97 research outputs found

### On Discovering Electromagnetic Emission from Neutron Star Mergers: The Early Years of Two Gravitational Wave Detectors

We present the first simulation addressing the prospects of finding an
electromagnetic (EM) counterpart to gravitational wave detections (GW) during
the early years of only two advanced interferometers. The perils of such a
search may have appeared insurmountable when considering the coarse ring-shaped
GW localizations spanning thousands of deg^2 using time-of-arrival information
alone. We show that leveraging the amplitude and phase information of the
predicted GW signal narrows the localization to arcs with a median area of only
~250 deg^2, thereby making an EM search tractable. Based on the locations and
orientations of the two LIGO detectors, we find that the GW sensitivity is
limited to one polarization and thus to only two sky quadrants. Thus, the rates
of GW events with two interferometers is only ~40% of the rate with three
interferometers of similar sensitivity. Another important implication of the
sky quadrant bias is that EM observatories in North America and Southern Africa
would be able to systematically respond to GW triggers several hours sooner
than Russia and Chile. Given the larger sky areas and the relative proximity of
detected mergers, 1m-class telescopes with very wide-field cameras are well
positioned for the challenge of finding an EM counterpart. Identification of
the EM counterpart amidst the even larger numbers of false positives further
underscores the importance of building a comprehensive catalog of foreground
stellar sources, background AGN and potential host galaxies in the local
universe.Comment: Submitted to ApJL, 8 pages, 4 figures, 1 tabl

### Gravitational Waves and Time Domain Astronomy

The gravitational wave window onto the universe will open in roughly five
years, when Advanced LIGO and Virgo achieve the first detections of high
frequency gravitational waves, most likely coming from compact binary mergers.
Electromagnetic follow-up of these triggers, using radio, optical, and high
energy telescopes, promises exciting opportunities in multi-messenger time
domain astronomy. In the next decade, space-based observations of low frequency
gravitational waves from massive black hole mergers, and their electromagnetic
counterparts, will open up further vistas for discovery. This two-part workshop
at featured brief presentations and stimulating discussions on the challenges
and opportunities presented by gravitational wave astronomy. Highlights from
the workshop, with the emphasis on strategies for electromagnetic follow-up,
are presented in this report.Comment: Submitted to Proc. IAU 285, "New Horizons in Transient Astronomy",
Oxford, Sept. 201

### Remnant baryon mass outside of the black hole after a neutron star-black hole merger

Gravitational-wave (GW) and electromagnetic (EM) signals from the merger of a
Neutron Star (NS) and a Black Hole (BH) are a highly anticipated discovery in
extreme gravity, nuclear-, and astrophysics. We develop a simple formula that
distinguishes between merger outcomes and predicts the post-merger remnant
mass, validated with 75 simulations. Our formula improves on existing results
by describing critical unexplored regimes: comparable masses and higher BH
spins. These are important to differentiate NSNS from NSBH mergers, and to
infer source physics from EM signals.Comment: 9 pages, 5 figures, 2 table

### Suitability of post-Newtonian/numerical-relativity hybrid waveforms for gravitational wave detectors

This article presents a study of the sufficient accuracy of post-Newtonian
and numerical relativity waveforms for the most demanding usage case: parameter
estimation of strong sources in advanced gravitational wave detectors. For
black hole binaries, these detectors require accurate waveform models which can
be constructed by fusing an analytical post-Newtonian inspiral waveform with a
numerical relativity merger-ringdown waveform. We perform a comprehensive
analysis of errors that enter such "hybrid waveforms". We find that the
post-Newtonian waveform must be aligned with the numerical relativity waveform
to exquisite accuracy, about 1/100 of a gravitational wave cycle. Phase errors
in the inspiral phase of the numerical relativity simulation must be controlled
to less than about 0.1rad. (These numbers apply to moderately optimistic
estimates about the number of GW sources; exceptionally strong signals require
even smaller errors.) The dominant source of error arises from the inaccuracy
of the investigated post-Newtonian Taylor-approximants. Using our error
criterium, even at 3.5-th post-Newtonian order, hybridization has to be
performed significantly before the start of the longest currently available
numerical waveforms which cover 30 gravitational wave cycles. The current
investigation is limited to the equal-mass, zero-spin case and does not take
into account calibration errors of the gravitational wave detectors.Comment: 32 pages, 12 figures, submitted to CQG for the NRDA2010 conference
proceedings, added new figure (fig. 5) since last versio

