95 research outputs found
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
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
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-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
Exploring thermal effects of the hadron-quark matter transition in neutron star mergers
We study the importance of the thermal behavior of the hadron-quark phase
transition in neutron star (NS) mergers. To this end, we devise a new scheme
approximating thermal effects to supplement any cold, barotropic hybrid
equation of state (EoS) model, i.e. two-phase EoS constructions with a hadronic
regime and a phase of deconfined quark matter. The consideration of
temperature-dependent phase boundaries turns out to be critical for a
quantitative description of quark matter effects in NS mergers, since the
coexistence phase can introduce a strong softening of the EoS at finite
temperature, which is even more significant than the change of the EoS by the
phase transition at T=0. We validate our approach by comparing to existing
fully temperature-dependent EoS models and find a very good quantitative
agreement of postmerger gravitational-wave (GW) features. Simulations with the
commonly-used thermal ideal-gas approach exhibit sizable differences compared
to full hybrid models implying that its use in NS merger simulations with quark
matter is problematic. Our new scheme provides the means to isolate thermal
effects of quark matter from the properties of the cold hybrid EoS and thus
allows an assessment of the thermal behavior alone. We show that different
shapes of the phase boundaries at finite temperature can have a large impact on
the postmerger dynamics and GW signal for the same cold hybrid model. This
finding demonstrates that postmerger GW emission contains important
complementary information compared to properties extracted from cold stars. We
also show by concrete examples that it is even possible for quark matter to
only occur and thus be detectable in finite-temperature systems like merger
remnants but not in cold NSs (abbreviated).Comment: 28 pages, 16 figures, revised version, published by Phys. Rev.
Impact of weak interactions of free nucleons on the r-process in dynamical ejecta from neutron-star mergers
We investigate beta-interactions of free nucleons and their impact on the
electron fraction (Y_e) and r-process nucleosynthesis in ejecta characteristic
of binary neutron star mergers (BNSMs). For that we employ trajectories from a
relativistic BNSM model to represent the density-temperature evolutions in our
parametric study. In the high-density environment, positron captures decrease
the neutron richness at the high temperatures predicted by the hydrodynamic
simulation. Circumventing the complexities of modelling three-dimensional
neutrino transport, (anti)neutrino captures are parameterized in terms of
prescribed neutrino luminosities and mean energies, guided by published results
and assumed as constant in time. Depending sensitively on the adopted
neutrino-antineutrino luminosity ratio, neutrino processes increase Y_e to
values between 0.25 and 0.40, still allowing for a successful r-process
compatible with the observed solar abundance distribution and a significant
fraction of the ejecta consisting of r-process nuclei. If the electron neutrino
luminosities and mean energies are relatively large compared to the
antineutrino properties, the mean Y_e might reach values >0.40 so that neutrino
captures seriously compromise the success of the r-process. In this case, the
r-abundances remain compatible with the solar distribution, but the total
amount of ejected r-material is reduced to a few percent, because the
production of iron-peak elements is favored. Proper neutrino physics, in
particular also neutrino absorption, have to be included in BNSM simulations
before final conclusions can be drawn concerning r-processing in this
environment and concerning observational consequences like kilonovae, whose
peak brightness and color temperature are sensitive to the
composition-dependent opacity of the ejecta.Comment: 12 pages, 9 figures; submitted to MNRA
Nucleosynthesis in dynamical and torus ejecta of compact binary mergers
We present a comprehensive study of r-process element nucleosynthesis in the
ejecta of compact binary mergers (CBMs) and their relic black-hole (BH)-torus
systems. The evolution of the BH-accretion tori is simulated for seconds with a
Newtonian hydrodynamics code including viscosity effects, pseudo-Newtonian
gravity for rotating BHs, and an energy-dependent two-moment closure scheme for
the transport of electron neutrinos and antineutrinos. The investigated cases
are guided by relativistic double neutron star (NS-NS) and NS-BH merger models,
producing ~3-6 Msun BHs with rotation parameters of A~0.8 and tori of 0.03-0.3
Msun. Our nucleosynthesis analysis includes the dynamical (prompt) ejecta
expelled during the CBM phase and the neutrino and viscously driven outflows of
the relic BH-torus systems. While typically ~20-25% of the initial
accretion-torus mass are lost by viscously driven outflows, neutrino-powered
winds contribute at most another ~1%, but neutrino heating enhances the viscous
ejecta significantly. Since BH-torus ejecta possess a wide distribution of
electron fractions (0.1-0.6) and entropies, they produce heavy elements from
A~80 up to the actinides, with relative contributions of A>130 nuclei being
subdominant and sensitively dependent on BH and torus masses and the exact
treatment of shear viscosity. The combined ejecta of CBM and BH-torus phases
can reproduce the solar abundances amazingly well for A>90. Varying
contributions of the torus ejecta might account for observed variations of
lighter elements with 40<Z<56 relative to heavier ones, and a considerable
reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly
asymmetric NS-BH mergers, might explain the composition of heavy-element
deficient stars.Comment: 7 pages, 4 figures, only changed title compared to previous version,
accepted for publication in Proceedings of Science (Nuclei in the Cosmos
XIII, Debrecen
Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers
We present the first comprehensive study of r-process element nucleosynthesis
in the ejecta of compact binary mergers (CBMs) and their relic black-hole
(BH)-torus systems. The evolution of the BH-accretion tori is simulated for
seconds with a Newtonian hydrodynamics code including viscosity effects,
pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment
closure scheme for the transport of electron neutrinos and antineutrinos. The
investigated cases are guided by relativistic double neutron star (NS-NS) and
NS-BH merger models, producing ~3-6 Msun BHs with rotation parameters of A~0.8
and tori of 0.03-0.3 Msun. Our nucleosynthesis analysis includes the dynamical
(prompt) ejecta expelled during the CBM phase and the neutrino and viscously
driven outflows of the relic BH-torus systems. While typically ~20-25% of the
initial accretion-torus mass are lost by viscously driven outflows,
neutrino-powered winds contribute at most another ~1%, but neutrino heating
enhances the viscous ejecta significantly. Since BH-torus ejecta possess a wide
distribution of electron fractions (0.1-0.6) and entropies, they produce heavy
elements from A~80 up to the actinides, with relative contributions of A>130
nuclei being subdominant and sensitively dependent on BH and torus masses and
the exact treatment of shear viscosity. The combined ejecta of CBM and BH-torus
phases can reproduce the solar abundances amazingly well for A>90. Varying
contributions of the torus ejecta might account for observed variations of
lighter elements with 40<Z<56 relative to heavier ones, and a considerable
reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly
asymmetric NS-BH mergers, might explain the composition of heavy-element
deficient stars.Comment: 30 pages, 22 figures; revised version, accepted by MNRAS; appendix
added with test results for neutrino transpor
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