43 research outputs found
Binary black hole spins: model selection with GWTC-3
The origin of the spins of stellar-mass black holes is still controversial,
and angular momentum transport inside massive stars is one of the main sources
of uncertainty. Here, we apply hierarchical Bayesian inference to derive
constraints on spin models from the 59 most confident binary black hole merger
events in the third gravitational-wave transient catalogue (GWTC-3). We
consider up to five parameters: chirp mass, mass ratio, redshift, effective
spin, and precessing spin. For model selection, we use a set of binary
population synthesis simulations spanning drastically different assumptions for
black hole spins and natal kicks. In particular, our spin models range from
maximal to minimal efficiency of angular momentum transport in stars. We find
that, if we include the precessing spin parameter into our analysis, models
predicting only vanishingly small spins are in tension with GWTC-3 data. On the
other hand, models in which most spins are vanishingly small, but that also
include a sub-population of tidally spun-up black holes are a good match to the
data. Our results show that the precessing spin parameter has a crucial impact
on model selection.Comment: 11 pages, submitted to mnras. arXiv admin note: text overlap with
arXiv:2102.12495 by same author
The cosmic merger rate density of compact objects: impact of star formation, metallicity, initial mass function and binary evolution
We evaluate the redshift distribution of binary black hole (BBH), black hole
- neutron star binary (BHNS) and binary neutron star (BNS) mergers, exploring
the main sources of uncertainty: star formation rate (SFR) density, metallicity
evolution, common envelope, mass transfer via Roche lobe overflow, natal kicks,
core-collapse supernova model and initial mass function. Among binary evolution
processes, uncertainties on common envelope ejection have a major impact: the
local merger rate density of BNSs varies from to
Gpc yr if we change the common envelope efficiency parameter from
to 0.5, while the local merger rates of BBHs and BHNSs vary
by a factor of . The BBH merger rate changes by one order of
magnitude, when uncertainties on metallicity evolution are taken
into account. In contrast, the BNS merger rate is almost insensitive to
metallicity. Hence, BNSs are the ideal test bed to put constraints on uncertain
binary evolution processes, such as common envelope and natal kicks. Only
models assuming values of and moderately low natal
kicks (depending on the ejected mass and the SN mechanism), result in a local
BNS merger rate density within the 90% credible interval inferred from the
second gravitational-wave transient catalogue.Comment: 14 pages, 12 figures, 2 tables, accepted for publication in MNRA
Massive binary black holes from Population II and III stars
Population III stars, born from the primordial gas in the Universe, lose a
negligible fraction of their mass via stellar winds and possibly follow a
top-heavy mass function. Hence, they have often been regarded as the ideal
progenitors of massive black holes (BHs), even above the pair instability mass
gap. Here, we evolve a large set of Population III binary stars (metallicity
) with our population-synthesis code SEVN, and compare them with
Population II binary stars (). In our models, the lower edge of the
pair-instability mass gap corresponds to a BH mass of
() M for single Population III (II) stars. Overall, we
find only mild differences between the properties of binary BHs (BBHs) born
from Population III and II stars, especially if we adopt the same initial mass
function and initial orbital properties. Most BBH mergers born from Population
III and II stars have primary BH mass below the pair-instability gap, and the
maximum secondary BH mass is M. Only up to %
(%) BBH mergers from Population III (II) progenitors have
primary mass above the gap. Unlike metal-rich binary stars, the main formation
channel of BBH mergers from Population III and II stars involves only stable
mass transfer episodes in our fiducial model.Comment: 15 pages, 17 figures, comments are welcom
Perspectives for multi-messenger astronomy with the next generation of gravitational-wave detectors and high-energy satellites
The Einstein Telescope (ET) is going to bring a revolution for the future of
multi-messenger astrophysics. In order to detect the counterparts of binary
neutron star (BNS) mergers at high redshift, the high-energy observations will
play a crucial role. Here, we explore the perspectives of ET, as single
observatory and in a network of gravitational-wave (GW) detectors, operating in
synergy with future -ray and X-ray satellites. We predict the
high-energy emission of BNS mergers and its detectability in a theoretical
framework which is able to reproduce the properties of the current sample of
observed short GRBs (SGRB). We estimate the joint GW and high-energy detection
rate for both the prompt and afterglow emissions, testing several combinations
of instruments and observational strategies. We find that the vast majority of
SGRBs detected in -rays will have a detectable GW counterpart; the
joint detection efficiency approaches considering a network of third
generation GW observatories. The probability of identifying the electromagnetic
counterpart of BNS mergers is significantly enhanced if the sky localisation
provided by GW instruments is observed by wide field X-ray monitors. We
emphasize that the role of the future X-ray observatories will be very crucial
for the detection of the fainter emission outside the jet core, which will
allow us to probe the yet unexplored population of low-luminosity SGRBs in the
nearby Universe, as well as to unveil the nature of the jet structure and the
connections with the progenitor properties.Comment: Submitted to the journa
New insights on binary black hole formation channels after GWTC-2: young star clusters versus isolated binaries
With the recent release of the second gravitational-wave transient catalogue
(GWTC-2), which introduced dozens of new detections, we are at a turning point
of gravitational wave astronomy, as we are now able to directly infer
constraints on the astrophysical population of compact objects. Here, we tackle
the burning issue of understanding the origin of binary black hole (BBH)
mergers. To this effect, we make use of state-of-the-art population synthesis
and N-body simulations, to represent two distinct formation channels: BBHs
formed in the field (isolated channel) and in young star clusters (dynamical
channel). We then use a Bayesian hierarchical approach to infer the
distribution of the mixing fraction , with () in the pure
dynamical (isolated) channel. %that controls the proportion of isolated and
dynamical BBHs. We explore the effects of additional hyper-parameters of the
model, such as the spread in metallicity and the parameter
, describing the distribution of spin magnitudes. We find
that the dynamical model is slightly favoured with a median value of ,
when and . Models with higher
spin magnitudes tend to strongly favour dynamically formed BBHs ( if
). Furthermore, we show that hyper-parameters
controlling the rates of the model, such as , have a large
impact on the inference of the mixing fraction, which rises from to
when we increase from 0.2 to 0.6, for a fixed value
of . Finally, our current set of observations is better
described by a combination of both formation channels, as a pure dynamical
scenario is excluded at the credible interval, except when the spin
magnitude is high.Comment: 13 pages, 10 figures, 2 tables, published in MNRA
Binary black hole mergers from Population III stars: uncertainties from star formation and binary star properties
Population III (Pop. III) binary stars likely produced the first stellar-born
binary black hole (BBH) mergers in the Universe. Here, we quantify the main
sources of uncertainty for the merger rate density evolution and mass spectrum
of Pop. III BBHs by considering four different formation histories of Pop. III
stars and 11 models of the initial orbital properties of their binary systems.
