204 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.
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
Testing Approximations of Thermal Effects in Neutron Star Merger Simulations
We perform three-dimensional relativistic hydrodynamical calculations of
neutron star mergers to assess the reliability of an approximate treatment of
thermal effects in such simulations by combining an ideal-gas component with
zero-temperature, micro-physical equations of state. To this end we compare the
results of simulations that make this approximation to the outcome of models
with a consistent treatment of thermal effects in the equation of state. In
particular we focus on the implications for observable consequences of merger
events like the gravitational-wave signal. It is found that the characteristic
gravitational-wave oscillation frequencies of the post-merger remnant differ by
about 50 to 250 Hz (corresponding to frequency shifts of 2 to 8 per cent)
depending on the equation of state and the choice of the characteristic index
of the ideal-gas component. In addition, the delay time to black hole collapse
of the merger remnant as well as the amount of matter remaining outside the
black hole after its formation are sensitive to the description of thermal
effects.Comment: 10 pages, 6 figures, 9 eps files; revised with minor additions due to
referee comments; accepted by Phys.Rev.
Extensive study of nuclear uncertainties and their impact on the r-process nucleosynthesis in neutron star mergers
Theoretically predicted yields of elements created by the rapid neutron
capture (r-) process carry potentially large uncertainties associated with
incomplete knowledge of nuclear properties as well as approximative
hydrodynamical modelling of the matter ejection processes. We present an
in-depth study of the nuclear uncertainties by systematically varying
theoretical nuclear input models that describe the experimentally unknown
neutron-rich nuclei. This includes two frameworks for calculating the radiative
neutron capture rates and six, four and four models for the nuclear masses,
-decay rates and fission properties, respectively. Our r-process nuclear
network calculations are based on detailed hydrodynamical simulations of
dynamically ejected material from NS-NS or NS-BH binary mergers plus the
secular ejecta from BH-torus systems. The impact of nuclear uncertainties on
the r-process abundance distribution and early radioactive heating rate is
found to be modest (within a factor for individual nuclei and
a factor 2 for the heating rate), however the impact on the late-time heating
rate is more significant and depends strongly on the contribution from fission.
We witness significantly larger sensitivity to the nuclear physics input if
only a single trajectory is used compared to considering ensembles of
200-300 trajectories, and the quantitative effects of the nuclear
uncertainties strongly depend on the adopted conditions for the individual
trajectory. We use the predicted Th/U ratio to estimate the cosmochronometric
age of six metal-poor stars to set a lower limit of the age of the Galaxy and
find the impact of the nuclear uncertainties to be up to 2 Gyr.Comment: 26 pages, 22 figures, submitted to MNRA
Equation-of-state dependence of the gravitational-wave signal from the ring-down phase of neutron-star mergers
Neutron-star (NS) merger simulations are conducted for 38 representative
microphysical descriptions of high-density matter in order to explore the
equation-of-state dependence of the postmerger ring-down phase. The formation
of a deformed, oscillating, differentially rotating very massive NS is the
typical outcome of the coalescence of two stars with 1.35 for most
candidate EoSs. The oscillations of this object imprint a pronounced peak in
the gravitational-wave (GW) spectra, which is used to characterize the emission
for a given model. The peak frequency of this postmerger GW signal correlates
very well with the radii of nonrotating NSs, and thus allows to constrain the
high-density EoS by a GW detection. In the case of 1.35-1.35
mergers the peak frequency scales particularly well with the radius of a NS
with 1.6 , where the maximum deviation from this correlation is only
60 meters for fully microphysical EoSs which are compatible with NS
observations. Combined with the uncertainty in the determination of the peak
frequency it appears likely that a GW detection can measure the radius of a 1.6
NS with an accuracy of about 100 to 200 meters. We also uncover
relations of the peak frequency with the radii of nonrotating NSs with 1.35
or 1.8 , with the radius or the central energy density
of the maximum-mass Tolman-Oppenheimer-Volkoff configuration, and with the
pressure or sound speed at a fiducial rest-mass density of about twice nuclear
saturation density. Furthermore, it is found that a determination of the
dominant postmerger GW frequency can provide an upper limit for the maximum
mass of nonrotating NSs. The prospects for a detection of the postmerger GW
signal and a determination of the dominant GW frequency are estimated to be in
the range of 0.015 to 1.2 events per year with the upcoming Advanced LIGO
detector.Comment: 29 pages, 28 figures, accepted for publication in Phys. Rev.
Self-consistent 3D radiative transfer for kilonovae: directional spectra from merger simulations
We present three-dimensional radiative transfer calculations for the ejecta
from a neutron star merger that include line-by-line opacities for tens of
millions of bound-bound transitions, composition from an r-process nuclear
network, and time-dependent thermalization of decay products from individual
and decay reactions. In contrast to expansion opacities and
other wavelength-binned treatments, a line-by-line treatment enables us include
fluorescence effects and associate spectral features with the emitting and
absorbing lines of individual elements. We find variations in the synthetic
observables with both the polar and azimuthal viewing angles. The spectra
exhibit blended features with strong interactions by Ce III, Sr II, Y II, and
Zr II that vary with time and viewing direction. We demonstrate the importance
of wavelength-calibration of atomic data using a model with calibrated Sr, Y,
and Zr data, and find major differences in the resulting spectra, including a
better agreement with AT2017gfo. The synthetic spectra for near-polar
inclination show a feature at around 8000 A, similar to AT2017gfo. However,
they evolve on a more rapid timescale, likely due to the low ejecta mass (0.005
M) as we take into account only the early ejecta. The comparatively
featureless spectra for equatorial observers gives a tentative prediction that
future observations of edge-on kilonovae will appear substantially different
from AT2017gfo. We also show that 1D models obtained by spherically averaging
the 3D ejecta lead to dramatically different direction-integrated luminosities
and spectra compared to full 3D calculations.Comment: 12 pages, 5 figures. Accepted by ApJ
Mass Ejection by Strange Star Mergers and Observational Implications
We determine the Galactic production rate of strangelets as a canonical input
to calculations of the measurable cosmic ray flux of strangelets by performing
simulations of strange star mergers and combining the results with recent
estimates of stellar binary populations. We find that the flux depends
sensitively on the bag constant of the MIT bag model of QCD and disappears for
high values of the bag constant and thus more compact strange stars. In the
latter case strange stars could coexist with ordinary neutron stars as they are
not converted by the capture of cosmic ray strangelets. An unambiguous
detection of an ordinary neutron star would then not rule out the strange
matter hypothesis.Comment: 5 pages, 2 eps figures; referee comments included, accepted by Phys.
Rev. Let
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