T. Bulik

THE LOWEST-MASS STELLAR BLACK HOLES : CATASTROPHIC DEATH OF NEUTRON STARS IN GAMMA-RAY BURSTS

Mergers of double neutron stars are considered the most likely progenitors for short gamma-ray bursts. Indeed, such a merger can produce a black hole with a transient accreting torus of nuclear matter, and the conversion of a fraction of the torus mass-energy to radiation can power a gamma-ray burst. Using available binary pulsar observations supported by our extensive evolutionary calculations of double neutron star formation, we demonstrate that the fraction of mergers that can form a black hole-torus system depends very sensitively on the (largely unknown) maximum neutron star mass. We show that the available observations and models put a very stringent constraint on this maximum mass under the assumption that black hole formation is required to produce a short gamma-ray burst in a double neutron star merger. Specifically, we find that the maximum neutron star mass must be within 2–2.5 M☉. Moreover, a single unambiguous measurement of a neutron star mass above 2.5 M☉ would exclude a black hole-torus central engine model of short gamma-ray bursts in double neutron star mergers. Such an observation would also indicate that if in fact short gamma-ray bursts are connected to neutron star mergers, the gamma-ray burst engine is best explained by the lesser known model invoking a highly magnetized massive neutron star.

The Effect of Pair-Instability Mass Loss on Black Hole Mergers

Context. Mergers of two stellar-origin black holes are a prime source of gravitational waves and are under intensive investigation. One crucial ingredient in their modeling has been neglected: pair-instability pulsation supernovae with associated severe mass loss may suppress the formation of massive black holes, decreasing black-hole-merger rates for the highest black-hole masses. Aims. We demonstrate the effects of pair-instability pulsation supernovae on merger rate and mass using populations of double black-hole binaries formed through the isolated binary classical evolution channel. Methods. The mass loss from pair-instability pulsation supernova is estimated based on existing hydrodynamical calculations. This mass loss is incorporated into the StarTrack population synthesis code. StarTrack is used to generate double black-hole populations with and without pair-instability pulsation supernova mass loss. Results. The mass loss associated with pair-instability pulsation supernovae limits the Population I/II stellar-origin black-hole mass to 50 M ⊙ , in tension with earlier predictions that the maximum black-hole mass could be as high as 100 M ⊙ . In our model, neutron stars form with mass 1−2 M ⊙ . We then encounter the first mass gap at 2−5 M ⊙ with the compact object absence due to rapid supernova explosions, followed by the formation of black holes with mass 5−50 M ⊙ , with a second mass gap at 50−135 M ⊙ created by pair-instability pulsation supernovae and by pair-instability supernovae. Finally, black holes with masses above 135 M ⊙ may potentially form to arbitrarily high mass limited only by the extent of the initial mass function and the strength of stellar winds. Suppression of double black-hole-merger rates by pair-instability pulsation supernovae is negligible for our evolutionary channel. Our standard evolutionary model, with the inclusion of pair-instability pulsation supernovae and pair-instability supernovae, is fully consistent with the Laser Interferometric Gravitational-wave Observatory (LIGO) observations of black-hole mergers: GW150914, GW151226, and LVT151012. The LIGO results are inconsistent with high (≳ 400 km s -1 ) black hole (BH) natal kicks. We predict the detection of several, and up to as many as ~60, BH-BH mergers with a total mass of 10−150 M ⊙ (most likely range: 20−80 M ⊙ ) in the forthcoming ~60 effective days of the LIGO O2 observations, assuming the detectors reach the optimistic target O2 sensitivity.

Population synthesis of double neutron stars

Using the StarTrack binary population synthesis code we model the population of double neutron stars in the Galaxy. We include a detailed treatment of the spin evolution of each pulsar due to processes such as spin-down and spin-up during accretion events as well as magnetic field decay. We also model the spatial distribution of double neutron stars by including their natal kicks and subsequent propagation in the Galactic gravitational potential. This synthetic pulsar population is compared to the observed sample of double neutron stars taking into account the selection effects of detection in the radio band, to determine the most likely evolutionary parameters. With these parameters we determine the properties of the double neutron star binaries detectable in gravitational waves by the high-frequency interferometers LIGO and VIRGO. In particular, we discuss the distributions of chirp masses and mass ratios in samples selected by their radio or gravitational wave emission.

