915 research outputs found
The correlation of field binary black hole mergers and how 3G gravitational-wave detectors can constrain it
Understanding the origin of merging binary black holes is currently one of
the most pressing quests in astrophysics. We show that if isolated binary
evolution dominates the formation mechanism of merging binary black holes, one
should expect a correlation between the effective spin parameter,
, and the redshift of the merger, , of binary black
holes. This correlation comes from tidal spin-up systems preferentially forming
and merging at higher redshifts due to the combination of weaker orbital
expansion from low metallicity stars given their reduced wind mass loss rate,
delayed expansion and have smaller maximal radii during the supergiant phase
compared to stars at higher metallicity. As a result, these tightly bound
systems merge with short inspiral times. Given our fiducial model of isolated
binary evolution, we show that the origin of a
correlation in the detectable LIGO--Virgo binary black hole population is
different from the intrinsic population, which will become accessible only in
the future by third-generation gravitational-wave detectors such as Einstein
Telescope and Cosmic Explorer. Finally, we compare our model predictions with
population predictions based on the current catalog of binary black hole
mergers and find that current data favor a positive correlation of
as predicted by our model of isolated binary evolution.Comment: 14 pages, 10 figures, submitted to A&
Classifying the unknown: discovering novel gravitational-wave detector glitches using similarity learning
The observation of gravitational waves from compact binary coalescences by
LIGO and Virgo has begun a new era in astronomy. A critical challenge in making
detections is determining whether loud transient features in the data are
caused by gravitational waves or by instrumental or environmental sources. The
citizen-science project \emph{Gravity Spy} has been demonstrated as an
efficient infrastructure for classifying known types of noise transients
(glitches) through a combination of data analysis performed by both citizen
volunteers and machine learning. We present the next iteration of this project,
using similarity indices to empower citizen scientists to create large data
sets of unknown transients, which can then be used to facilitate supervised
machine-learning characterization. This new evolution aims to alleviate a
persistent challenge that plagues both citizen-science and instrumental
detector work: the ability to build large samples of relatively rare events.
Using two families of transient noise that appeared unexpectedly during LIGO's
second observing run (O2), we demonstrate the impact that the similarity
indices could have had on finding these new glitch types in the Gravity Spy
program
Elastic Spin Relaxation Processes in Semiconductor Quantum Dots
Electron spin decoherence caused by elastic spin-phonon processes is
investigated comprehensively in a zero-dimensional environment. Specifically, a
theoretical treatment is developed for the processes associated with the
fluctuations in the phonon potential as well as in the electron procession
frequency through the spin-orbit and hyperfine interactions in the
semiconductor quantum dots. The analysis identifies the conditions (magnetic
field, temperature, etc.) in which the elastic spin-phonon processes can
dominate over the inelastic counterparts with the electron spin-flip
transitions. Particularly, the calculation results illustrate the potential
significance of an elastic decoherence mechanism originating from the
intervalley transitions in semiconductor quantum dots with multiple equivalent
energy minima (e.g., the X valleys in SiGe). The role of lattice anharmonicity
and phonon decay in spin relaxation is also examined along with that of the
local effective field fluctuations caused by the stochastic electronic
transitions between the orbital states. Numerical estimations are provided for
typical GaAs and Si-based quantum dots.Comment: 57 pages, 14 figure
Black Hole Mergers from Hierarchical Triples in Dense Star Clusters
Hierarchical triples are expected to be produced by the frequent binary-mediated interactions in the cores of globular clusters. In some of these triples, the tertiary companion can drive the inner binary to merger following large eccentricity oscillations, as a result of the eccentric Kozai–Lidov mechanism. In this paper, we study the dynamics and merger rates of black hole (BH) hierarchical triples, formed via binary–binary encounters in the CMC Cluster Catalog, a suite of cluster simulations with present-day properties representative of the Milky Way's globular clusters. We compare the properties of the mergers from triples to the other merger channels in dense star clusters, and show that triple systems do not produce significant differences in terms of mass and effective spin distribution. However, they represent an important pathway for forming eccentric mergers, which could be detected by LIGO–Virgo/Kamioka Gravitational-Wave Detector (LVK), and future missions such as LISA and the DECi-hertz Interferometer Gravitational wave Observatory. We derive a conservative lower limit for the merger rate from this channel of 0.35 Gpc⁻³ yr⁻¹ in the local universe and up to ~9% of these events may have a detectable eccentricity at LVK design sensitivity. Additionally, we find that triple systems could play an important role in retaining second-generation BHs, which can later merge again in the core of the host cluster
One Channel to Rule Them All? Constraining the Origins of Binary Black Holes using Multiple Formation Pathways
The second LIGO-Virgo catalog of gravitational wave transients has more than
quadrupled the observational sample of binary black holes. We analyze this
catalog using a suite of five state-of-the-art binary black hole population
models covering a range of isolated and dynamical formation channels and infer
branching fractions between channels as well as constraints on uncertain
physical processes that impact the observational properties of mergers. Given
our set of formation models, we find significant differences between the
branching fractions of the underlying and detectable populations, and that the
diversity of detections suggests that multiple formation channels are at play.
