1,295 research outputs found
IC10~X-1/NGC300~X-1: the very immediate progenitors of BH-BH binaries
We investigate the future evolution of two extragalactic X-ray binaries: IC10
X-1 and NGC300 X-1. Each of them consists of a high mass BH (\sim 20-30
\msun) accreting from a massive WR star companion (\gtrsim 20 \msun), and
both are located in low metallicity galaxies. We analyze the current state of
the systems and demonstrate that both systems will very quickly (
Myr) form close BH-BH binaries with the short coalescence time ( Gyr)
and large chirp mass (\sim 15 \msun). The formation of BH-BH system seems
unavoidable, as {\em (i)} WR companions are well within their Roche lobes and
they do not expand so no Roche lobe overflow is expected, {\em (ii)} even
intense WR wind mass loss does not remove sufficient mass to prohibit the
formation of the second BH, {\em (ii)} even if BH receives the large natal
kick, the systems are very closely bound and are almost impossible to disrupt.
As there are two such immediate BH-BH progenitor systems within 2 Mpc and as
the current gravitational wave instruments LIGO/VIRGO (initial stage) can
detect such massive BH-BH mergers out to Mpc, the empirically
estimated detection rate of such inspirals is at the
99% confidence level. If there is no detection in the current LIGO/VIRGO data
(unreleased year of run), the existence of these two massive BH systems
poses an interesting challenge. Either the gravitational radiation search is
not sensitive to massive inspirals or there is some fundamental
misunderstanding of stellar evolution physics leading directly to the formation
of BH-BH binaries.Comment: 9 pages, resubmitted to ApJ with major extensio
The effect of metallicity on the detection prospects for gravitational waves
Data from the SDSS (300,000 galaxies) indicates that recent star formation
(within the last 1 billion years) is bimodal: half the stars form from gas with
high amounts of metals (solar metallicity), and the other half form with small
contribution of elements heavier than Helium (10-30% solar). Theoretical
studies of mass loss from the brightest stars derive significantly higher
stellar-origin BH masses (30-80 Msun) than previously estimated for sub-solar
compositions. We combine these findings to estimate the probability of
detecting gravitational waves (GWs) arising from the inspiral of double compact
objects. Our results show that a low metallicity environment significantly
boosts the formation of double compact object binaries with at least one BH. In
particular, we find the GW detection rate is increased by a factor of 20 if the
metallicity is decreased from solar (as in all previous estimates) to a 50-50
mixture of solar and 10% solar metallicity. The current sensitivity of the two
largest instruments to NS-NS binary inspirals (VIRGO: 9 Mpc; LIGO: 18) is not
high enough to ensure a first detection. However, our results indicate that if
a future instrument increased the sensitivity to 50-100 Mpc, a detection of GWs
would be expected within the first year of observation. It was previously
thought that NS-NS inspirals were the most likely source for GW detection. Our
results indicate that BH-BH binaries are 25-times more likely sources than
NS-NS systems and that we are on the cusp of GW detection.Comment: 4 pages of text, 2 figures, 2 tables (ApJ Letters, accepted
The fate of Cyg X-1: an empirical lower limit on BH-NS merger rate
The recent distance determination allowed precise estimation of the orbital
parameters of Cyg X-1, which contains a massive 14.8 Msun BH with a 19.2 Msun O
star companion. This system appears to be the clearest example of a potential
progenitor of a BH-NS system. We follow the future evolution of Cyg X-1, and
show that it will soon encounter a Roche lobe overflow episode, followed
shortly by a Type Ib/c supernova and the formation of a NS. It is demonstrated
that in majority of cases (70%) the supernova and associated natal kick
disrupts the binary due to the fact that the orbit expanded significantly in
the Roche lobe overflow episode. In the reminder of cases (30%) the newly
formed BH-NS system is too wide to coalesce in the Hubble time. Only
sporadically (1%) a Cyg X-1 like binary may form a coalescing BH-NS system
given a favorable direction and magnitude of the natal kick. If Cyg X-1 like
channel (comparable mass BH-O star bright X-ray binary) is the only or dominant
way to form BH-NS binaries in the Galaxy we can estimate the empirical BH-NS
