440 research outputs found
Search strategies for long gravitational-wave transients: hidden Markov model tracking and seedless clustering
A number of detections have been made in the past few years of gravitational
waves from compact binary coalescences. While there exist well-understood
waveform models for signals from compact binary coalescences, many sources of
gravitational waves are not well modeled, including potential long-transient
signals from a binary neutron star post-merger remnant. Searching for these
sources requires robust detection algorithms that make minimal assumptions
about any potential signals. In this paper, we compare two unmodeled search
schemes for long-transient gravitational waves, operating on cross-power
spectrograms. One is an efficient algorithm first implemented for continuous
wave searches, based on a hidden Markov model. The other is a seedless
clustering method, which has been used in transient gravitational wave analysis
in the past. We quantify the performance of both algorithms, including
sensitivity and computational cost, by simulating synthetic signals with a
special focus on sources like binary neutron star post-merger remnants. We
demonstrate that the hidden Markov model tracking is a good option in
model-agnostic searches for low signal-to-noise ratio signals. We also show
that it can outperform the seedless method for certain categories of signals
while also being computationally more efficient.Comment: 10 pages, 7 figure
Can a black hole–neutron star merger explain GW170817, AT2017gfo, and GRB170817A?
The discovery of the compact binary coalescence in both gravitational waves and electromagnetic radiation marks a breakthrough in the field of multimessenger astronomy and has improved our knowledge in a number of research areas. However, an open question is the exact origin of the observables and if one can confirm reliably that GW170817 and its electromagnetic counterparts resulted from a binary neutron star merger. To answer the question if the observation of GW170817, GRB170817A, and AT2017gfo could be explained by the merger of a neutron star with a black hole, we perform a joint multimessenger analysis of the gravitational waves, the short gamma-ray burst, and the kilonova. Assuming a black hole–neutron star system, we derive multimessenger constraints for the tidal deformability of the neutron star of Λ>425 and for the mass ratio of q<2.03 at 90% confidence, with peaks in the likelihood near Λ=830 and q=1.0. Overall, we find that a black hole–neutron star merger could explain the observed signatures; however, our analysis shows that a binary neutron star origin of GW170817 seems more plausible
Exploring a search for long-duration transient gravitational waves associated with magnetar bursts
Soft gamma repeaters and anomalous X-ray pulsars are thought to be magnetars,
neutron stars with strong magnetic fields of order --. These objects emit intermittent bursts
of hard X-rays and soft gamma rays. Quasiperiodic oscillations in the X-ray
tails of giant flares imply the existence of neutron star oscillation modes
which could emit gravitational waves powered by the magnetar's magnetic energy
reservoir. We describe a method to search for transient gravitational-wave
signals associated with magnetar bursts with durations of 10s to 1000s of
seconds. The sensitivity of this method is estimated by adding simulated
waveforms to data from the sixth science run of Laser Interferometer
Gravitational-wave Observatory (LIGO). We find a search sensitivity in terms of
the root sum square strain amplitude of for a half sine-Gaussian waveform with a central
frequency and a characteristic time . This corresponds to a gravitational wave energy of
, the same order of
magnitude as the 2004 giant flare which had an estimated electromagnetic energy
of , where is the distance to SGR 1806-20. We
present an extrapolation of these results to Advanced LIGO, estimating a
sensitivity to a gravitational wave energy of for a magnetar at a distance of .
These results suggest this search method can probe significantly below the
energy budgets for magnetar burst emission mechanisms such as crust cracking
and hydrodynamic deformation
Testing of the LSST's photometric calibration strategy at the CTIO 0.9 meter telescope
The calibration hardware system of the Large Synoptic Survey Telescope (LSST)
is designed to measure two quantities: a telescope's instrumental response and
atmospheric transmission, both as a function of wavelength. First of all, a
"collimated beam projector" is designed to measure the instrumental response
function by projecting monochromatic light through a mask and a collimating
optic onto the telescope. During the measurement, the light level is monitored
with a NIST-traceable photodiode. This method does not suffer from stray light
effects or the reflections (known as ghosting) present when using a flat-field
screen illumination, which has a systematic source of uncertainty from
uncontrolled reflections. It allows for an independent measurement of the
throughput of the telescope's optical train as well as each filter's
transmission as a function of position on the primary mirror. Second, CALSPEC
stars can be used as calibrated light sources to illuminate the atmosphere and
measure its transmission. To measure the atmosphere's transfer function, we use
the telescope's imager with a Ronchi grating in place of a filter to configure
it as a low resolution slitless spectrograph. In this paper, we describe this
calibration strategy, focusing on results from a prototype system at the Cerro
Tololo Inter-American Observatory (CTIO) 0.9 meter telescope. We compare the
instrumental throughput measurements to nominal values measured using a
laboratory spectrophotometer, and we describe measurements of the atmosphere
made via CALSPEC standard stars during the same run
Observational implications of lowering the LIGO-Virgo alert threshold
The recent detection of the binary-neutron-star merger associated with GW170817 by both the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo and the network of electromagnetic-spectrum observing facilities around the world has made the multi-messenger detection of gravitational-wave (GW) events a reality. These joint detections allow us to probe GW sources in greater detail and provide us with the possibility of confidently establishing events that would not have been detected in GW data alone. In this Letter, we explore the prospects of using the electromagnetic (EM) follow-up of low-significance GW event candidates to increase the sample of confident detections with EM counterparts. We find that the GW-alert threshold change that would roughly double the number of detectable astrophysical events would increase the false-alarm rate (FAR) by more than five orders of magnitude from 1 per 100 years to more than 1000 per year. We find that the localization costs of following up low-significance candidates are marginal, as the same changes to FAR only increase distance/area localizations by less than a factor of 2 and increase volume localization by less than a factor of 4. We argue that EM follow-up thresholds for low-significance candidates should be set on the basis of alert purity (P_(astro)) and not FAR. Ideally, such estimates of P_(astro) would be provided by LIGO-Virgo, but in their absence we provide estimates of the average purity of the GW candidate alerts issued by LIGO-Virgo as a function of FAR for various LIGO-Virgo observing epochs
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