50 research outputs found
Black hole beasts and where to find them
Gravity rules the Universe. It can form enormous cosmic webs of matter and hold together planets, stars, solar systems, and even galaxies. Yet, gravity itself is not directly visible.
However, we can get a glimpse into this dark sector by listening to gravity's own messengers -- gravitational waves. Since 2015, humankind has heard gravitational waves from at least 50 collisions involving black holes and neutron stars. This leaves us with the burning question: what causes these black holes and neutron stars to collide? In the first part of this thesis, we ask questions related to the origin of these colliding objects.
How will the inventory of merging black holes and neutron stars explode as our detectors improve? Can black holes merge repeatedly? If so, where do such repeated mergers happen? Can we tell apart which events are products of repeated mergers?
We have just started listening to a full symphony produced by some of the most violent events: mergers of black holes. This full symphony is encoded in the higher harmonics that accompany the cosmic melody produced by the spacetime's resonating vibrations. In the second part of this thesis, we focus on the final stages of the binary coalescence -- the so-called ringdown phase -- when this melody is the loudest. We try to understand how the signal heard by our detectors changes when we change the properties of the black holes that play this tune. We also try to understand whether the future detectors can listen to the full symphony, or if they can record only some of the notes. We develop methods to harness the vast potential of ringdown harmonics to estimate of the properties of the black holes that produced the signal
Dropping Anchor: Understanding the Populations of Binary Black Holes with Random and Aligned Spin Orientations
The relative spin orientations of black holes (BHs) in binaries encode their
evolutionary history: BHs assembled dynamically should have isotropically
distributed spins, while spins of the BHs originating in the field should be
aligned with the orbital angular momentum. In this article, we introduce a
simple population model for these dynamical and field binaries that uses spin
orientations as an anchor to disentangle these two evolutionary channels. We
then analyze binary BH mergers in the Third Gravitational-Wave Transient
Catalog (GWTC-3) and ask whether BHs from the isotropic-spin population possess
different distributions of mass ratio, spin magnitudes, or redshifts from the
preferentially-aligned-spin population. We find no compelling evidence that
binary BHs in GWTC-3 have different source-property distributions depending on
their spin alignment, but we do find that the dynamical and field channels
cannot both have mass-ratio distributions that strongly favor equal masses. We
give an example of how this can be used to provide insights into the various
processes that drive these BHs to merge. We also find that the current
detections are insufficient in extracting differences in spin magnitude or
redshift distributions of isotropic and aligned spin populations.Comment: 15 pages, 5 figures; accepted for publication in The Astrophysical
Journa
The role of supernova convection for the lower mass gap and the isolated binary formation of gravitational wave sources
Understanding astrophysical phenomena involving compact objects requires an
insight about the engine behind core-collapse supernovae (SNe) and the fate of
the stellar collapse of massive stars. In particular, this insight is crucial
in developing an understanding of the origin and formation channels of detected
population of BH-BH, BH-NS and NS-NS mergers. To gain this understanding, we
must tie our current knowledge of pre-SN stars properties and their potential
explosions to the final NS or BH mass distribution. The timescale of convection
growth may have a large effect on the strength of SN explosion and therefore
also on the mass distribution of stellar remnants. In this study we adopt the
new formulas for the relation between the pre-SN star properties and its
remnant from Fryer et al. 2022 in prep. into StarTrack population synthesis
code and check how they impact double compact object (DCO) mergers formed via
isolated binary evolution. The new formulas give one ability to test a wide
spectrum of assumptions on the convection growth time. In particular, different
variants allow for a smooth transition between having a deep lower mass gap and
a remnant mass distribution filled by massive NSs and low mass BHs. In this
paper we present distribution of masses, mass ratios and the local merger rate
densities of DCO mergers for different variants of new remnant mass formulas.
We test them together with different approaches to other highly uncertain
processes. We find that mass distribution of DCO mergers up to m_1+m_2 < 35
Msun is sensitive to adopted assumption on SN convection growth timescale.
