1,814 research outputs found
Probing the Mass Fraction of MACHOs in Extragalactic Halos
Current microlensing searches calibrate the mass fraction of the Milky Way
halo which is in the form of Massive Compact Halo Objects (MACHOs). We show
that surveys like the Sloan Digital Sky Survey (SDSS) can probe the same
quantity in halos of distant galaxies. Microlensing of background quasars by
MACHOs in intervening galaxies would distort the equivalent width distribution
of the quasar emission lines by an amplitude that depends on the projected
quasar-galaxy separation. For a statistical sample of detectable at the >2sigma
level out to a quasar-galaxy impact parameter of several tens of kpc, as long
as extragalactic halos are made of MACHOs. Detection of this signal would test
whether the MACHO fraction inferred for the Milky-Way halo is typical of other
galaxies.Comment: 12 pages, 2 figures, submitted to ApJ Letter
X-Ray Absorption by the Hot Intergalactic Medium
The current census of observed baryons in the local Universe is still missing
a significant fraction of them according to standard Big-Bang nucleosynthesis.
Numerical simulations predict that most of the missing baryons are in a hot
intergalactic medium, which is difficult to observe through its X-ray emission
or Sunyaev-Zel'dovich effect. We show that the next generation of X-ray
satellites will be able to detect this gas through the X-ray absorption lines
imprinted by its highly-ionized metals on the spectrum of a background quasar.
For the metallicity typically found in intracluster gas, up to 70% of the
baryons produce O VIII absorption lines with an equivalent width >0.1eV. The
spectrum of any high redshift quasar is expected to show several such lines per
unit redshift due to intervening gaseous halos of galaxy groups. These lines
will be detectable at a signal-to-noise ratio >5 after a day of integration
with the future Constellation-X telescope for any of the several hundred
brightest quasars across the sky.Comment: 9 pages, 2 figures, submitted to ApJ
Hyper-Eddington accretion in GRB
Popularmo dels of the GRB origin associate this event with a cosmic explosion, birth of a stellar mass black hole and jet ejection. Due to the shock collisions that happen in the jet, the gamma rays are produced and we detect a burst of duration up to several tens of seconds. This burst duration is determined by the lifetime of the central engine, which may be different in various scenarios.
Characteristically, the observed bursts have a bimodal distribution and constitute the two classes: short (t < 2 s) and long bursts. Theoretical models invoke the mergers of two neutron stars or a neutron star with a black hole, or, on the other hand, a massive starexplosion (collapsar). In any of these models we have a phase of disc accretion onto a newly born black hole: the disc is formed from the disrupted neutron star or fed by the material fallback from the ejected collapsar envelope. The disc is extremely hot and dense, and the accretion rate is orders of magnitude higher than the Eddington rate. In such physical conditions the main cooling mechanism is neutrino emission, and one of possible ways of energy extraction from the accretion disc is the neutrino-antineutrino annihilation
The rest-frame ultraviolet spectra of GRBs from massive rapidly-rotating stellar progenitors
The properties of a massive star prior to its final explosion are imprinted
in the circumstellar medium (CSM) created by its wind and termination shock. We
perform a detailed, comprehensive calculation of the time-variable and
angle-dependent transmission spectra of an average-luminosity Gamma-Ray Burst
(GRB) which explodes in the CSM structure produced by the collapse of a 20
Msun, rapidly rotating, Z=0.001 progenitor star. We study both the case in
which metals are initially in the gaseous phase, as well as the situation in
which they are heavily depleted into dust. We find that high-velocity lines
from low-ionization states of silicon, carbon, and iron are initially present
in the spectrum only if the metals are heavily depleted into dust prior to the
GRB explosion. However, such lines disappear on timescales of a fraction of a
second for a burst observed on-axis, and of a few seconds for a burst seen at
high-latitude, making their observation virtually impossible. Rest-frame lines
produced in the termination shock are instead clearly visible in all
conditions. We conclude that time-resolved, early-time spectroscopy is not a
promising way in which the properties of the GRB progenitor wind can be
routinely studied. Previous detections of high velocity features in GRB UV
spectra must have been due either due to a superposition of a physically
unrelated absorber or to a progenitor star with very unusual properties.Comment: Published in MNRAS; higher resolution figures in published version
The Atg1-kinase complex tethers Atg9-vesicles to initiate autophagy
