99 research outputs found
Massive disk formation in the tidal disruption of a neutron star by a nearly extremal black hole
Black hole-neutron star (BHNS) binaries are important sources of
gravitational waves for second-generation interferometers, and BHNS mergers are
also a proposed engine for short, hard gamma-ray bursts. The behavior of both
the spacetime (and thus the emitted gravitational waves) and the neutron star
matter in a BHNS merger depend strongly and nonlinearly on the black hole's
spin. While there is a significant possibility that astrophysical black holes
could have spins that are nearly extremal (i.e. near the theoretical maximum),
to date fully relativistic simulations of BHNS binaries have included
black-hole spins only up to =0.9, which corresponds to the black hole
having approximately half as much rotational energy as possible, given the
black hole's mass. In this paper, we present a new simulation of a BHNS binary
with a mass ratio and black-hole spin =0.97, the highest simulated
to date. We find that the black hole's large spin leads to the most massive
accretion disk and the largest tidal tail outflow of any fully relativistic
BHNS simulations to date, even exceeding the results implied by extrapolating
results from simulations with lower black-hole spin. The disk appears to be
remarkably stable. We also find that the high black-hole spin persists until
shortly before the time of merger; afterwards, both merger and accretion spin
down the black hole.Comment: 20 pages, 10 figures, submitted to Classical and Quantum Gravit
Improved methods for simulating nearly extremal binary black holes
Astrophysical black holes could be nearly extremal (that is, rotating nearly
as fast as possible); therefore, nearly extremal black holes could be among the
binaries that current and future gravitational-wave observatories will detect.
Predicting the gravitational waves emitted by merging black holes requires
numerical-relativity simulations, but these simulations are especially
challenging when one or both holes have mass and spin exceeding the
Bowen-York limit of . We present improved methods that enable us to
simulate merging, nearly extremal black holes more robustly and more
efficiently. We use these methods to simulate an unequal-mass, precessing
binary black hole coalescence, where the larger black hole has . We
also use these methods to simulate a non-precessing binary black hole
coalescence, where both black holes have , nearly reaching the
Novikov-Thorne upper bound for holes spun up by thin accretion disks. We
demonstrate numerical convergence and estimate the numerical errors of the
waveforms; we compare numerical waveforms from our simulations with
post-Newtonian and effective-one-body waveforms; we compare the evolution of
the black-hole masses and spins with analytic predictions; and we explore the
effect of increasing spin magnitude on the orbital dynamics (the so-called
"orbital hangup" effect).Comment: 18 pages, 18 figure
Bcl11b—A Critical Neurodevelopmental Transcription Factor—Roles in Health and Disease
B cell leukemia 11b (Bcl11b) is a zinc finger protein transcription factor with a multiplicity of functions. It works as both a genetic suppressor and activator, acting directly, attaching to promoter regions, as well as indirectly, attaching to promoter-bound transcription factors. Bcl11b is a fundamental transcription factor in fetal development, with important roles for the differentiation and development of various neuronal subtypes in the central nervous system (CNS). It has been used as a specific marker of layer V subcerebral projection neurons as well as striatal interneurons. Bcl11b also has critical developmental functions in the immune, integumentary and cardiac systems, to the extent that Bcl11b knockout mice are incompatible with extra-uterine life. Bcl11b has been implicated in a number of disease states including Huntington’s disease, Alzheimer’s disease, HIV and T-Cell malignancy, amongst others. Bcl11b is a fascinating protein whose critical roles in the CNS and other parts of the body are yet to be fully explicated. This review summarizes the current literature on Bcl11b and its functions in development, health, and disease as well as future directions for research
The ‘Facebook\u27 Effect: College Students\u27 Perceptions of Online Discussions in the Age of Social Networking