Between the two extreme tested variants the probability of compact object
formation within the lower mass gap may differ up to 2 orders of magnitude. The
mass ratio distribution of DCO mergers is significantly influenced by SN model
only for our standard mass transfer stability criteria.Comment: 20 pages, submitted to MNRAS, comments welcom
Third post-Newtonian effective-one-body Hamiltonian in scalar-tensor and Einstein-scalar-Gauss-Bonnet gravity
We build an effective-one-body (EOB) Hamiltonian at third post-Newtonian
(3PN) order in scalar-tensor (ST) and Einstein-scalar-Gauss-Bonnet (ESGB)
theories of gravity. The latter is an extension of general relativity that
predicts scalar hair for black holes. We start from the known two-body
Lagrangian at 3PN order, and use order-reduction methods to construct its
ordinary Hamiltonian counterpart. We then reduce the conservative two-body
dynamics to the (non-geodesic) motion of a test particle in an effective metric
by means of canonical transformations. The resulting EOB Hamiltonian is a
modification of the general relativistic Hamiltonian, and already at 3PN order,
it must account for nonlocal-in-time tail contributions. We include the latter
beyond circular orbits and up to sixth order in the binary's orbital
eccentricity. We finally calculate the orbital frequency at the innermost
stable circular orbit (ISCO) of binary black holes in the shift-symmetric ESGB
model. Our work extends F.L. Juli\'e and N. Deruelle [Phys. Rev. D95, 124054
(2017)], and it is an essential step towards the accurate modeling of
gravitational waveforms beyond general relativity.Comment: 25 pages, 1 figur
Extracting linear and nonlinear quasinormal modes from black hole merger simulations
In general relativity, when two black holes merge they produce a rotating
(Kerr) black hole remnant. According to perturbation theory, the remnant emits
"ringdown" radiation: a superposition of exponentials with characteristic
complex frequencies that depend only on the remnant's mass and spin. While the
goal of the black hole spectroscopy program is to measure the quasinormal mode
frequencies, a knowledge of their amplitudes and phases is equally important to
determine which modes are detectable, and possibly to perform additional
consistency checks. Unlike the complex frequencies, the amplitudes and phases
depend on the properties of the binary progenitors, such as the binary mass
ratio and component spins. In this paper we develop a fitting algorithm
designed to reliably identify the modes present in numerical simulations and to
extract their amplitudes and phases. We apply the algorithm to over 500 binary
black hole simulations from the public SXS numerical relativity simulation
catalog, and we present fitting formulas for the resulting mode amplitudes and
phases as functions of the properties of the progenitors. Crucially, our
algorithm allows for the extraction of not only prograde fundamental modes and
overtones, but also retrograde modes and second-order modes. We unveil
interesting relations for the amplitude ratios of different modes. The fitting
code and interactive versions of some of the plots are publicly available. The
results presented in this paper can be updated as more and better simulations
become available.Comment: 37 pages, 22 figures, 2 tables. Interactive plots and code usage
examples available at https://mhycheung.github.io/jaxqualin
Constraining the fraction of binary black holes formed in isolation and young star clusters with gravitational-wave data
Ten binary black-hole mergers have already been detected during the first two
observing runs of advanced LIGO and Virgo, and many more are expected to be
observed in the near future. This opens the possibility for gravitational-wave
astronomy to better constrain the properties of black hole binaries, not only
as single sources, but as a whole astrophysical population. In this paper, we
address the problem of using gravitational-wave measurements to estimate the
proportion of merging black holes produced either via isolated binaries or
binaries evolving in young star clusters. To this end, we use a Bayesian
hierarchical modeling approach applied to catalogs of merging binary black
holes generated using state-of-the-art population synthesis and N-body codes.
In particular, we show that, although current advanced LIGO/Virgo observations
only mildly constrain the mixing fraction between the two
formation channels, we expect to narrow down the fractional errors on to
after a few hundreds of detections.Comment: 17 pages, 4 figure