393 research outputs found
Revealing the high-density equation of state through binary neutron star mergers
We present a novel method for revealing the equation of state of high-density
neutron star matter through gravitational waves emitted during the postmerger
phase of a binary neutron star system. The method relies on a small number of
detections of the peak frequency in the postmerger phase for binaries of
different (relatively low) masses, in the most likely range of expected
detections. From such observations, one can construct the derivative of the
peak frequency versus the binary mass, in this mass range. Through a detailed
study of binary neutron star mergers for a large sample of equations of state,
we show that one can extrapolate the above information to the highest possible
mass (the threshold mass for black hole formation in a binary neutron star
merger). In turn, this allows for an empirical determination of the maximum
mass of cold, nonrotating neutron stars to within 0.1 M_sun, while the
corresponding radius is determined to within a few percent. Combining this with
the determination of the radius of cold, nonrotating neutron stars of 1.6 M_sun
(to within a few percent, as was demonstrated in Bauswein et al., PRD, 86,
063001, 2012), allows for a clear distinction of a particular candidate
equation of state among a large set of other candidates. Our method is
particularly appealing because it reveals simultaneously the moderate and very
high-density parts of the equation of state, enabling the distinction of
mass-radius relations even if they are similar at typical neutron star masses.
Furthermore, our method also allows to deduce the maximum central energy
density and maximum central rest-mass density of cold, nonrotating neutron
stars with an accuracy of a few per cent.Comment: 14 pages, 12 figures, 2 tables, accepted for publication in Phys.
Rev.
Exploring properties of high-density matter through remnants of neutron-star mergers
Remnants of neutron-star mergers are essentially massive, hot, differentially
rotating neutron stars, which are initially strongly oscillating. They
represent a unique probe for high-density matter because the oscillations are
detectable via gravitational-wave measurements and are strongly dependent on
the equation of state. The impact of the equation of state is apparent in the
frequency of the dominant oscillation mode of the remnant. For a fixed total
binary mass a tight relation between the dominant postmerger frequency and the
radii of nonrotating neutron stars exists. Inferring observationally the
dominant postmerger frequency thus determines neutron star radii with high
accuracy of the order of a few hundred meters. By considering symmetric and
asymmetric binaries of the same chirp mass, we show that the knowledge of the
binary mass ratio is not critical for this kind of radius measurements. We
summarize different possibilities to deduce the maximum mass of nonrotating
neutron stars. We clarify the nature of the three most prominent features of
the postmerger gravitational-wave spectrum and argue that the merger remnant
can be considered to be a single, isolated, self-gravitating object that can be
described by concepts of asteroseismology. The understanding of the different
mechanisms shaping the gravitational-wave signal yields a physically motivated
analytic model of the gravitational-wave emission, which may form the basis for
template-based gravitational-wave data analysis. We explore the observational
consequences of a scenario of two families of compact stars including hadronic
and quark stars. We find that this scenario leaves a distinctive imprint on the
postmerger gravitational-wave signal. In particular, a strong discontinuity in
the dominant postmerger frequency as function of the total mass will be a
strong indication for two families of compact stars. (abridged)Comment: 22 pages, 17 figures; accepted for publication in EPJ
Prompt merger collapse and the maximum mass of neutron stars
We perform hydrodynamical simulations of neutron-star mergers for a large
sample of temperature-dependent, nuclear equations of state, and determine the
threshold mass above which the merger remnant promptly collapses to form a
black hole. We find that, depending on the equation of state, the threshold
mass is larger than the maximum mass of a non-rotating star in isolation by
between 30 and 70 per cent. Our simulations also show that the ratio between
the threshold mass and maximum mass is tightly correlated with the compactness
of the non-rotating maximum-mass configuration. We speculate on how this
relation can be used to derive constraints on neutron-star properties from
future observations.Comment: 6 pages, 3 figures, accepted for publication in Phys. Rev. Let
Measuring neutron-star properties via gravitational waves from binary mergers
We demonstrate by a large set of merger simulations for symmetric binary
neutron stars (NSs) that there is a tight correlation between the frequency
peak of the postmerger gravitational-wave (GW) emission and the physical
properties of the nuclear equation of state (EoS), e.g. expressed by the radius
of the maximum-mass Tolman-Oppenheimer-Volkhoff configuration. Therefore, a
single measurement of the peak frequency of the postmerger GW signal will
constrain the NS EoS significantly. For plausible optimistic merger-rate
estimates a corresponding detection with Advanced LIGO is likely to happen
within an operation time of roughly a year.Comment: 5 pages, 4 figures, accepted by Phys. Rev. Lett., revised version
including referee comment
Nucleosynthesis constraints on the neutron star-black hole merger rate
We derive constraints on the time-averaged event rate of neutron star-black
hole (NS-BH) mergers by using estimates of the population-integrated production
of heavy rapid neutron-capture (r-process) elements with nuclear mass numbers A
> 140 by such events in comparison to the Galactic repository of these chemical
species. Our estimates are based on relativistic hydrodynamical simulations
convolved with theoretical predictions of the binary population. This allows us
to determine a strict upper limit of the average NS-BH merger rate of ~6*10^-5
per year. We quantify the uncertainties of this estimate to be within factors
of a few mostly because of the unknown BH spin distribution of such systems,
the uncertain equation of state of NS matter, and possible errors in the
Galactic content of r-process material. Our approach implies a correlation
between the merger rates of NS-BH binaries and of double NS systems.
