636 research outputs found
Prospects of Finding Detached Black Hole-Star Binaries with TESS
We discuss prospects of identifying and characterizing black hole (BH)
companions to normal stars on tight but detached orbits, using photometric data
from the Transiting Exoplanet Survey Satellite (TESS). We focus on the
following two periodic signals from the visible stellar component: (i)
in-eclipse brightening of the star due to gravitational microlensing by the BH
(self-lensing), and (ii) a combination of ellipsoidal variations due to tidal
distortion of the star and relativistic beaming due to its orbital motion
(phase-curve variation). We evaluate the detectability of each signal in the
light curves of stars in the TESS input catalog, based on a pre-launch noise
model of TESS photometry as well as the actual light curves of spotted stars
from the prime Kepler mission to gauge the potential impact of stellar activity
arising from the tidally spun-up stellar components. We estimate that the
self-lensing and phase-curve signals from BH companions, if exist, will be
detectable in the light curves of effectively and
low-mass stars, respectively, taking into account orbital
inclination dependence of the signals. These numbers could be large enough to
actually detect signals from BHs: simple population models predict some 10 and
100 detectable BHs among these "searchable" stars, although the latter may be
associated with a comparable number of false-positives due to stellar
variabilities and additional vetting with radial velocity measurements would be
essential. Thus the TESS data could serve as a resource to study nearby BHs
with stellar companions on shorter-period orbits than will potentially be
probed with Gaia.Comment: 16 pages, 7 figures, accepted for publication in Ap
Radiative Transfer Simulations for Neutron Star Merger Ejecta
The merger of binary neutron stars (NSs) is among the most promising
gravitational wave (GW) sources. Next-generation GW detectors are expected to
detect signals from the NS merger within 200 Mpc. Detection of electromagnetic
wave (EM) counterpart is crucial to understand the nature of GW sources. Among
possible EM emission from the NS merger, emission powered by radioactive
r-process nuclei is one of the best targets for follow-up observations.
However, prediction so far does not take into account detailed r-process
element abundances in the ejecta. We perform radiative transfer simulations for
the NS merger ejecta including all the r-process elements from Ga to U for the
first time. We show that the opacity in the NS merger ejecta is about kappa =
10 cm^2 g^{-1}, which is higher than that of Fe-rich Type Ia supernova ejecta
by a factor of ~ 100. As a result, the emission is fainter and longer than
previously expected. The spectra are almost featureless due to the high
expansion velocity and bound-bound transitions of many different r-process
elements. We demonstrate that the emission is brighter for a higher mass ratio
of two NSs and a softer equation of states adopted in the merger simulations.
Because of the red color of the emission, follow-up observations in red optical
and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc,
expected brightness of the emission is i = 22 - 25 AB mag, z = 21 - 23 AB mag,
and 21 - 24 AB mag in NIR JHK bands. Thus, observations with wide-field 4m- and
8m-class optical telescopes and wide-field NIR space telescopes are necessary.
We also argue that the emission powered by radioactive energy can be detected
in the afterglow of nearby short gamma-ray bursts.Comment: 18 pages, 16 figures, accepted for publication in ApJ, computed light
curves are available at
http://th.nao.ac.jp/MEMBER/tanaka/nsmerger_lightcurve.htm
Mass ejection from neutron star mergers: different components and expected radio signals
In addition to producing a strong gravitational signal, a short gamma-ray
burst (GRB), and a compact remnant, neutron star mergers eject significant
masses at significant kinetic energies. This mass ejection takes place via
dynamical mass ejection and a GRB jet but other processes have also been
suggested: a shock-breakout material, a cocoon resulting from the interaction
of the jet with other ejecta, and viscous and neutrino driven winds from the
central remnant or the accretion disk. The different components of the ejected
masses include up to a few percent of a solar mass, some of which is ejected at
relativistic velocities. The interaction of these ejecta with the surrounding
interstellar medium will produce a long lasting radio flare, in a similar way
to GRB afterglows or to radio supernovae. The relative strength of the
different signals depends strongly on the viewing angle. An observer along the
jet axis or close to it will detect a strong signal at a few dozen days from
the radio afterglow (or the orphan radio afterglow) produced by the highly
relativistic GRB jet. For a generic observer at larger viewing angles, the
dynamical ejecta, whose contribution peaks a year or so after the event, will
generally dominate. Depending on the observed frequency and the external
density, other components may also give rise to a significant contribution. We
also compare these estimates with the radio signature of the short GRB 130603B.
