947 research outputs found
Short Gamma-Ray Bursts and Binary Mergers in Spiral and Elliptical Galaxies: Redshift Distribution and Hosts
To test whether the short GRB rates, redshift distribution and host galaxies
are consistent with current theoretical predictions, we use avery large
database of population synthesis calculations to examine BH-NS and NS-NS merger
rates in the universe, factoring in (i) the star formation history of the
universe, (ii) a heterogeneous population of star-forming galaxies, including
spirals and ellipticals, and (iii) a simple flux-limited selection model for
short GRB detection. When we require our models reproduce the known short GRB
rates and redshift measurements (and, for NS-NS, the merger rates extrapolated
from binary pulsars in the Galaxy), a small fraction of models reproduce all
observations, both when we assume a NS-NS and a BH-NS origin for bursts. Most
commonly models produce mergers preferentially in spiral galaxies if short GRBs
arise from NS-NS mergers alone. Model universes where present-day binary
mergers occur preferentially in elliptical galaxies necessarily include a
significant fraction of binaries with long delay times between birth and merger
(often ). Though long delays occur, almost all of our models
predict that a higher proportion of short GRBs should occur at moderate to high
redshift (e.g., ) than has presently been observed, in agreement with
recent observations which suggest a selection bias towards successful follow-up
of low-redshift short GRBs. Finally, if only a fraction of BH-NS mergers have
the right combination of masses and spins to make GRBs, then at best only a
small fraction of BH-NS models could be consistent with all {\em current}
available data. (Abridged)Comment: 14 figures, using bitmapped fonts (via eps2eps) to fit in archive
space restrictions; better resolution figures are available from the author.
Accepted for publication in ApJ. v3 updates reference
Constraining population synthesis models via the binary neutron star population
The observed sample of double neutron-star (NS-NS) binaries presents a
challenge to population-synthesis models of compact object formation: the
parameters entering into these models must be carefully chosen so as to match
(i) the observed star formation rate and (ii) the formation rate of NS-NS
binaries, which can be estimated from the observed sample and the selection
effects related to the discoveries with radio-pulsar surveys. In this paper, we
select from an extremely broad family of possible population synthesis models
those few (2%) which are consistent with the observed sample of NS-NS binaries.
To further sharpen the constraints the observed NS-NS population places upon
our understanding of compact-object formation processes, we separate the
observed NS-NS population into two channels: (i) merging NS-NS binaries, which
will inspiral and merge through the action of gravitational waves within
Gyr, and (ii) wide NS-NS binaries, consisting of all the rest. With the subset
of astrophysically consistent models, we explore the implications for the rates
at which double black hole (BH-BH), black hole-neutron star (BH-NS), and NS-NS
binaries will merge through the emission of gravitational waves.Comment: (v1) Submitted to ApJ. Uses emulateapj.cls. 8 pages, 7 figures. (v2)
Minor textual changes in response to referee queries. Substantial additions
in appendicies, including a detailed discussion of sample multidimensional
population synthesis fit
The Brightest Point X-Ray Sources in Elliptical Galaxies and the Mass Spectrum of Accreting Black Holes
We propose that the shape of the upper-end X-ray luminosity function observed
in elliptical galaxies for point sources is a footprint of the black-hole (BH)
mass spectrum among old X-ray transients formed in the galaxies. We show that
this underlying BH mass spectrum is modified by a weighting factor that is
related to the transient duty cycle and it generally depends on the BH mass and
XRB donor type (main-sequence, red-giant, or white-dwarf donors). A duty cycle
that depends on the binary mass-transfer rate relative to the critical rate for
transient behavior is most probably favored. We also find that the derived BH
mass spectrum slope depends on the strength of angular momentum loss due to
magnetic braking for main-sequence donors. More specifically, we find that, for
``standard'' magnetic braking, BH XRBs with red-giant donors dominate the
upper-end XLF; for weaker magnetic braking prescriptions main-sequence donors
are found to be dominant. In both cases the BH mass spectrum has a differential
slope of ~2.5 and an upper BH mass cut-off at ~20 Msun is needed to understand
the very brightest of the BH XRBs in elliptical galaxies. We expect that our
result will help to constrain binary population synthesis models and the
adopted relations between black holes and the masses of their progenitors.Comment: 10 pages in ApJ format, 4 figures, accepted for publication in Ap
Polar kicks and the spin period - eccentricity relation in double neutron stars
We present results of a population synthesis study aimed at examining the
role of spin-kick alignment in producing a correlation between the spin period
of the first-born neutron star and the orbital eccentricity of observed double
neutron star binaries in the Galactic disk. We find spin-kick alignment to be
compatible with the observed correlation, but not to alleviate the requirements
for low kick velocities suggested in previous population synthesis studies. Our
results furthermore suggest low- and high-eccentricity systems may form through
two distinct formation channels distinguished by the presence or absence of a
stable mass transfer phase before the formation of the second neutron star. The
presence of highly eccentric systems in the observed sample of double neutron
stars may furthermore support the notion that neutron stars accrete matter when
moving through the envelope of a giant companion.Comment: To appear in the proceedings of "40 Years of Pulsars: Millisecond
Pulsars, Magnetars, and More", August 12-17, 2007, McGill University,
Montreal, Canad
Merger Sites of Double Neutron Stars and their Host Galaxies
Using the StarTrack population synthesis code we analyze the formation
channels possibly available to double neutron star binaries and find that they