merger rate in the Galaxy at the level of 0.001 per Myr. This rate is so low
that the detection of BH-NS systems in gravitational radiation is highly
unlikely, generating Advanced LIGO/VIRGO detection rates at the level of only 1
per century. If BH-NS inspirals are in fact detected, it will indicate that the
formation of these systems proceeds via some alternative and yet unobserved
channels.Comment: 5 pages, ApJ Letters (accepted
Double Compact Objects as Low-frequency Gravitational Wave Sources
We study the Galactic field population of double compact objects (NS-NS,
BH-NS, BH-BH binaries) to investigate the number (if any) of these systems that
can potentially be detected with LISA at low gravitational-wave frequencies. We
calculate the Galactic numbers and physical properties of these binaries and
show their relative contribution from the disk, bulge and halo. Although the
Galaxy hosts 10^5 double compact object binaries emitting low-frequency
gravitational waves, only a handful of these objects in the disk will be
detectable with LISA, but none from the halo or bulge. This is because the bulk
of these binaries are NS-NS systems with high eccentricities and long orbital
periods (weeks/months) causing inefficient signal accumulation (small number of
signal bursts at periastron passage in 1 yr of LISA observations) rendering
them undetectable in the majority of these cases. We adopt two evolutionary
models that differ in their treatment of the common envelope phase that is a
major (and still mostly unknown) process in the formation of close double
compact objects. Depending on the adopted evolutionary model, our calculations
indicate the likely detection of about 4 NS-NS binaries and 2 BH-BH systems
(model A; likely survival of progenitors through CE) or only a couple of NS-NS
binaries (model B; suppression of the double compact object formation due to CE
mergers).Comment: 12 pages, ApJ accepted, major change
Double Compact Objects I: The Significance Of The Common Envelope On Merger Rates
The last decade of observational and theoretical developments in stellar and
binary evolution provides an opportunity to incorporate major improvements to
the predictions from populations synthesis models. We compute the Galactic
merger rates for NS-NS, BH-NS, and BH-BH mergers with the StarTrack code. The
most important revisions include: updated wind mass loss rates (allowing for
stellar mass black holes up to 80 \msun), a realistic treatment of the common
envelope phase (a process that can affect merger rates by 2--3 orders of
magnitude), and a qualitatively new neutron star/black hole mass distribution
(consistent with the observed "mass gap"). Our findings include: (i) The
binding energy of the envelope plays a pivotal role in determining whether a
binary merges within a Hubble time. (ii) Our description of natal kicks from
supernovae plays an important role, especially for the formation of BH-BH
systems. (iii) The masses of BH-BH systems can be substantially increased in
the case of low metallicities or weak winds. (iv) Certain combinations of
parameters underpredict the Galactic NS-NS merger rate, and can be ruled out.
{\em (v)} Models incorporating delayed supernovae do not agree with the
observed NS/BH "mass gap", in accordance with our previous work. This is the
first in a series of three papers. The second paper will study the merger rates
of double compact objects as a function of redshift, star formation rate, and
metallicity. In the third paper we will present the detection rates for
gravitational wave observatories, using up-to-date signal waveforms and
sensitivity curves.Comment: 41 pages, 20 figures, accepted for ApJ & new model
Binary compact object coalescence rates: The role of elliptical galaxies
We estimate binary compact object merger detection rates for LIGO, including
the binaries formed in ellipticals long ago. Specifically, we convolve hundreds
of model realizations of elliptical- and spiral-galaxy population syntheses
with a model for elliptical- and spiral-galaxy star formation history as a
function of redshift. Our results favor local merger rate densities of 4\times
10^{-3} {Mpc}^{-3}{Myr}^{-1} for binary black holes (BH), 3\times 10^{-2}
{Mpc}^{-3}{Myr}^{-1} for binary neutron stars (NS), and 10^{-2}
{Mpc}^{-3}{Myr}^{-1} for BH-NS binaries. Mergers in elliptical galaxies are a
significant fraction of our total estimate for BH-BH and BH-NS detection rates;
NS-NS detection rates are dominated by the contribution from spiral galaxies.
