1,192 research outputs found
On Dust Extinction of Gamma-ray Burst Host Galaxies
Although it is well recognized that gamma-ray burst (GRB) afterglows are
obscured and reddened by dust in their host galaxies, the wavelength-dependence
and quantity of dust extinction are still poorly known. Current studies on this
mostly rely on fitting the afterglow spectral energy distributions (SEDs) with
template extinction models. The inferred extinction (both quantity and
wavelength-dependence) and dust-to-gas ratios are often in disagreement with
that obtained from dust depletion and X-ray spectroscopy studies. We argue that
this discrepancy could result from the prior assumption of a template
extinction law. We propose an analytical formula to approximate the GRB host
extinction law. With the template extinction laws self-contained, and the
capability of revealing extinction laws differing from the conventional ones,
it is shown that this is a powerful approach in modeling the afterglow SEDs to
derive GRB host extinction.Comment: 9 pages, 4 figures; The Astrophysical Journal, in press (2008 Oct 1
issue
Differential spot-size focus servo
We describe performance of a differential spot-size (wax-wane) focus servo. Crosstalk from the tracks are analyzed in the single detector and differential focus circuits. Magnitude of the crosstalk is reduced by a factor of three in the differential circuit. A false focus-error signal (FES) is present when the spot crosses sector marks at an angle
Another short-burst host galaxy with an optically obscured high star formation rate: The case of GRB 071227
We report on radio continuum observations of the host galaxy of the short
gamma-ray burst 071227 (z=0.381) with the Australia Telescope Compact Array
(ATCA). We detect the galaxy in the 5.5 GHz band with an integrated flux
density of Fnu = 43 +/- 11 microJy, corresponding to an unobscured
star-formation rate (SFR) of about 24 Msun/yr, forty times higher than what was
found from optical emission lines. Among the ~30 well-identified and studied
host galaxies of short bursts this is the third case where the host is found to
undergo an episode of intense star formation. This suggests that a fraction of
all short-burst progenitors hosted in star-forming galaxies could be physically
related to recent star formation activity, implying a relatively short merger
time scale.Comment: 6 pages, ApJ, accepted for publicatio
Super-Maxwellian Helium Evaporation from Pure and Salty Water
Helium atoms evaporate from pure water and salty solutions in super-Maxwellian speed distributions, as observed experimentally and modeled theoretically.The experiments are performed by monitoring the velocities of dissolved He atoms that evaporate from microjets of pure water at 252 K and 4–8.5 molal LiCl and LiBr at 232–252 K. The average He atom energies exceed the flux-weighted Maxwell-Boltzmann average of 2RT by 30% for pure water and 70% for 8.5m LiBr. Classical molecular dynamics simulations closely reproduce the observed speed distributions and provide microscopic insight into the forces that eject the He atoms from solution. Comparisons of the density profile and He kinetic energies across the water-vacuum interface indicate that the He atoms are accelerated by He–water collisions within the top 1-2 layers of the liquid. We also find that the average He atom kinetic energy scales with the free energy of solvation of this sparingly soluble gas. This free-energydifference reflects the steeply decreasing potential of mean force on the He atoms in the interfacial region, whose gradient is the repulsive force that tends to expel the atoms. The accompanying sharp decrease in water density suppresses the He–water collisions that would otherwise maintain a Maxwell-Boltzmanndistribution, allowing the He atom to escape at high energies. Helium is especially affected by this reduction in collisions because its weak interactions make energy transfer inefficient
The Mammalian Cell Cycle Regulates Parvovirus Nuclear Capsid Assembly.
It is unknown whether the mammalian cell cycle could impact the assembly of viruses maturing in the nucleus. We addressed this question using MVM, a reference member of the icosahedral ssDNA nuclear parvoviruses, which requires cell proliferation to infect by mechanisms partly understood. Constitutively expressed MVM capsid subunits (VPs) accumulated in the cytoplasm of mouse and human fibroblasts synchronized at G0, G1, and G1/S transition. Upon arrest release, VPs translocated to the nucleus as cells entered S phase, at efficiencies relying on cell origin and arrest method, and immediately assembled into capsids. In synchronously infected cells, the consecutive virus life cycle steps (gene expression, proteins nuclear translocation, capsid assembly, genome replication and encapsidation) proceeded tightly coupled to cell cycle progression from G0/G1 through S into G2 phase. However, a DNA synthesis stress caused by thymidine irreversibly disrupted virus life cycle, as VPs became increasingly retained in the cytoplasm hours post-stress, forming empty capsids in mouse fibroblasts, thereby impairing encapsidation of the nuclear viral DNA replicative intermediates. Synchronously infected cells subjected to density-arrest signals while traversing early S phase also blocked VPs transport, resulting in a similar misplaced cytoplasmic capsid assembly in mouse fibroblasts. In contrast, thymidine and density arrest signals deregulating virus assembly neither perturbed nuclear translocation of the NS1 protein nor viral genome replication occurring under S/G2 cycle arrest. An underlying mechanism of cell cycle control was identified in the nuclear translocation of phosphorylated VPs trimeric assembly intermediates, which accessed a non-conserved route distinct from the importin α2/β1 and transportin pathways. The exquisite cell cycle-dependence of parvovirus nuclear capsid assembly conforms a novel paradigm of time and functional coupling between cellular and virus life cycles. This junction may determine the characteristic parvovirus tropism for proliferative and cancer cells, and its disturbance could critically contribute to persistence in host tissues
Searching for differences in Swift's intermediate GRBs
Gamma-ray bursts are usually classified through their high-energy emission
into short-duration and long-duration bursts, which presumably reflect two
different types of progenitors. However, it has been shown on statistical
grounds that a third, intermediate population is needed in this classification
scheme, although an extensive study of the properties of this class has so far
not been done. The large amount of follow-up studies generated during the Swift
era allows us to have a suficient sample to attempt a study of this third
population through the properties of their prompt emission and their
afterglows. Our study is focused on a sample of GRBs observed by Swift during
its first four years of operation. The sample contains those bursts with
measured redshift since this allows us to derive intrinsic properties.