can be richer than previously thought. We identify a group of short lived,
tight binaries, which do not live long enough to escape their host galaxies,
despite their large center-of-mass velocities. We present our most recent
results on all possible evolutionary paths leading to the formation of double
neutron stars, calculate their coalescence rates, and also revisit the question
of the distribution of merger sites around host galaxies. For a wide variety of
binary evolution models and galaxy potentials, we find that most of neutron
star mergers take place within galaxies. Our results stem from allowing for
radial and common envelope evolution of helium-rich stars (testable in the
future with detailed stellar-structure and hydrodynamic calculations) and
indicate that double neutron star binaries may not be excluded as Gamma-Ray
Burst (GRB) progenitors solely on the basis of their spatial distribution
around host galaxies. We also find, in contrast to Bethe & Brown (1998), that
in a significant fraction of common envelope (CE) phases neutron stars do not
accrete enough material to become black holes, and thus the channels involving
CEs are still open for the formation of double neutron stars.Comment: 12 pages, 3 figures, ApJ Letters 2002, accepte
Eccentric double white dwarfs as LISA sources in globular clusters
We consider the formation of double white dwarfs (DWDs) through dynamical
interactions in globular clusters. Such interactions can give rise to eccentric
DWDs, in contrast to the exclusively circular population expected to form in
the Galactic disk. We show that for a 5-year Laser Interferometer Space Antenna
(LISA) mission and distances as far as the Large Magellanic Cloud, multiple
harmonics from eccentric DWDs can be detected at a signal-to-noise ratio higher
than 8 for at least a handful of eccentric DWDs, given their formation rate and
typical lifetimes estimated from current cluster simulations. Consequently the
association of eccentricity with stellar-mass LISA sources does not uniquely
involve neutron stars, as is usually assumed. Due to the difficulty of
detecting (eccentric) DWDs with present and planned electromagnetic
observatories, LISA could provide unique dynamical identifications of these
systems in globular clusters.Comment: Published in ApJ 665, L5
Interacting Binaries with Eccentric Orbits. III. Orbital Evolution due to Direct Impact and Self-Accretion
The rapid circularization and synchronization of the stellar components in an
eccentric binary system at the onset of Roche lobe overflow (RLO) is a
fundamental assumption common to all binary stellar evolution and population
synthesis codes, even though the validity of this assumption is questionable
both theoretically and observationally. Here we calculate the evolution of the
orbital elements of an eccentric binary through the direct three-body
integration of a massive particle ejected through the inner Lagrangian point of
the donor star at periastron. The trajectory of this particle leads to three
possible outcomes: direct accretion (DA) onto the companion star within a
single orbit, self-accretion (SA) back onto the donor star within a single
orbit, or a quasi-periodic orbit around the companion star. We calculate the
secular evolution of the binary orbit in the first two cases and conclude that
DA can increase or decrease the orbital semi-major axis and eccentricity, while
SA always decreases the orbital both orbital elements. In cases where mass
overflow contributes to circularizing the orbit, circularization can set in on
timescales as short as a few per cent of the mass transfer timescale. In cases
where mass overflow increases the eccentricity, the orbital evolution is
governed by competition between mass overflow and tidal torques. In the absence
of tidal torques, mass overflow resulting in DI can lead to substantially
subsynchronously rotating donor stars. Contrary to common assumptions, DI
furthermore does not always provide a strong sink of orbital angular momentum
in close mass-transferring binaries; in fact we instead find that a significant
part can be returned to the orbit during the particle orbit. The formulation
presented here can be combined with stellar and binary evolution codes to
generate a better picture of the evolution of eccentric, RLO binary star
systems.Comment: 15 pages, 10 figures, Accepted for publication in Ap
Modeling the Redshift Evolution of the Normal Galaxy X-ray Luminosity Function
Emission from X-ray binaries (XRBs) is a major component of the total X-ray
luminosity of normal galaxies, so X-ray studies of high redshift galaxies allow
us to probe the formation and evolution of X-ray binaries on very long
timescales. In this paper, we present results from large-scale population
synthesis models of binary populations in galaxies from z = 0 to 20. We use as
input into our modeling the Millennium II Cosmological Simulation and the
updated semi-analytic galaxy catalog by Guo et al. (2011) to self-consistently
account for the star formation history (SFH) and metallicity evolution of each
galaxy. We run a grid of 192 models, varying all the parameters known from
previous studies to affect the evolution of XRBs. We use our models and
observationally derived prescriptions for hot gas emission to create
theoretical galaxy X-ray luminosity functions (XLFs) for several redshift bins.
Models with low CE efficiencies, a 50% twins mass ratio distribution, a steeper
IMF exponent, and high stellar wind mass loss rates best match observational
results from Tzanavaris & Georgantopoulos (2008), though they significantly
underproduce bright early-type and very bright (Lx > 10d41) late-type galaxies.
These discrepancies are likely caused by uncertainties in hot gas emission and
SFHs, AGN contamination, and a lack of dynamically formed Low-mass XRBs. In our
highest likelihood models, we find that hot gas emission dominates the emission
for most bright galaxies. We also find that the evolution of the normal galaxy
X-ray luminosity density out to z = 4 is driven largely by XRBs in galaxies
with X-ray luminosities between 10d40 and 10d41 erg/s.Comment: Accepted into ApJ, 17 pages, 3 tables, 7 figures. Text updated to
address referee's comment
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