The uncertainty on the orbital properties affects the BBH merger rate density
by up to two orders of magnitude; models with shorter initial orbital periods
lead to higher BBH merger rates, because they favour the merger via stable mass
transfer episodes. The uncertainty on the star formation history also has a
substantial impact on both the shape and the normalisation of the BBH merger
rate density: the peak of the merger rate density shifts from up to
depending on the assumed star formation rate, while the maximum BBH
merger rate density for our fiducial binary population model spans from
to Gpc yr. The typical BBH masses are not
affected by the star formation rate model and only mildly influenced by the
binary population parameters. The primary black holes born from Pop. III stars
tend to be rather massive ( M) with respect to those born from
metal-rich stars ( M). However, we expect that Pop. III BBH
mergers with primary mass M are rare ( Gpc
yr). Finally, we estimate that the Einstein Telescope will detect
Pop. III BBH mergers per year, depending on the star formation
history and binary star properties.Comment: 16 pages, 16 figures, 1 table. Comments are welcome. Submitted to
MNRA
Detecting VHE prompt emission from binary neutron-star mergers: ET and CTA synergies
The current generation of very-high-energy ray (VHE; E above 30 GeV)
detectors (MAGIC and H.E.S.S.) have recently demonstrated the ability to detect
the afterglow emission of GRBs. However, the GRB prompt emission, typically
observed in the 10 keV-10 MeV band, has so far remained undetected at higher
energies. Here, we investigate the perspectives of multi-messenger observations
to detect the prompt emission of short GRBs in VHE. Considering binary neutron
star mergers as progenitors of short GRBs, we evaluate the joint detection
efficiency of the Cherenkov Telescope Array (CTA) observing in synergy with the
third generation of gravitational wave detectors, such as the Einstein
Telescope (ET) and Cosmic Explorer (CE). In particular, we evaluate the
expected capabilities to detect and localize gravitational wave events in the
inspiral phase and to provide an early warning alert able to drive the VHE
search. We compute the amount of possible joint detections by considering
several observational strategies, and demonstrate that the sensitivities of CTA
make the detection of the VHE emission possible even if it is several orders
fainter than the one observed at 10 keV-10 MeV. We discuss the results in terms
of possible scenarios of production of VHE photons from binary neutron star
mergers by considering GRB prompt and afterglow emissions
Compact object mergers: exploring uncertainties from stellar and binary evolution with SEVN
Population-synthesis codes are an unique tool to explore the parameter space
of massive binary star evolution and binary compact object (BCO) formation.
Most population-synthesis codes are based on the same stellar evolution model,
limiting our ability to explore the main uncertainties. Our code SEVN overcomes
this issue by interpolating the main stellar properties from a set of
pre-computed evolutionary tracks. With SEVN, we evolved
binaries in the metallicity range , exploring a number
of models for electron-capture, core-collapse and pair-instability supernovae,
different assumptions for common envelope, stability of mass transfer,
quasi-homogeneous evolution and stellar tides. We find that stellar evolution
has a dramatic impact on the formation of single and binary compact objects.
Just by slightly changing the overshooting parameter () and the pair-instability model, the maximum mass of a black hole
can vary from to . Furthermore,
the formation channels of BCOs and the merger efficiency we obtain with SEVN
show significant differences with respect to the results of other
population-synthesis codes, even when the same binary-evolution parameters are
used. For example, the main traditional formation channel of BCOs is strongly
suppressed in our models: at high metallicity () only % of
the merging binary black holes and binary neutron stars form via this channel,
while other authors found fractions %. The local BCO merger rate density
of our fiducial models is consistent with the most recent estimates by the
LIGO--Virgo--KAGRA collaboration.Comment: Submitted to MNRAS, comments welcome! The SEVN code is available at
https://gitlab.com/sevncodes/sevn.git. All the data underlying this article
are available in Zenodo at the link https://doi.org/10.5281/zenodo.7260771.
All the Jupyter notebooks used to produce the plots in the paper are
available in the gitlab repository https://gitlab.com/iogiul/iorio22_plot.gi