### Binary black hole merger: symmetry and the spin expansion

We regard binary black hole (BBH) merger as a map from a simple initial state
(two Kerr black holes, with dimensionless spins {\bf a} and {\bf b}) to a
simple final state (a Kerr black hole with mass m, dimensionless spin {\bf s},
and kick velocity {\bf k}). By expanding this map around {\bf a} = {\bf b} = 0
and applying symmetry constraints, we obtain a simple formalism that is
remarkably successful at explaining existing BBH simulations. It also makes
detailed predictions and suggests a more efficient way of mapping the parameter
space of binary black hole merger. Since we rely on symmetry rather than
dynamics, our expansion complements previous analytical techniques.Comment: 4 pages, 4 figures, matches Phys. Rev. Lett. versio

### Identifying Elusive Electromagnetic Counterparts to Gravitational Wave Mergers: An End-to-end Simulation

Combined gravitational wave (GW) and electromagnetic (EM) observations of compact binary mergers should enable detailed studies of astrophysical processes in the strong-field gravity regime. This decade, ground-based GW interferometers promise to routinely detect compact binary mergers. Unfortunately, networks of GW interferometers have poor angular resolution on the sky and their EM signatures are predicted to be faint. Therefore, a challenging goal will be to unambiguously pinpoint the EM counterparts of GW mergers. We perform the first comprehensive end-to-end simulation that focuses on: (1) GW sky localization, distance measures, and volume errors with two compact binary populations and four different GW networks; (2) subsequent EM detectability by a slew of multiwavelength telescopes; and (3) final identification of the merger counterpart amidst a sea of possible astrophysical false positives. First, we find that double neutron star binary mergers can be detected out to a maximum distance of 400 Mpc (or 750 Mpc) by three (or five) detector GW networks, respectively. Neutron-star-black-hole binary mergers can be detected a factor of 1.5 further out; their median to maximum sky localizations are 50-170 deg^2 (or 6-65 deg^2) for a three (or five) detector GW network. Second, by optimizing depth, cadence, and sky area, we quantify relative fractions of optical counterparts that are detectable by a suite of different aperture-size telescopes across the globe. Third, we present five case studies to illustrate the diversity of scenarios in secure identification of the EM counterpart. We discuss the case of a typical binary, neither beamed nor nearby, and the challenges associated with identifying an EM counterpart at both low and high Galactic latitudes. For the first time, we demonstrate how construction of low-latency GW volumes in conjunction with local universe galaxy catalogs can help solve the problem of false positives. We conclude with strategies that would best prepare us for successfully identifying the elusive EM counterpart of a GW merger

### What to do when things get crowded? Scalable joint analysis of overlapping gravitational wave signals

The gravitational wave sky is starting to become very crowded, with the
fourth science run (O4) at LIGO expected to detect $\mathcal{O}(100)$ compact
object coalescence signals. Data analysis issues start to arise as we look
further forwards, however. In particular, as the event rate increases in e.g.
next generation detectors, it will become increasingly likely that signals
arrive in the detector coincidentally, eventually becoming the dominant source
class. It is known that current analysis pipelines will struggle to deal with
this scenario, predominantly due to the scaling of traditional methods such as
Monte Carlo Markov Chains and nested sampling, where the time difference
between analysing a single signal and multiple can be as significant as days to
months. In this work, we argue that sequential simulation-based inference
methods can solve this problem by breaking the scaling behaviour. Specifically,
we apply an algorithm known as (truncated marginal) neural ratio estimation
(TMNRE), implemented in the code peregrine and based on swyft. To demonstrate
its applicability, we consider three case studies comprising two overlapping,
spinning, and precessing binary black hole systems with merger times separated
by 0.05 s, 0.2 s, and 0.5 s. We show for the first time that we can recover,
with full precision (as quantified by a comparison to the analysis of each
signal independently), the posterior distributions of all 30 model parameters
in a full joint analysis. Crucially, we achieve this with only $\sim 15\%$ of
the waveform evaluations that would be needed to analyse even a single signal
with traditional methods.Comment: 6 pages. 3 figures. Codes: peregrine is publicly available at
https://github.com/PEREGRINE-GW/peregrine/tree/overlapping, swyft is
available at https://github.com/undark-lab/swyf

