810 research outputs found
X-ray Destruction of Dust Along the Line of Sight to Gamma-Ray Bursts
We show that if all gamma-ray bursts emit X-rays in a way similar to those
observed by BeppoSax, much of the extinction along the line of sight in the
host galaxy of the burst can be destroyed. Two mechanisms are principally
responsible for dust destruction: grain heating and grain charging. The latter,
which can lead to electrostatic stresses greater than the tensile strength of
the grains, is often the more important. Grains may regularly be destroyed at
distances as large as 100 parsecs. This dust destruction can permit us to see
the UV/optical afterglow even when the burst is embedded deep within a
highly-obscured region. Because the destruction rate depends on grain
composition and size, it may be possible to observe the amount and
wavelength-dependence of extinction change during the course of the burst and
first few minutes of the afterglow. It may also be possible to detect
interstellar absorption lines in the afterglow spectrum that would not exist
but for the return of heavy elements to the gas phase.Comment: Accepted for publication in The Astrophysical Journal. Uses AASTeX.
31 pages, including a six-panel figur
Perinatal Hypoxia-Ischemia Disrupts Striatal High-Affinity [ 3 H]Glutamate Uptake into Synaptosomes
: We examined the impact of hypoxia-ischemia on high-affinity [ 3 H]glutamate uptake into a synaptosomal fraction prepared from immature rat corpus striatum. In 7-day-old pups the right carotid artery was ligated, and pups were exposed to 8% oxygen for 0, 0.5, 1, or 2.5 h, and allowed to recover for up to 24 h before they were killed. High-affinity glutamate uptakes in striatal synaptosomes derived from tissue ipsilateral and contralateral to ligation were compared. After 1 h of hypoxia plus ischemia, high-affinity glutamate uptake in the striatum was reduced by 54 ± 13% compared with values from the opposite (nonischemic) side of the brain (p < 0.01, t test versus ligates not exposed to hypoxia). There were similar declines after 2.5 h of hypoxiaischemia. Activity remained low after a 1 h recovery period in room air, but after 24 h of recovery, high-affinity glutamate uptake was equal bilaterally. Kinetic analysis revealed that loss of activity could be attributed primarily to a 40% reduction in the number of uptake sites. Hypoxia alone had no effect on high-affinity glutamate uptake although it reduced synaptosomal uptake of [ 3 H]3,4-dihydroxyphenyl-ethylamine. Addition of 1 mg/ml of bovine serum albumin to the incubation medium preferentia'ly stimulated high-affinity glutamate uptake in hypoxic-ischemic brain compared with its effects in normal tissue. These studies demonstrate that hypoxia-ischemia reversibly inhibits high-affinity glutamate uptake and this occurs earlier than the time required to produce neuronal damage in the model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66361/1/j.1471-4159.1986.tb00803.x.pd
SN 2009kf : a UV bright type IIP supernova discovered with Pan-STARRS 1 and GALEX
We present photometric and spectroscopic observations of a luminous type IIP
Supernova 2009kf discovered by the Pan-STARRS 1 (PS1) survey and detected also
by GALEX. The SN shows a plateau in its optical and bolometric light curves,
lasting approximately 70 days in the rest frame, with absolute magnitude of M_V
= -18.4 mag. The P-Cygni profiles of hydrogen indicate expansion velocities of
9000km/s at 61 days after discovery which is extremely high for a type IIP SN.