Despite the growing prominence of Facebook in the lives of college students, few studies have investigated the potential of these innovative web-based communication tools for engaging students in academic discussions. This study used a pre-test, post-test design in two introductory-level courses at a large public university to compare students’ (n = 107) perceptions of, attitudes toward, and perceived learning associated with two different online discussion tools: the Facebook group forum and a university-sponsored online tool. Although pre-course surveys indicated that few students enjoyed online discussions, postcourse analysis revealed significant changes in students’ opinions regarding the value and functionality of web-based discussion forums, with Facebook as their clear preference. Students who participated in Facebook discussions enjoyed the site’s familiarity, navigability, and aesthetically appealing interface. Facebook users also reported that they were able to become better acquainted with classmates, felt like valued participants in the course, and learned more course material. This study suggests that, if used appropriately, Facebook may help to increase college student engagement in certain learning contexts by cultivating classroom community and stimulating intellectual discourse
The SXS Collaboration catalog of binary black hole simulations
Accurate models of gravitational waves from merging black holes are necessary
for detectors to observe as many events as possible while extracting the
maximum science. Near the time of merger, the gravitational waves from merging
black holes can be computed only using numerical relativity. In this paper, we
present a major update of the Simulating eXtreme Spacetimes (SXS) Collaboration
catalog of numerical simulations for merging black holes. The catalog contains
2018 distinct configurations (a factor of 11 increase compared to the 2013 SXS
catalog), including 1426 spin-precessing configurations, with mass ratios
between 1 and 10, and spin magnitudes up to 0.998. The median length of a
waveform in the catalog is 39 cycles of the dominant
gravitational-wave mode, with the shortest waveform containing 7.0 cycles and
the longest 351.3 cycles. We discuss improvements such as correcting for moving
centers of mass and extended coverage of the parameter space. We also present a
thorough analysis of numerical errors, finding typical truncation errors
corresponding to a waveform mismatch of . The simulations provide
remnant masses and spins with uncertainties of 0.03% and 0.1% (
percentile), about an order of magnitude better than analytical models for
remnant properties. The full catalog is publicly available at
https://www.black-holes.org/waveforms .Comment: 33+18 pages, 13 figures, 4 tables, 2,018 binaries. Catalog metadata
in ancillary JSON file. v2: Matches version accepted by CQG. Catalog
available at https://www.black-holes.org/waveform
Cosmic Explorer: The U.S. Contribution to Gravitational-Wave Astronomy beyond LIGO
This white paper describes the research and development needed over the next decade to realize "Cosmic Explorer," the U.S. node of a future third-generation detector network that will be capable of observing and characterizing compact gravitational-wave sources to cosmological redshifts
Populating the Galaxy with pulsars I: stellar & binary evolution
The computation of theoretical pulsar populations has been a major component
of pulsar studies since the 1970s. However, the majority of pulsar population
synthesis has only regarded isolated pulsar evolution. Those that have examined
pulsar evolution within binary systems tend to either treat binary evolution
poorly or evolve the pulsar population in an ad-hoc manner. Thus no complete
and direct comparison with observations of the pulsar population within the
Galactic disk has been possible to date. Described here is the first component
of what will be a complete synthetic pulsar population survey code. This
component is used to evolve both isolated and binary pulsars. Synthetic
observational surveys can then be performed on this population for a variety of
radio telescopes. The final tool used for completing this work will be a code
comprised of three components: stellar/binary evolution, Galactic kinematics
and survey selection effects. Results provided here support the need for
further (apparent) pulsar magnetic field decay during accretion, while they
conversely suggest the need for a re-evaluation of the assumed \textit{typical}
MSP formation process. Results also focus on reproducing the observed
diagram for Galactic pulsars and how this precludes short timescales
for standard pulsar exponential magnetic field decay. Finally, comparisons of
bulk pulsar population characteristics are made to observations displaying the
predictive power of this code, while we also show that under standard binary
evolutionary assumption binary pulsars may accrete much mass.Comment: 26 pages, 16 figures, 1 table, accepted for publication in MNRA
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