. Employing
three-dimensional numerical simulations of a toy model, we investigate the
impact of such density perturbations on the post-shock turbulence. We consider
a wide range of perturbation parameters. The spatial scale and the amplitude of
the perturbations are found to be of comparable importance. The turbulence is
particularly enhanced when the perturbation frequency is close to that of the
convective turnovers in the gain region. Our analysis confirms that the buoyant
density perturbations is indeed the main source of the additional turbulence in
the gain region, validating the previous order-of-magnitude estimates.Comment: 15 pages, 16 figures. Minor revisions following referee comments
including additional 3D simulations and updated Figs. 5 and 10. Matches
version published in MNRA
Radiation from an inertial mirror horizon
The purpose of this study is to investigate radiation from asymptotic zero
acceleration motion where a horizon is formed and subsequently detected by an
outside witness. A perfectly reflecting moving mirror is used to model such a
system and compute the energy and spectrum. The trajectory is asymptotically
inertial (zero proper acceleration)-ensuring negative energy flux (NEF), yet
approaches light-speed with a null ray horizon at a finite advanced time. We
compute the spectrum and energy analytically.Comment: 10 pages, 6 figures, 1 table; Accepted for publication in Universe;
Selected Papers from the 17th Russian Gravitational Conference -International
Conference on Gravitation, Cosmology and Astrophysics (RUSGRAV-17
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
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