The radio flare from the dynamical ejecta might be detectable with the EVLA and
the LOFAR for the higher range of external densities .Comment: 12 pages, 5 figures, submitted to MNRA
Short-lived Pu Points to Compact Binary Mergers as Sites for Heavy r-process Nucleosynthesis
Measurements of the radioactive Pu abundances can break the
degeneracy between high-rate/low-yield and low-rate/high-yield scenarios for
the production of heavy -process elements. The first corresponds to
production by core collapse supernovae (cc-SNe) while the latter corresponds to
production by e.g. compact binary mergers. The estimated Pu abundance
in the current interstellar medium inferred from deep-sea measurements (Wallner
et al. 2015) is significantly lower than that corresponding Early Solar System
abundances (Turner et al 2007). We estimate the expected median value of the
Pu abundances and fluctuations around this value in both models. We
show that while the current and Early Solar System abundances are naturally
explained within the low-rate/high-yield (e.g. merger) scenario, they are
incompatible with the high-rate/low-yield (cc-SNe) model. The inferred event
rate remarkably agrees with compact binary merger rates estimated from Galactic
neutron star binaries and from short gamma-ray bursts. Furthermore, the ejected
mass of -process elements per event agrees with both theoretical and
observational macronova/kilonova estimates.Comment: 11 pages, 7 figure
Rapidly Rising Optical Transients from the Birth of Binary Neutron Stars
We study optical counterparts of a new-born pulsar in a double neutron star
system like PSR J0737-3039A/B. This system is believed to eject a small amount
of mass of at the second core-collapse supernova.
We argue that the initial spin of the new-born pulsar can be determined by the
orbital period at the time when the second supernova occurs. The spin angular
momentum of the progenitor is expected to be similar to that of the He-burning
core, which is tidally synchronized with the orbital motion, and then the
second remnant may be born as a millisecond pulsar. If the dipole magnetic
field strength of the nascent pulsar is comparable to that inferred from the
current spin-down rate of PSR J0737-3039B, the initial spin-down luminosity is
comparable to the luminosity of super-luminous supernovae. We consider thermal
emission arising from the supernova ejecta driven by the relativistic wind from
such a new-born pulsar. The resulting optical light curves have a rising time
days and peak luminosity erg/s. The optical emission
may last for a month to several months, due to the reprocessing of X-rays and
UV photons via photoelectric absorption. These features are broadly consistent
with those of the rapidly-rising optical transients. The high spin-down
luminosity and small ejecta mass are favorable for the progenitor of the
repeating fast radio burst, FRB 121102. We discuss a possible connection
between newborn double pulsars and fast radio bursts.Comment: 10 pages, 2 figures, accepted for publication in Ap
Exploring tidal effects of coalescing binary neutron stars in numerical relativity
We study gravitational waves emitted in the late inspiral stage of binary
neutron stars by analyzing the waveform obtained in numerical-relativity
simulations. For deriving the physical gravitational waveforms from the
numerical results, the resolution extrapolation plays an essential role for our
simulations. The extrapolated gravitational-wave phases are compared with those
calculated in the post-Newtonian (PN) and effective-one-body (EOB) formalisms
including corrections of tidal effects. We show that the extrapolated
gravitational-wave phases in numerical relativity agree well with those by the
PN and EOB calculations for most of the inspiral stage except for a
tidally-dominated, final inspiral stage, in which the PN and EOB results
underestimate the tidal effects. Nevertheless, the accumulated phase difference
between our extrapolated results and the results by the PN/EOB calculations is
at most 1--3 radian in the last 15 cycles.Comment: 17 pages, 12 figures, accepted for publication in Physical Review
Exploring tidal effects of coalescing binary neutron stars in numerical relativity II: Longterm simulations
We perform new longterm (15-16 orbits) simulations of coalescing binary
neutron stars in numerical relativity using an updated Einstein's equation
solver, employing low-eccentricity initial data, and modeling the neutron stars
by a piecewise polytropic equation of state. A convergence study shows that our
new results converge more rapidly than the third order and using the determined
convergence order, we construct an extrapolated waveform for which the
estimated total phase error should be less than 1 radian. We then compare the
extrapolated waveforms with those calculated by the latest effective-one-body
(EOB) formalism in which the so-called tidal deformability, higher
post-Newtonian corrections, and gravitational self-force effects are taken into
account. We show that for a binary of compact neutron stars with their radius
11.1 km, the waveform by the EOB formalism agrees quite well with the numerical
waveform so that the total phase error is smaller than 1 radian for the total
phase of radian up to the merger. By contrast, for a binary of less
compact neutron stars with their radius 13.6 km, the EOB and numerical
waveforms disagree with each other in the last few wave cycles, resulting in
the total phase error of radian.Comment: 12 pages, 4 figures, submitted to PR
A cocoon shock breakout as the origin of the -ray emission in GW170817
The short Gamma-Ray Burst, GRB170817A, that followed the binary neutron star
merger gravitational waves signal, GW170817, is not a usual sGRB. It is weaker
by three orders of magnitude than the weakest sGRB seen before and its spectra,
showing a hard early signal followed by a softer thermal spectrum, is unique.
