180 research outputs found
Effects of Luminosity Functions Induced by Relativistic Beaming on Statistics of Cosmological Gamma-Ray Bursts
We study the effects of the beaming-induced luminosity function on statistics
of observed GRBs, assuming the cosmological scenario. We select and divide the
BATSE 4B data into 588 long bursts (T sec) and 149 short bursts
(T sec), and compare the statistics calculated in each subgroup. The
of the long bursts is $ 0.2901\pm 0.0113$, and that of the
short bursts is $0.4178\pm 0.0239$, which is a Euclidean value. For luminosity
function models, we consider a cylindrical-beam and a conic-beam. We take into
account the spatial distribution of GRB sources as well. A broad luminosity
function is naturally produced when one introduces beaming of GRBs. We
calculate the maximum detectable redshift of GRBs, $z_{\rm max}$. The estimated
$z_{\rm max}$ for the cylindrical-beam case is as high as $\sim 14$ for the
long bursts and $\sim 3$ for the short bursts. The large $z_{\rm max}$ value
for the short bursts is rather surprising in that the for
this subgroup is close to the so-called Euclidean value, 0.5. We calculate the
fraction of bursts whose redshifts are larger than a certain redshift ,
i.e. . When we take and apply the luminosity function
derived for the cylindrical-beam, the expected is
for long bursts. When we increase the opening angle of the conic beam to
, decreases to at . We conclude that the beaming-induced luminosity functions are
compatible with the redshift distribution of observed GRBs and that the
apparent Euclidean value of may not be due to the Euclidean
space distribution but to the luminosity distribution.Comment: Accepted for publication in the Astronomical Journal (vol. 548, Feb.
20 2001
Gravitational lensing aided luminosity distance estimation for compact binary coalescences
The luminosity distance is a key observable of gravitational-wave
observations. We demonstrate how one can correctly retrieve the luminosity
distance of compact binary coalescences if the gravitational-wave signal is
strongly lensed. We perform a proof-of-concept parameter estimation for the
luminosity distance supposing (i) strong lensing produces two lensed
gravitational-wave signals, (ii) the advanced LIGO-Virgo network detects both
lensed signals as independent events, and (iii) the two events are identified
as strongly lensed signals originated from a single compact binary coalescence.
Focusing on the maximum magnification allowed in the given lensing scenario, we
find that the strong lensing can improve the precision of the distance
estimation by up to a factor of two compared to that can be expected for the
signal experiencing no lensing. Our results imply that strong lensing of
gravitational waves can be helpful for better constraining the distance to the
source, and furthermore, the Hubble constant.Comment: 7 pages, 4 figures, 2 table
Gravitational waves from BH-NS binaries: Effective Fisher matrices and parameter estimation using higher harmonics
Inspiralling black hole-neutron star (BH-NS) binaries emit a complicated
gravitational wave signature, produced by multiple harmonics sourced by their
strong local gravitational field and further modulated by the orbital plane's
precession. Some features of this complex signal are easily accessible to
ground-based interferometers (e.g., the rate of change of frequency); others
less so (e.g., the polarization content); and others unavailable (e.g.,
features of the signal out of band). For this reason, an ambiguity function (a
diagnostic of dissimilarity) between two such signals varies on many parameter
scales and ranges. In this paper, we present a method for computing an
approximate, effective Fisher matrix from variations in the ambiguity function
on physically pertinent scales which depend on the relevant signal to noise
ratio. As a concrete example, we explore how higher harmonics improve parameter
measurement accuracy. As previous studies suggest, for our fiducial BH-NS
binaries and for plausible signal amplitudes, we see that higher harmonics at
best marginally improve our ability to measure parameters. For non-precessing
binaries, these Fisher matrices separate into intrinsic (mass, spin) and
extrinsic (geometrical) parameters; higher harmonics principally improve our
knowledge about the line of sight. For the precessing binaries, the extra
information provided by higher harmonics is distributed across several
parameters. We provide concrete estimates for measurement accuracy, using
coordinates adapted to the precession cone in the detector's sensitive band.Comment: 19 pages, 11 figure
The KaVA and KVN Pulsar Project
We present our work towards using the Korean VLBI (Very Long Baseline
Interferometer) Network (KVN) and VLBI Exploration of Radio Astronomy (VERA)
arrays combined into the KVN and VERA Array (KaVA) for observations of radio
pulsars at high frequencies (22-GHz). Pulsar astronomy is generally
focused at frequencies approximately 0.3 to several GHz and pulsars are usually
discovered and monitored with large, single-dish, radio telescopes. For most
pulsars, reduced radio flux is expected at high frequencies due to their steep
spectrum, but there are exceptions where high frequency observations can be
useful. Moreover, some pulsars are observable at high frequencies only, such as
those close to the Galactic Center. The discoveries of a radio-bright magnetar
and a few dozen extended Chandra sources within 15 arc-minute of the Galactic
Center provide strong motivations to make use of the KaVA frequency band for
searching pulsars in this region. Here, we describe the science targets and
report progresses made from the KVN test observations for known pulsars. We
then discuss why KaVA pulsar observations are compelling.Comment: To appear in PASJ KaVA Special Issu
The Probability Distribution of Binary Pulsar Coalescence Rate Estimates. II. Neutron Star-White Dwarf Binaries
We consider the statistics of pulsar binaries with white dwarf companions
(NS-WD). Using the statistical analysis method developed by Kim et al. (2003)
we calculate the Galactic coalescence rate of NS-WD binaries due to
gravitational-wave emission. We find that the most likely values for the total
Galactic coalescence rate (R_tot) of NS-WD binaries lie in the range 0.2--10
per Myr depending on different assumed pulsar population models. For our
reference model, we obtain R_tot=4.11_(-2.56)^(+5.25) per Myr at a 68%
statistical confidence level. These rate estimates are not corrected for pulsar
beaming and as such they are found to be about a factor of 20 smaller than the
Galactic coalescence rate estimates for double neutron star systems. Based on
our rate estimates, we calculate the gravitational-wave background due to
coalescing NS-WD binaries out to extragalactic distances within the frequency
band of the Laser Interferometer Space Antenna. We find the contribution from
NS-WD binaries to the gravitational-wave background to be negligible.Comment: 20 pages, 2 figures, 2 tables, Accepted for publication in Ap
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