Autophagosomes are double-membrane vesicles that sequester cytoplasmic material for lysosomal degradation. Their biogenesis is initiated by recruitment of Atg9-vesicles to the phagophore assembly site. This process depends on the regulated activation of the Atg1-kinase complex. However, the underlying molecular mechanism remains unclear. Here we reconstitute this early step in autophagy from purified components in vitro. We find that on assembly from its cytoplasmic subcomplexes, the Atg1-kinase complex becomes activated, enabling it to recruit and tether Atg9-vesicles. The scaffolding protein Atg17 targets the Atg1-kinase complex to autophagic membranes by specifically recognizing the membrane protein Atg9. This interaction is inhibited by the two regulatory subunits Atg31 and Atg29. Engagement of the Atg1-Atg13 subcomplex restores the Atg9-binding and membrane-tethering activity of Atg17. Our data help to unravel the mechanism that controls Atg17-mediated tethering of Atg9-vesicles, providing the molecular basis to understand initiation of autophagosome-biogenesis
Recent Advances of the HalogenâZinc Exchange Reaction
For the preparation of zinc organometallics bearing highly sensitive functional groups such as ketones, aldehydes or nitro groups, especially mild halogenâzinc exchange reagents have proven to be of great potential. In this Minireview, the latest research in the area of the halogenâzinc exchange reaction is reported, with a special focus lying on novel dialkylzinc reagents complexed with lithium alkoxides. Additionally, the preparation and application of organofluorine zinc reagents and transitionâmetalâcatalyzed halogenâzinc exchange reactions are reviewed
Preliminary Results of Using a Student Success Facilitator in an Accelerated Baccalaureate Nursing Program
Competing demands of work, family, friends, and the academic rigor while in an accelerated baccalaureate nursing program often lead to high levels of anxiety and may contribute to unsuccessful student progression. This phenomena is not unique to the UNLV School of Nursing but is an issue that nursing programs grapple with nationwide. While there is nursing research literature related to student success programs there are limited studies that describe the use of a Student Success Facilitator (SSF) coupled with a Student to Student Mentoring Program (SSMP). IRB Exempt #1205-416
Characterisation of spatial network-like patterns from junctions' geometry
We propose a new method for quantitative characterization of spatial
network-like patterns with loops, such as surface fracture patterns, leaf vein
networks and patterns of urban streets. Such patterns are not well
characterized by purely topological estimators: also patterns that both look
different and result from different morphogenetic processes can have similar
topology. A local geometric cue -the angles formed by the different branches at
junctions- can complement topological information and allow to quantify the
large scale spatial coherence of the pattern. For patterns that grow over time,
such as fracture lines on the surface of ceramics, the rank assigned by our
method to each individual segment of the pattern approximates the order of
appearance of that segment. We apply the method to various network-like
patterns and we find a continuous but sharp dichotomy between two classes of
spatial networks: hierarchical and homogeneous. The first class results from a
sequential growth process and presents large scale organization, the latter
presents local, but not global organization.Comment: version 2, 14 page
Special Issue: Advances in SARS-CoV-2 Infection
Since the beginning of the pandemic, and particularly during the first and second waves, the goal of this Special Issue has been to highlight the crucial aspects of SARS- CoV-2 and the disease in light of the new knowledge that was emerging. A total of 15 manuscripts have been published in this Special Issue. These papers provided insights into epidemiology, pathogenesis, epigenetics COVID-19 emergencies in hospital settings, advanced diagnosis, vaccination, and SARS-CoV-2 infection in the experimental setting. The high scientific rigor, originality, and, for some of them, the high number of citations obtained, are well evident
Effects of Dust on Gravitational Lensing by Spiral Galaxies
Gravitational lensing of an optical QSO by a spiral galaxy is often
counteracted by dust obscuration, since the line-of-sight to the QSO passes
close to the center of the galactic disk. The dust in the lens is likely to be
correlated with neutral hydrogen, which in turn should leave a Lyman-alpha
absorption signature on the QSO spectrum. We use the estimated dust-to-gas
ratio of the Milky-Way galaxy as a mean and allow a spread in its values to
calculate the effects of dust on lensing by low redshift spiral galaxies. Using
a no-evolution model for spirals at z<1 we find (in Lambda=0 cosmologies) that
the magnification bias due to lensing is stronger than dust obscuration for QSO
samples with a magnitude limit B<16. The density parameter of neutral hydrogen,
Omega_HI, is overestimated in such samples and is underestimated for fainter
QSOs.Comment: 18 pages, 4 figures, ApJ, in pres
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