Intermediate bursts are less energetic and have dimmer afterglows than long
GRBs, especially when considering the X-ray light curves, which are on average
one order of magnitude fainter than long bursts. There is a less significant
trend in the redshift distribution that places intermediate bursts closer than
long bursts. Except for this, intermediate bursts show similar properties to
long bursts. In particular, they follow the Epeak vs. Eiso correlation and
have, on average, positive spectral lags with a distribution similar to that of
long bursts. Like long GRBs, they normally have an associated supernova,
although some intermediate bursts have shown no supernova component. This study
shows that intermediate bursts are different from short bursts and, in spite of
sharing many properties with long bursts, there are some differences between
them as well. We suggest that the physical difference between intermediate and
long bursts could be that for the first the ejecta are thin shells while for
the latter they are thick shells.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 17
figures, 5 table
GRB 130427A Afterglow: A Test for GRB Models
Gamma-ray Burst 130427A had the largest fluence for almost 30 years. With an isotropic energy output of 8.5×1053 erg and redshift of 0.34, it combined a very high energy release with a relative proximity to Earth in an unprecedented fashion. Sensitive X-ray facilities such as {\it XMM-Newton} and {\it Chandra} detected the afterglow of this event for a record-breaking baseline of 90 Ms. We show the X-ray light curve of GRB 130427A of this event over such an interval. The light curve shows an unbroken power law decay with a slope of α=1.31 over more than three decades in time. In this presentation, we investigate the consequences of this result for the scenarios proposed to interpret GRB 130427A and the implications in the context of the forward shock model (jet opening angle, energetics, surrounding medium). We also remark the chance of extending GRB afterglow observations for several hundreds of Ms with {\it Athena}
A quiescent galaxy at the position of the long GRB 050219A
Long-duration gamma-ray bursts (LGRBs) are produced by the collapse of very
massive stars. Due to the short lifetime of their progenitors, LGRBs pinpoint
star-forming galaxies. We present here a multi-band search for the host galaxy
of the long dark GRB 050219A within the enhanced Swift/XRT error circle. We
used spectroscopic observations acquired with VLT/X-shooter to determine the
redshift and star-formation rate of the putative host galaxy. We compared the
results with the optical/IR spectral energy distribution obtained with
different facilities. Surprisingly, the host galaxy is a old and quiescent
early-type galaxy at z = 0.211 characterised by an unprecedentedly low specific
star-formation rate. It is the first LGRB host to be also an early-type
post-starburst galaxy. This is further evidence that GRBs can explode in all
kind of galaxies, with the only requirement being an episode of star-formation.Comment: 11 pages, 10 figures, 3 tables; accepted for publication in Astronomy
& Astrophysic
Test of Lorentz Invariance in Electrodynamics Using Rotating Cryogenic Sapphire Microwave Oscillators
We present the first results from a rotating Michelson-Morley experiment that
uses two orthogonally orientated cryogenic sapphire resonator-oscillators
operating in whispering gallery modes near 10 GHz. The experiment is used to
test for violations of Lorentz Invariance in the frame-work of the photon
sector of the Standard Model Extension (SME), as well as the isotropy term of
the Robertson-Mansouri-Sexl (RMS) framework. In the SME we set a new bound on
the previously unmeasured component of
, and set more stringent bounds by up to a factor of 7
on seven other components. In the RMS a more stringent bound of
on the isotropy parameter, is set, which is more than a factor of 7 improvement. More detailed
description of the experiment and calculations can be found in: hep-ph/0506200Comment: Final published version, 4 pages, references adde
The distribution of equivalent widths in long GRB afterglow spectra
The extreme brightness of gamma-ray burst (GRB) afterglows and their simple
spectral shape make them ideal beacons to study the interstellar medium of
their host galaxies through absorption line spectroscopy. Using 69
low-resolution GRB afterglow spectra, we conduct a study of the rest-frame
equivalent width (EW) distribution of features with an average rest-frame EW
larger than 0.5 A. To compare an individual GRB with the sample, we develop EW
diagrams as a graphical tool, and we give a catalogue with diagrams for the 69
spectra. We introduce a line strength parameter (LSP) that allows us to
quantify the strength of the absorption features as compared to the sample by a
single number. Using the distributions of EWs of single-species features, we
derive the distribution of column densities by a curve of growth (CoG) fit. We
find correlations between the LSP and the extinction of the GRB, the UV
brightness of the host galaxies and the neutral hydrogen column density.
However, we see no significant evolution of the LSP with the redshift. There is
a weak correlation between the ionisation of the absorbers and the energy of
the GRB, indicating that, either the GRB event is responsible for part of the
ionisation, or that galaxies with high-ionisation media produce more energetic
GRBs. Spectral features in GRB spectra are, on average, 2.5 times stronger than
those seen in QSO intervening damped Lyman-alpha (DLA) systems and slightly
more ionised. In particular we find larger excess in the EW of CIV1549 relative
to QSO DLAs, which could be related to an excess of Wolf-Rayet stars in the
environments of GRBs. From the CoG fitting we obtain an average number of
components in the absorption features of GRBs of 6.00(-1.25,+1.00). The most
extreme ionisation ratios in our sample are found for GRBs with low neutral
hydrogen column density, which could be related to ionisation by the GRB
emission.Comment: 37 pages, 31 figures, 15 tables. Accepted for publication in Astonomy
and Astrophysic
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