Astrophysical significance of the detection of coalescing binaries with gravitational waves

We use the StarTrack stellar population synthesis code to analyze properties of double compact object binaries as sources of gravitational waves. Since the distribution of lifetimes of these objects extends up to the Hubble time we conclude that a proper calculation of the expected rate must include a full cosmological model. We present such model, calculate the expected coalescence rates, and analyze the intrinsic sensitivity of these rates to the model assumptions. We find that the rate alone is a very poor estimator of the underlying stellar evolution model. However we show that the distribution of observed chirp masses is very sensitive to the underlying stellar evolution model, while it is very insensitive to the underlying cosmology, star formation rate history and variation of detector sensitivity.

Gravitational wave background from Population III binaries

Context. Current star formation models imply that the binary fraction of Population III stars is non-zero. The evolution of these binaries must have led to the formation of compact object binaries. Aims. We estimate the gravitational wave background originating in these binaries and discuss its observability. Methods. The properties of the Population III binaries are investigated using a binary population synthesis code. We numerically model the background and take into account the evolution of eccentric binaries. Results. The gravitational wave background from Population III binaries dominates the spectrum below 100 Hz. If the binary fraction is larger than 10 −2 , the background will be detectable by Einstein Telescope (ET), Laser Interferometer Space Antenna (LISA), and DECi-Hertz Interferometer Gravitational wave Observatory (DECIGO). Conclusions. The gravitational wave background from Population III binaries will dominate the spectrum below 100 Hz. The instruments LISA, ET, and DECIGO should either see it easily or, in the case of non-detection, provide very strong constraints on the properties of the Population III stars.

Effect of metallicity on the gravitational-wave signal from the cosmological population of compact binary coalescences

Recent studies on stellar evolution have shown that the properties of compact objects strongly depend on metallicity of the environment in which they were formed. In this work, we study how the metallicity of the stellar population can affect unresolved gravitational waves background from extragalactic compact binaries. We obtain a suit of models using population synthesis code, estimate the gravitational wave background they produce and discuss its detectability with second (Advanced LIGO, Advanced Virgo) and third (Einstein Telescope) generation detectors. Our results show that the background is dominated by binary black holes for all considered models in the frequency range of terrestrial detectors, and that it could be detected in most cases by Advanced LIGO/Virgo, and with Einstein Telescope with a very high signal to noise ratio.

VIRGO sensitivity to binary coalescences and the Population III black hole binaries

Aims. We analyze the properties of VIRGO detector with the aim of studying its ability to search for coalescing black hole binaries. We focus on the remnants of the Population III stars, which currently should be massive black holes ( ∼

GRB 030406 - an extremely hard burst outside of the INTEGRAL field of view

100{-}1000~M_\odot

Expected masses of merging compact object binaries observed in gravitational waves

), some of them bound in binary systems. The coalescence of such binaries due to emission of gravitational waves may be currently observable. Methods. We use a binary population synthesis to model the evolution of Population III binaries. Results. We calculate the signal to noise ratios of gravitational waves emitted by the system in each of the coalescence phase: inspiral, merger and ringdown, and provide simple formulae for the signal to noise ratio as a function of masses of the binaries. We estimate the detection rates for the VIRGO interferometer and also compare them with the estimates for the current LIGO. We show that these expected rates are similar to, or larger than the expected rates from coalescences of Population I and II compact object binaries.

The eccentricity distribution of compact binaries

Using the IBIS Compton mode, the INTEGRAL satellite is able to detect and localize bright and hard GRBs, which happen outside of the nominal INTEGRAL telescopes field of view. We have developed a method of analyzing such INTEGRAL data to obtain the burst location and spectra. We present the results for the case of GRB030406. The burst is localized with the Compton events, and the location is consistent with the previous Interplanetary Network position. A spectral analysis is possible with the detailed modeling of the detector response for such a far off-axis source with the offset of 36.9