48 research outputs found
Contribution of pulsars to the gamma-ray background and their observation with the space telescopes GLAST and AGILE
Luminosities and uxes of the expected population of galactic gamma-ray
pulsars become foreseeable if physical distributions at birth and evolutive
history are assigned. In this work we estimate the contribution of pulsar uxes
to the gamma-ray background, which has been measured by the EGRET experiment on
board of the CGRO. For pulsar luminosities we select some of the most important
gamma-ray emission models, taking into account both polar cap and outer gap
scenarios. We nd that this contribution strongly depends upon controversial
neutron star birth properties. A comparison between our simulation results and
EGRET data is presented for each model, nding an average contribution of about
10%. In addition, we perform the calculation of the number of new gamma-ray
pulsars detectable by GLAST and AGILE, showing a remarkable di erence between
the two classes of models. Finally, we suggest some improvements in the
numerical code, including more sophisticated galactic m odels and di erent
populations of pulsars like binaries, milliseconds, anomalous pulsars and
magnetars.Comment: 6 pages, 6 figures, to be published in the Proceedings of the 6th
International Symposium ''Frontiers of Fundamental and Computational
Physics'' (FFP6), Udine (Italy), Sep. 26-29, 200
Direct Measurement of the Positive Acceleration of the Universe and Testing Inhomogeneous Models under Gravitational Wave Cosmology
One possibility for explaining the apparent accelerating expansion of the
universe is that we live in the center of a spherically inhomogeneous universe.
Although current observations cannot fully distinguish CDM and these
inhomogeneous models, direct measurement of the acceleration of the universe
can be a powerful tool in probing them. We have shown that, if CDM is
the correct model, DECIGO/BBO would be able to detect the positive redshift
drift (which is the time evolution of the source redshift ) in 3--5 year
gravitational wave (GW) observations from neutron-star binaries, which enables
us to rule out any Lema\^itre-Tolman-Bondi (LTB) void model with monotonically
increasing density profile. We may even be able to rule out any LTB model
unless we allow unrealistically steep density profile at . This test
can be performed with GW observations alone, without any reference to
electromagnetic observations, and is more powerful than the redshift drift
measurement using Lyman forest.Comment: 5 pages, 2 figure
Cosmological perturbations of self-accelerating universe in nonlinear massive gravity
We study cosmological perturbations of self-accelerating universe solutions
in the recently proposed nonlinear theory of massive gravity, with general
matter content. While the broken diffeomorphism invariance implies that there
generically are 2 tensor, 2 vector and 2 scalar degrees of freedom in the
gravity sector, we find that the scalar and vector degrees have vanishing
kinetic terms and nonzero mass terms. Depending on their nonlinear behavior,
this indicates either nondynamical nature of these degrees or strong couplings.
Assuming the former, we integrate out the 2 vector and 2 scalar degrees of
freedom. We then find that in the scalar and vector sectors, gauge-invariant
variables constructed from metric and matter perturbations have exactly the
same quadratic action as in general relativity. The difference from general
relativity arises only in the tensor sector, where the graviton mass modifies
the dispersion relation of gravitational waves, with a time-dependent effective
mass. This may lead to modification of stochastic gravitational wave spectrum.Comment: 32 pages, 1 figure; v2: minor update to match the published versio
Merger of binary neutron stars of unequal mass in full general relativity
We present results of three dimensional numerical simulations of the merger
of unequal-mass binary neutron stars in full general relativity. A -law
equation of state is adopted, where , ,
\varep, and are the pressure, rest mass density, specific internal
energy, and the adiabatic constant, respectively. We take and the
baryon rest-mass ratio to be in the range 0.85--1. The typical grid size
is for . We improve several implementations since the
latest work. In the present code, the radiation reaction of gravitational waves
is taken into account with a good accuracy. This fact enables us to follow the
coalescence all the way from the late inspiral phase through the merger phase
for which the transition is triggered by the radiation reaction. It is found
that if the total rest-mass of the system is more than times of the
maximum allowed rest-mass of spherical neutron stars, a black hole is formed
after the merger irrespective of the mass ratios. The gravitational waveforms
and outcomes in the merger of unequal-mass binaries are compared with those in
equal-mass binaries. It is found that the disk mass around the so formed black
holes increases with decreasing rest-mass ratios and decreases with increasing
compactness of neutron stars. The merger process and the gravitational
waveforms also depend strongly on the rest-mass ratios even for the range --1.Comment: 32 pages, PRD68 to be publishe
