339 research outputs found
Using gravitational-wave standard sirens
Gravitational waves (GWs) from supermassive binary black hole (BBH) inspirals
are potentially powerful standard sirens (the GW analog to standard candles)
(Schutz 1986, 2002). Because these systems are well-modeled, the space-based GW
observatory LISA will be able to measure the luminosity distance (but not the
redshift) to some distant massive BBH systems with 1-10% accuracy. This
accuracy is largely limited by pointing error: GW sources generally are poorly
localized on the sky. Localizing the binary independently (e.g., through
association with an electromagnetic counterpart) greatly reduces this
positional error. An electromagnetic counterpart may also allow determination
of the event's redshift. In this case, BBH coalescence would constitute an
extremely precise (better than 1%) standard candle visible to high redshift. In
practice, gravitational lensing degrades this precision, though the candle
remains precise enough to provide useful information about the
distance-redshift relation. Even if very rare, these GW standard sirens would
complement, and increase confidence in, other standard candles.Comment: 10 pages, 8 figures. ApJ, in pres
Lensing and high-z supernova surveys
Gravitational lensing causes the distribution of observed brightnesses of
standard candles at a given redshift to be highly non-gaussian. The
distribution is strongly, and asymmetrically, peaked at a value less than the
expected value in a homogeneous Robertson-Walker universe. Therefore, given any
small sample of observations in an inhomogeneous universe, the most likely
observed luminosity is at flux values less than the Robertson-Walker value.
This paper explores the impact of this systematic error due to lensing upon
surveys predicated on measuring standard candle brightnesses. We re-analyze
recent results from the high-z supernova team (Riess et al. 1998), both when
most of the matter in the universe is in the form of compact objects
(represented by the empty-beam expression, corresponding to the maximal case of
lensing), and when the matter is continuously distributed in galaxies. We find
that the best-fit model remains unchanged (at Omega_m=0, Omega_Lambda=0.45),
but the confidence contours change size and shape, becoming larger (and thus
allowing a broader range of parameter space) and dropping towards higher values
of matter density, Omega_m (or correspondingly, lower values of the
cosmological constant, Omega_Lambda). These effects are slight when the matter
is continuously distributed. However, the effects become considerably more
important if most of the matter is in compact objects. For example, neglecting
lensing, the Omega_m=0.5, Omega_Lambda=0.5 model is more than 2 sigma away from
the best fit. In the empty-beam analysis, this cosmology is at 1 sigma.Comment: 11 pages, 3 ps figures. uses aaspp4.sty. accepted to ApJ Letters.
includes analysis of lensing due to matter continuously distributed in
galaxie
GRB beaming and gravitational-wave observations
Using the observed rate of short-duration gamma-ray bursts (GRBs) it is
possible to make predictions for the detectable rate of compact binary
coalescences in gravitational-wave detectors. These estimates rely crucially on
the growing consensus that short gamma-ray bursts are associated with the
merger of two neutron stars or a neutron star and a black hole, but otherwise
make no assumptions beyond the observed rate of short GRBs. In particular, our
results do not assume coincident gravitational wave and electromagnetic
observations. We show that the non-detection of mergers in the existing
LIGO/Virgo data constrains the progenitor masses and beaming angles of
gamma-ray bursts. For future detectors, we find that the first detection of a
NS-NS binary coalescence associated with the progenitors of short GRBs is
likely to happen within the first 16 months of observation, even in the case of
a modest network of observatories (e.g., only LIGO-Hanford and LIGO-Livingston)
operating at modest sensitivities (e.g., advanced LIGO design sensitivity, but
without signal recycling mirrors), and assuming a conservative distribution of
beaming angles (e.g. all GRBs beamed at \theta=30 deg). Less conservative
assumptions reduce the waiting time until first detection to weeks to months.
Alternatively, the compact binary coalescence model of short GRBs can be ruled
out if a binary is not seen within the first two years of operation of a
LIGO-Hanford, LIGO-Livingston, and Virgo network at advanced design
sensitivity. We also demonstrate that the rate of GRB triggered sources is less
than the rate of untriggered events if \theta<30 deg, independent of the noise
curve, network configuration, and observed GRB rate. Thus the first detection
in GWs of a binary GRB progenitor is unlikely to be associated with a GRB
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