SN 2009kf is also remarkably bright in the near-ultraviolet (NUV) and shows a
slow evolution 10-20 days after optical discovery. The NUV and optical
luminosity at these epochs can be modelled with a black-body with a hot
effective temperature (T ~16,000 K) and a large radius (R ~1x10^{15} cm). The
bright bolometric and NUV luminosity, the lightcurve peak and plateau duration,
the high velocities and temperatures suggest that 2009kf is a type IIP SN
powered by a larger than normal explosion energy. Recently discovered high-z
SNe (0.7 < z < 2.3) have been assumed to be IIn SNe, with the bright UV
luminosities due to the interaction of SN ejecta with a dense circumstellar
medium (CSM). UV bright SNe similar to SN 2009kf could also account for these
high-z events, and its absolute magnitude M_NUV = -21.5 +/- 0.5 mag suggests
such SNe could be discovered out to z ~2.5 in the PS1 survey.Comment: Accepted for publication in APJ
The extraordinarily bright optical afterglow of GRB 991208 and its host galaxy
Observations of the extraordinarily bright optical afterglow (OA) of GRB
991208 started 2.1 d after the event. The flux decay constant of the OA in the
R-band is -2.30 +/- 0.07 up to 5 d, which is very likely due to the jet effect,
and after that it is followed by a much steeper decay with constant -3.2 +/-
0.2, the fastest one ever seen in a GRB OA. A negative detection in several
all-sky films taken simultaneously to the event implies either a previous
additional break prior to 2 d after the occurrence of the GRB (as expected from
the jet effect). The existence of a second break might indicate a steepening in
the electron spectrum or the superposition of two events. Once the afterglow
emission vanished, contribution of a bright underlying SN is found, but the
light curve is not sufficiently well sampled to rule out a dust echo
explanation. Our determination of z = 0.706 indicates that GRB 991208 is at 3.7
Gpc, implying an isotropic energy release of 1.15 x 10E53 erg which may be
relaxed by beaming by a factor > 100. Precise astrometry indicates that the GRB
coincides within 0.2" with the host galaxy, thus given support to a massive
star origin. The absolute magnitude is M_B = -18.2, well below the knee of the
galaxy luminosity function and we derive a star-forming rate of 11.5 +/- 7.1
Mo/yr. The quasi-simultaneous broad-band photometric spectral energy
distribution of the afterglow is determined 3.5 day after the burst (Dec 12.0)
implying a cooling frequency below the optical band, i.e. supporting a jet
model with p = -2.30 as the index of the power-law electron distribution.Comment: Accepted for publication in Astronomy and Astrophysics, 9 pages, 6
figures (Fig. 3 and Fig. 4 have been updated
Swift panchromatic observations of the bright gamma-ray burst GRB050525a
The bright gamma-ray burst GRB050525a has been detected with the Swift
observatory, providing unique multiwavelength coverage from the very earliest
phases of the burst. The X-ray and optical/UV afterglow decay light curves both
exhibit a steeper slope ~0.15 days after the burst, indicative of a jet break.
This jet break time combined with the total gamma-ray energy of the burst
constrains the opening angle of the jet to be 3.2 degrees. We derive an
empirical `time-lag' redshift from the BAT data of z_hat = 0.69 +/- 0.02, in
good agreement with the spectroscopic redshift of 0.61.
Prior to the jet break, the X-ray data can be modelled by a simple power law
with index alpha = -1.2. However after 300 s the X-ray flux brightens by about
30% compared to the power-law fit. The optical/UV data have a more complex
decay, with evidence of a rapidly falling reverse shock component that
dominates in the first minute or so, giving way to a flatter forward shock
component at later times.
The multiwavelength X-ray/UV/Optical spectrum of the afterglow shows evidence
for migration of the electron cooling frequency through the optical range
within 25000 s. The measured temporal decay and spectral indices in the X-ray
and optical/UV regimes compare favourably with the standard fireball model for
Gamma-ray bursts assuming expansion into a constant density interstellar
medium.Comment: 31 pages, 7 figures, referee comments implemented, typo corrected in
author list, accepted by Ap
Long gamma-ray bursts and core-collapse supernovae have different environments
When massive stars exhaust their fuel they collapse and often produce the
extraordinarily bright explosions known as core-collapse supernovae. On
occasion, this stellar collapse also powers an even more brilliant relativistic
explosion known as a long-duration gamma-ray burst. One would then expect that
long gamma-ray bursts and core-collapse supernovae should be found in similar
galactic environments. Here we show that this expectation is wrong. We find
that the long gamma-ray bursts are far more concentrated on the very brightest
regions of their host galaxies than are the core-collapse supernovae.
Furthermore, the host galaxies of the long gamma-ray bursts are significantly
fainter and more irregular than the hosts of the core-collapse supernovae.
Together these results suggest that long-duration gamma-ray bursts are
associated with the most massive stars and may be restricted to galaxies of
limited chemical evolution. Our results directly imply that long gamma-ray
bursts are relatively rare in galaxies such as our own Milky Way.Comment: 27 pages, 4 figures, submitted to Nature on 22 August 2005, revised 9
February 2006, online publication 10 May 2006. Supplementary material
referred to in the text can be found at
http://www.stsci.edu/~fruchter/GRB/locations/supplement.pdf . This new
version contains minor changes to match the final published versio
Implementation and testing of the first prompt search for gravitational wave transients with electromagnetic counterparts
Aims. A transient astrophysical event observed in both gravitational wave
(GW) and electromagnetic (EM) channels would yield rich scientific rewards. A
first program initiating EM follow-ups to possible transient GW events has been
developed and exercised by the LIGO and Virgo community in association with
several partners. In this paper, we describe and evaluate the methods used to
promptly identify and localize GW event candidates and to request images of
targeted sky locations.