Using only models that reproduce current observations of Galactic NS-NS
binaries, we find slightly higher rates for NS-NS and largely similar ranges
for BH-NS and BH-BH binaries. Assuming a detection signal-to-noise ratio
threshold of 8 for a single detector (as part of a network), corresponding to
radii \Cv of the effective volume inside of which a single LIGO detector could
observe the inspiral of two 1.4 M_\sun neutron stars of 14 Mpc and 197 Mpc, for
initial and advanced LIGO, we find event rates of any merger type of 2.9*
10^{-2} -- 0.46 and 25-400 per year (at 90% confidence level), respectively. We
also find that the probability P_{detect} of detecting one or more mergers with
this single detector can be approximated by (i) P_{detect}\simeq 0.4+0.5\log
(T/0.01{yr}), assuming \Cv=197 {Mpc} and it operates for T years, for T between
2 days and 0.1 {yr}); or by (ii) P_{detect}\simeq 0.5 + 1.5 \log \Cv/32{Mpc},
for one year of operation and for \Cv between 20 and 70 Mpc. [ABRIDGED]Comment: 22 pages, 11 figures. Accepted by ApJ. v2 adds several figures, an
electronic-only table of all intermediate binary evolution simulations
(tab1.txt here), and new subsections outlining broader significance (e.g.,
5.4; 4.6; 6.1
Double and single recycled pulsars: an evolutionary puzzle?
We investigate the statistics of isolated recycled pulsars and double neutron
star binaries in the Galactic disk. Since recycled pulsars are believed to form
through accretion and spinup in close binaries, the isolated objects presumably
originate from disrupted progenitors of double neutron stars. There are a
comparable number of double neutron star systems compared to isolated recycled
pulsars. We find that standard evolutionary models cannot explain this fact,
predicting several times the number of isolated recycled pulsars than those in
double neutron star systems. We demonstrate, through population synthesis
calculations, that the velocity distribution of isolated recycled pulsars is
broader than for binary systems. When this is accounted for in a model for
radio pulsar survey selection effects, which include the effects of Doppler
smearing for the double neutron star binaries, we find that there is a small
(25%) bias towards the detection of double neutron star systems. This bias,
however, is not significant enough to explain the observational discrepancy if
standard (sigma = 265 km/s) neutron star natal kick velocities are invoked in
binary population syntheses. Population syntheses in which the 1D Maxwellian
velocity dispersion of the natal kick is sigma=170 km/s are consistent with the
observations. These conclusions further support earlier findings the neutron
stars formed in close interacting binaries receive significantly smaller natal
kicks than the velocities of Galactic single pulsars would seem to indicate.Comment: 12 pages, MNRAS (accepted
The eccentricity distribution of compact binaries
The current gravitational wave detectors have reached their operational
sensitivity and are nearing detection of compact object binaries. In the coming
years, we expect that the Advanced LIGO/VIRGO will start taking data. At the
same time, there are plans for third generation ground-based detectors such as
the Einstein Telescope, and space detectors such as DECIGO. We discuss the
eccentricity distribution of inspiral compact object binaries during they
inspiral phase. We analyze the expected distributions of eccentricities at
three frequencies that are characteristic of three future detectors: Advanced
LIGO/VIRGO (30 Hz), Einstein Telescope (3 Hz), and DECIGO (0.3 Hz). We use the
StarTrack binary population code to investigate the properties of the
population of compact binaries in formation. We evolve their orbits until the
point that they enter a given detector sensitivity window and analyze the
eccentricity distribution at that time. We find that the eccentricities of
BH-BH and BH-NS binaries are quite small when entering the Advanced LIGO/VIRGO
detector window for all considered models of binary evolution. Even in the case
of the DECIGO detector, the typical eccentricities of BH-BH binaries are below
10^{-4}, and the BH-NS eccentricities are smaller than 10^{-3}. Some fraction
of NS-NS binaries may have significant eccentricities. Within the range of
considered models, we found that a fraction of between 0.2% and 2% NS-NS
binaries will have an eccentricity above 0.01 for the Advanced LIGO/VIRGO
detectors. For the ET detector, this fraction is between 0.4% and 4%, and for
the DECIGO detector it lies between 2% and 27%.Comment: 8 pages, 5 figures, accepted by A&
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