A mixture of channels is strongly preferred over any single channel dominating
the detected population: an individual channel does not contribute to more than
of the observational sample of binary black holes. We calculate
the preference between the natal spin assumptions and common envelope
efficiencies in our models, favoring natal spins of isolated black holes of
, and marginally preferring common envelope efficiencies of
while strongly disfavoring highly inefficient common envelopes.
We show that it is essential to consider multiple channels when interpreting
gravitational wave catalogs, as inference on branching fractions and physical
prescriptions becomes biased when contributing formation scenarios are not
considered or incorrect physical prescriptions are assumed. Although our
quantitative results can be affected by uncertain assumptions in model
predictions, our methodology is capable of including models with updated
theoretical considerations and additional formation channels.Comment: 27 pages (14 pages main text + 13 pages appendices/references), 8
figures, 1 table, published in Ap
Microbial translocation and microbiome dsybiosis in HIV-associated immune activation.
CAPRISA, 2016.Abstract available in pdf
Magnetic Impurity in a Metal with Correlated Conduction Electrons: An Infinite Dimensions Approach
We consider the Hubbard model with a magnetic Anderson impurity coupled to a
lattice site. In the case of infinite dimensions, one-particle correlations of
the impurity electron are described by the effective Hamiltonian of the
two-impurity system. One of the impurities interacts with a bath of free
electrons and represents the Hubbard lattice, and the other is coupled to the
first impurity by the bare hybridization interaction. A study of the effective
two-impurity Hamiltonian in the frame of the 1/N expansion and for the case of
a weak conduction-electron interaction (small U) reveals an enhancement of the
usual exponential Kondo scale. However, an intermediate interaction (U/D = 1 -
3), treated by the variational principle, leads to the loss of the exponential
scale. The Kondo temperature T_K of the effective two-impurity system is
calculated as a function of the hybridization parameter and it is shown that
T_K decreases with an increase of U. The non-Fermi-liquid character of the
Kondo effect in the intermediate regime at the half filling is discussed.Comment: 12 pages with 8 PS figures, RevTe
Stress Priming in Reading and the Selective Modulation of Lexical and Sub-Lexical Pathways
Four experiments employed a priming methodology to investigate different mechanisms of stress assignment and how they are modulated by lexical and sub-lexical mechanisms in reading aloud in Italian. Lexical stress is unpredictable in Italian, and requires lexical look-up. The most frequent stress pattern (Dominant) is on the penultimate syllable [laVOro (work)], while stress on the antepenultimate syllable [MAcchina (car)] is relatively less frequent (non-Dominant). Word and pseudoword naming responses primed by words with non-dominant stress – which require whole-word knowledge to be read correctly – were compared to those primed by nonwords. Percentage of errors to words and percentage of dominant stress responses to nonwords were measured. In Experiments 1 and 2 stress errors increased for non-dominant stress words primed by nonwords, as compared to when they were primed by words. The results could be attributed to greater activation of sub-lexical codes, and an associated tendency to assign the dominant stress pattern by default in the nonword prime condition. Alternatively, they may have been the consequence of prosodic priming, inducing more errors on trials in which the stress pattern of primes and targets was not congruent. The two interpretations were investigated in Experiments 3 and 4. The results overall suggested a limited role of the default metrical pattern in word pronunciation, and showed clear effect of prosodic priming, but only when the sub-lexical mechanism prevailed
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