Methods. During two observing periods (Dec 17 2009 to Jan 8 2010 and Sep 2 to
Oct 20 2010), a low-latency analysis pipeline was used to identify GW event
candidates and to reconstruct maps of possible sky locations. A catalog of
nearby galaxies and Milky Way globular clusters was used to select the most
promising sky positions to be imaged, and this directional information was
delivered to EM observatories with time lags of about thirty minutes. A Monte
Carlo simulation has been used to evaluate the low-latency GW pipeline's
ability to reconstruct source positions correctly.
Results. For signals near the detection threshold, our low-latency algorithms
often localized simulated GW burst signals to tens of square degrees, while
neutron star/neutron star inspirals and neutron star/black hole inspirals were
localized to a few hundred square degrees. Localization precision improves for
moderately stronger signals. The correct sky location of signals well above
threshold and originating from nearby galaxies may be observed with ~50% or
better probability with a few pointings of wide-field telescopes.Comment: 17 pages. This version (v2) includes two tables and 1 section not
included in v1. Accepted for publication in Astronomy & Astrophysic
Bioturbation in a Declining Oxygen Environment, in situ Observations from Wormcam
Bioturbation, the displacement and mixing of sediment particles by fauna or flora, facilitates life supporting processes by increasing the quality of marine sediments. In the marine environment bioturbation is primarily mediated by infaunal organisms, which are susceptible to perturbations in their surrounding environment due to their sedentary life history traits. Of particular concern is hypoxia, dissolved oxygen (DO) concentrations ≤2.8 mg l−1, a prevalent and persistent problem that affects both pelagic and benthic fauna. A benthic observing system (Wormcam) consisting of a buoy, telemetering electronics, sediment profile camera, and water quality datasonde was developed and deployed in the Rappahannock River, VA, USA, in an area known to experience seasonal hypoxia from early spring to late fall. Wormcam transmitted a time series of in situ images and water quality data, to a website via wireless internet modem, for 5 months spanning normoxic and hypoxic periods. Hypoxia was found to significantly reduce bioturbation through reductions in burrow lengths, burrow production, and burrowing depth. Although infaunal activity was greatly reduced during hypoxic and near anoxic conditions, some individuals remained active. Low concentrations of DO in the water column limited bioturbation by infaunal burrowers and likely reduced redox cycling between aerobic and anaerobic states. This study emphasizes the importance of in situ observations for understanding how components of an ecosystem respond to hypoxia
The Collimation and Energetics of the Brightest Swift Gamma-Ray Bursts
Long-duration gamma-ray bursts (GRBs) are widely believed to be
highly-collimated explosions (opening angle theta ~ 1-10 deg). As a result of
this beaming factor, the true energy release from a GRB is usually several
orders of magnitude smaller than the observed isotropic value. Measuring this
opening angle, typically inferred from an achromatic steepening in the
afterglow light curve (a "jet" break), has proven exceedingly difficult in the
Swift era. Here we undertake a study of five of the brightest (in terms of the
isotropic prompt gamma-ray energy release, E(gamma, iso)) GRBs in the Swift era
to search for jet breaks and hence constrain the collimation-corrected energy
release. We present multi-wavelength (radio through X-ray) observations of GRBs
050820A, 060418, and 080319B, and construct afterglow models to extract the
opening angle and beaming-corrected energy release for all three events.
Together with results from previous analyses of GRBs 050904 and 070125, we find
evidence for an achromatic jet break in all five events, strongly supporting
the canonical picture of GRBs as collimated explosions. The most natural
explanation for the lack of observed jet breaks from most Swift GRBs is
therefore selection effects. However, the opening angles for the events in our
sample are larger than would be expected if all GRBs had a canonical energy
release of ~ 10e51 erg. The total energy release we measure for those
"hyper-energetic" (E(total) >~ 10e52 erg) events in our sample is large enough
to start challenging models with a magnetar as the compact central remnant.Comment: Submitted to ApJ, comments welcom
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