Predictions of the detection rate of gravitational-wave signals from such
compact-object binaries by Advanced LIGO and Advanced Virgo on the optimistic
side are incompatible with the constraints set by our analysis.Comment: 5 pages, 3 figures; accepted for publication in ApJ
Neutron-star radius constraints from GW170817 and future detections
We introduce a new, powerful method to constrain properties of neutron stars
(NSs). We show that the total mass of GW170817 provides a reliable constraint
on the stellar radius if the merger did not result in a prompt collapse as
suggested by the interpretation of associated electromagnetic emission. The
radius R_1.6 of nonrotating NSs with a mass of 1.6 M_sun can be constrained to
be larger than 10.68_{-0.04}^{+0.15} km, and the radius R_max of the
nonrotating maximum mass configuration must be larger than 9.60_{-0.03}^{+0.14}
km. We point out that detections of future events will further improve these
constraints. Moreover, we show that a future event with a signature of a prompt
collapse of the merger remnant will establish even stronger constraints on the
NS radius from above and the maximum mass M_max of NSs from above. These
constraints are particularly robust because they only require a measurement of
the chirp mass and a distinction between prompt and delayed collapse of the
merger remnant, which may be inferred from the electromagnetic signal or even
from the presence/absence of a ringdown gravitational-wave (GW) signal. This
prospect strengthens the case of our novel method of constraining NS
properties, which is directly applicable to future GW events with accompanying
electromagnetic counterpart observations. We emphasize that this procedure is a
new way of constraining NS radii from GW detections independent of existing
efforts to infer radius information from the late inspiral phase or postmerger
oscillations, and it does not require particularly loud GW events.Comment: 7 pages, 5 figures, accepted for publication in ApJ
Neutron-powered precursors of kilonovae
The merger of binary neutron stars (NSs) ejects a small quantity of neutron
rich matter, the radioactive decay of which powers a day to week long thermal
transient known as a kilonova. Most of the ejecta remains sufficiently dense
during its expansion that all neutrons are captured into nuclei during the
r-process. However, recent general relativistic merger simulations by Bauswein
and collaborators show that a small fraction of the ejected mass (a few per
cent, or ~1e-4 Msun) expands sufficiently rapidly for most neutrons to avoid
capture. This matter originates from the shocked-heated interface between the
merging NSs. Here we show that the beta-decay of these free neutrons in the
outermost ejecta powers a `precursor' to the main kilonova emission, which
peaks on a timescale of a few hours following merger at U-band magnitude ~22
(for an assumed distance of 200 Mpc). The high luminosity and blue colors of
the neutron precursor render it a potentially important counterpart to the
gravitational wave source, that may encode valuable information on the
properties of the merging binary (e.g. NS-NS versus NS-black hole) and the NS
equation of state. Future work is necessary to assess the robustness of the
fast moving ejecta and the survival of free neutrons in the face of neutrino
absorptions, although the precursor properties are robust to a moderate amount
of leptonization. Our results provide additional motivation for short latency
gravitational wave triggers and rapid follow-up searches with sensitive ground
based telescopes.Comment: 6 pages, 5 figures, accepted to MNRAS main journa
Neutron-star merger ejecta as obstacles to neutrino-powered jets of gamma-ray bursts
We present the first special relativistic, axisymmetric hydrodynamic
simulations of black hole-torus systems (approximating general relativistic
gravity) as remnants of binary-neutron star (NS-NS) and neutron star-black hole
(NS-BH) mergers, in which the viscously driven evolution of the accretion torus
is followed with self-consistent energy-dependent neutrino transport and the
interaction with the cloud of dynamical ejecta expelled during the NS-NS
merging is taken into account. The modeled torus masses, BH masses and spins,
and the ejecta masses, velocities, and spatial distributions are adopted from
relativistic merger simulations. We find that energy deposition by neutrino
annihilation can accelerate outflows with initially high Lorentz factors along
polar low-density funnels, but only in mergers with extremely low baryon
pollution in the polar regions. NS-BH mergers, where polar mass ejection during
the merging phase is absent, provide sufficiently baryon-poor environments to
enable neutrino-powered, ultrarelativistic jets with terminal Lorentz factors
above 100 and considerable dynamical collimation, favoring short gamma-ray
bursts (sGRBs), although their typical energies and durations might be too
small to explain the majority of events. In the case of NS-NS mergers, however,
neutrino emission of the accreting and viscously spreading torus is too short
and too weak to yield enough energy for the outflows to break out from the
surrounding ejecta shell as highly relativistic jets. We conclude that neutrino
annihilation alone cannot power sGRBs from NS-NS mergers.Comment: 7 pages, 4 figures, minor revisions compared to original version,
accepted for publication in ApJ Letter
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