Gravitational waves from inspiralling compact binaries: Parameter estimation using second-post-Newtonian waveforms
The parameters of inspiralling compact binaries can be estimated using
matched filtering of gravitational-waveform templates against the output of
laser-interferometric gravitational-wave detectors. Using a recently calculated
formula, accurate to second post-Newtonian (2PN) order [order , where
is the orbital velocity], for the frequency sweep () induced by
gravitational radiation damping, we study the statistical errors in the
determination of such source parameters as the ``chirp mass'' , reduced
mass , and spin parameters and (related to spin-orbit and
spin-spin effects, respectively). We find that previous results using template
phasing accurate to 1.5PN order actually underestimated the errors in ,
, and . For two inspiralling neutron stars, the measurement errors
increase by less than 16 percent.Comment: 14 pages, ReVTe
Short Gamma Ray Bursts as possible electromagnetic counterpart of coalescing binary systems
Coalescing binary systems, consisting of two collapsed objects, are among the
most promising sources of high frequency gravitational waves signals
detectable, in principle, by ground-based interferometers. Binary systems of
Neutron Star or Black Hole/Neutron Star mergers should also give rise to short
Gamma Ray Bursts, a subclass of Gamma Ray Bursts. Short-hard-Gamma Ray Bursts
might thus provide a powerful way to infer the merger rate of two-collapsed
object binaries. Under the hypothesis that most short Gamma Ray Bursts
originate from binaries of Neutron Star or Black Hole/Neutron Star mergers, we
outline here the possibility to associate short Gamma Ray Bursts as
electromagnetic counterpart of coalescing binary systems.Comment: 4 pages, 1 figur
Comparison of advanced gravitational-wave detectors
We compare two advanced designs for gravitational-wave antennas in terms of
their ability to detect two possible gravitational wave sources. Spherical,
resonant mass antennas and interferometers incorporating resonant sideband
extraction (RSE) were modeled using experimentally measurable parameters. The
signal-to-noise ratio of each detector for a binary neutron star system and a
rapidly rotating stellar core were calculated. For a range of plausible
parameters we found that the advanced LIGO interferometer incorporating RSE
gave higher signal-to-noise ratios than a spherical detector resonant at the
same frequency for both sources. Spheres were found to be sensitive to these
sources at distances beyond our galaxy. Interferometers were sensitive to these
sources at far enough distances that several events per year would be expected
Post-Newtonian SPH calculations of binary neutron star coalescence. I. Method and first results
We present the first results from our Post-Newtonian (PN) Smoothed Particle
Hydrodynamics (SPH) code, which has been used to study the coalescence of
binary neutron star (NS) systems. The Lagrangian particle-based code
incorporates consistently all lowest-order (1PN) relativistic effects, as well
as gravitational radiation reaction, the lowest-order dissipative term in
general relativity. We test our code on sequences of single NS models of
varying compactness, and we discuss ways to make PN simulations more relevant
to realistic NS models. We also present a PN SPH relaxation procedure for
constructing equilibrium models of synchronized binaries, and we use these
equilibrium models as initial conditions for our dynamical calculations of
binary coalescence. Though unphysical, since tidal synchronization is not
expected in NS binaries, these initial conditions allow us to compare our PN
work with previous Newtonian results.
We compare calculations with and without 1PN effects, for NS with stiff
equations of state, modeled as polytropes with . We find that 1PN
effects can play a major role in the coalescence, accelerating the final
inspiral and causing a significant misalignment in the binary just prior to
final merging. In addition, the character of the gravitational wave signal is
altered dramatically, showing strong modulation of the exponentially decaying
waveform near the end of the merger. We also discuss briefly the implications
of our results for models of gamma-ray bursts at cosmological distances.Comment: RevTeX, 37 pages, 17 figures, to appear in Phys. Rev. D, minor
corrections onl
An -frequency dynamics algorithm for gravitational waves
Coalescence of low mass compact binaries of neutron stars and black holes are
primary burst sources for LIGO and VIRGO.Of importance in the early stages of
observations will be the classification of candidate detections by source-type.
The diversity in source parameters and serendipity in any new window of
observations suggest to consider model-independent detection algorithms. Here a
frequency dynamics algorithm is described which extracts a trajectory in the
-plane from the noisy signal. The algorithm is studied in simulated
binary coalescence. Robust results are obtained with experimental noise data.
Experiments show the method to be superior to matched filtering in the presence
of model imperfections.Comment: to appear in Rapid Commun, Phys Rev