We show, first, that the -rays must have emerged from at least mildly
relativistic outflow, implying that a relativistic jet was launched following
the merger. We then show that the observations are consistent with the
predictions of a mildly relativistic shock breakout: a minute -ray
energy as compared with the total energy and a rather smooth light curve with a
hard to soft evolution. We present here a novel analytic study and detailed
numerical 2D and 3D relativistic hydrodynamic and radiation simulations that
support the picture in which the observed -rays arose from a shock
breakout of a cocoon from the merger's ejecta (Kasliwal 2017). The cocoon can
be formed by either a choked jet which does not generate a sGRB (in any
direction) or by a successful jet which generates an undetected regular sGRB
along the system's axis pointing away from us. Remarkably, for the choked jet
model, the macronova signal produced by the ejecta (which is partially boosted
to high velocities by the cocoon's shock) and the radio that is produced by the
interaction of the shocked cocoon material with the surrounding matter, agree
with the observed UV/optical/IR emission and with current radio observations.
Finally, we discuss the possibility that the jet propagation within the ejecta
may photodissociate some of of the heavy elements and may affect the
composition of a fraction of ejecta and, in turn, the opacity and the early
macronova light
Measurability of the tidal deformability by gravitational waves from coalescing binary neutron stars
Combining new gravitational waveforms derived by long-term (14--16 orbits)
numerical-relativity simulations with waveforms by an effective-one-body (EOB)
formalism for coalescing binary neutron stars, we construct hybrid waveforms
and estimate the measurability for the dimensionless tidal deformability of the
neutron stars, , by advanced gravitational-wave detectors. We focus on
the equal-mass case with the total mass . We find that for an event
at a hypothetical effective distance of Mpc, the
distinguishable difference in the dimensionless tidal deformability will be
, 400, and 800 at 1-, 2-, and 3- levels,
respectively, for advanced LIGO. If the true equation of state is stiff and the
typical neutron-star radius is km, our analysis suggests that
the radius will be constrained within km at 2- level for an
event at Mpc. On the other hand, if the true equation of
state is soft and the typical neutron-star radius is km , it
will be difficult to narrow down the equation of state among many soft ones,
although it is still possible to discriminate the true one from stiff equations
of state with km. We also find that gravitational waves from
binary neutron stars will be distinguished from those from spinless binary
black holes at more than 2- level for an event at
Mpc. The validity of the EOB formalism, Taylor-T4, and Taylor-F2 approximants
as the inspiral waveform model is also examined.Comment: 18 pages, 9 figures, accepted for publication in PR
Binary Neutron Star Mergers: Dependence on the Nuclear Equation of State
We perform a numerical-relativity simulation for the merger of binary neutron
stars with 6 nuclear-theory-based equations of state (EOSs) described by
piecewise polytropes. Our purpose is to explore the dependence of the dynamical
behavior of the binary neutron star merger and resulting gravitational
waveforms on the EOS of the supernuclear-density matter. The numerical results
show that the merger process and the first outcome are classified into three
types; (i) a black hole is promptly formed, (ii) a short-lived hypermassive
neutron star (HMNS) is formed, (iii) a long-lived HMNS is formed. The type of
the merger depends strongly on the EOS and on the total mass of the binaries.
For the EOS with which the maximum mass is larger than 2Msun, the lifetime of
the HMNS is longer than 10 ms for a total mass m_0=2.7Msun. A recent radio
observation suggests that the maximum mass of spherical neutron stars is M_max
\geq 1.97\pm 0.04Msun in one \sigma level. This fact and our results support
the possible existence of a HMNS soon after the onset of the merger for a
typical binary neutron star with m_0=2.7Msun. We also show that the torus mass
surrounding the remnant black hole is correlated with the type of the merger
process; the torus mass could be large, \geq 0.1Msun, in the case that a
long-lived HMNS is formed. We also show that gravitational waves carry
information of the merger process, the remnant, and the torus mass surrounding
a black hole.Comment: 13 pages, 6 figures, to be published in PR
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