### Radio Counterparts of Compact Binary Mergers detectable in Gravitational Waves: A Simulation for an Optimized Survey

Mergers of binary neutron stars and black hole-neutron star binaries produce
gravitational-wave (GW) emission and outflows with significant kinetic
energies. These outflows result in radio emissions through synchrotron
radiation. We explore the detectability of these synchrotron generated radio
signals by follow-up observations of GW merger events lacking a detection of
electromagnetic counterparts in other wavelengths. We model radio light curves
arising from (i) sub-relativistic merger ejecta and (ii) ultra-relativistic
jets. The former produces radio remnants on timescales of a few years and the
latter produces $\gamma$-ray bursts in the direction of the jet and
orphan-radio afterglows extending over wider angles on timescales of weeks.
Based on the derived light curves, we suggest an optimized survey at $1.4$ GHz
with five epochs separated by a logarithmic time interval. We estimate the
detectability of the radio counterparts of simulated GW-merger events to be
detected by advanced LIGO and Virgo by current and future radio facilities. The
detectable distances for these GW merger events could be as high as 1 Gpc.
$20$--$60\%$ of the long-lasting radio remnants will be detectable in the case
of the moderate kinetic energy of $3\cdot 10^{50}$ erg and a circum-merger
density of $0.1 {\rm cm^{-3}}$ or larger, while $5$--$20\%$ of the orphan radio
afterglows with kinetic energy of $10^{48}$ erg will be detectable. The
detection likelihood increases if one focuses on the well-localizable GW
events. We discuss the background noise due to radio fluxes of host galaxies
and false positives arising from extragalactic radio transients and variable
Active Galactic Nuclei and we show that the quiet radio transient sky is of
great advantage when searching for the radio counterparts.Comment: 23 pages, 10 figures, accepted for publication in Ap

### A study of the agreement between binary neutron star ejecta models derived from numerical relativity simulations

Neutron star mergers have recently become a tool to study extreme gravity,
nucleosynthesis, and the chemical composition of the Universe. To date, there
has been one joint gravitational and electromagnetic observation of a binary
neutron star merger, GW170817, as well as a solely gravitational observation,
GW190425. In order to accurately identify and interpret electromagnetic signals
of neutron star mergers, better models of the matter outflows generated by
these mergers are required. We compare a series of ejecta models to see where
they provide strong constraints on the amount of ejected mass expected from a
system, and where systematic uncertainties in current models prevent us from
reliably extracting information from observed events. We also examine 2396
neutron star equations of state compatible with GW170817 to see whether a given
ejecta mass could be reasonably produced with a neutron star of said equation
of state, and whether different ejecta models provide consistent predictions.
We find that the difference between models is often comparable to or larger
than the error generally assumed for these models, implying better constraints
on the models are needed. We also note that the extrapolation of outflow models
outside of their calibration window, while commonly needed to analyze
gravitational wave events, is extremely unreliable and occasionally leads to
completely unphysical results.Comment: 12 pages, 5 figure

### Gravitational-wave emission from compact Galactic binaries

Compact Galactic binaries where at least one member is a white dwarf or
neutron star constitute the majority of individually detectable sources for
future low-frequency space-based gravitational-wave (GW) observatories; they
also form an unresolved continuum, the dominant Galactic foreground at
frequencies below a few mHz. Due to the paucity of electromagnetic
observations, the majority of studies of Galactic-binary populations so far
have been based on population-synthesis simulations. However, recent surveys
have reported several new detections of white-dwarf binaries, providing new
constraints for population estimates. In this article, we evaluate the impact
of revised local densities of interacting white-dwarf binaries on future GW
observations. Specifically: we consider five scenarios that explain these
densities with different assumptions on the formation of interacting systems;
we simulate corresponding populations of detached and interacting white-dwarf
binaries; we estimate the number of individually detectable GW sources and the
magnitude of the confusion-noise foreground, as observed by space-based
detectors with 5- and 1-Mkm arms. We confirm earlier estimates of thousands of
detached-binary detections, but project only few ten to few hundred detections
of interacting systems. This reduction is partly due to our assessment of
detection prospects, based on the iterative identification and subtraction of
bright sources with respect to both instrument and confusion noise. We also
confirm earlier estimates for the confusion-noise foreground, except in one
scenario that explains smaller local densities of interacting systems with
smaller numbers of progenitor detached systems.Comment: 17 pages, 3 figures, 5 tables, version matches the published
Astrophysical Journal pape

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