148 research outputs found
Collaborative analysis of solar maximum mission, Venera and Prognoz solar X-ray bursts
Efforts centered on cross-calibrating the Solar Maximum Mission (SMM) HXRBS detector with the Venera 13/14 cosmic ray burst detectors. The event was divided into six time intervals, and the best fitting SMM and Venera 13 spectra were calculated for each interval, using the individual fitting routines for the two instruments. The results are presented and discussed
Gamma-Ray Burster Counterparts: HST Blue and Ultraviolet Data
The surest solution of the Gamma Ray Burst (GRB) mystery is to find an
unambiguous low-energy quiescent counterpart. However, to date no reasonable
candidates have been identified in the x-ray, optical, infrared, or radio
ranges. The Hubble Space Telescope (HST) has now allowed for the first deep
ultraviolet searches for quiescent counterparts. This paper reports on
multiepoch ultraviolet searches of five GRB positions with HST. We found no
sources with significant ultraviolet excesses, variability, parallax, or proper
motion in any of the burst error regions. In particular, we see no sources
similar to that proposed as a counterpart to the GRB970228. While this negative
result is disappointing, it still has good utility for its strict limits on the
no-host-galaxy problem in cosmological models of GRBs. For most cosmological
models (with peak luminosity 6X10^50 erg/s), the absolute B magnitude of any
possible host galaxy must be fainter than -15.5 to -17.4. These smallest boxes
for some of the brightest bursts provide the most critical test, and our limits
are a severe problem for all published cosmological burst models.Comment: 15 pages, 2 ps figures, accepted for publication in the Astrophysical
Journa
Variable Spin-down in the Soft Gamma Repeater SGR 1900+14 and Correlations with Burst Activity
We have analyzed Rossi X-ray Timing Explorer Proportional Counter Array
observations of the pulsed emission from SGR 1900+14 during September 1996,
June - October 1998, and early 1999. Using these measurements and results
reported elsewhere, we construct a period history of this source for 2.5 years.
We find significant deviations from a steady spin-down trend during quiescence
and the burst active interval. Burst and Transient Source Experiment
observations of the burst emission are presented and correlations between the
burst activity and spin-down rate of SGR 1900+14 are discussed. We find an 80
day interval during the summer of 1998 when the average spin-down rate is
larger than the rate elsewhere by a factor ~ 2.3. This enhanced spin-down may
be the result of a discontinuous spin-down event or ``braking glitch'' at the
time of the giant flare on 27 August 1998. Furthermore, we find a large
discrepancy between the pulsar period and average spin-down rate in X-rays as
compared to radio observations for December 1998 and January 1999.Comment: 6 pages, 2 figures, submitted to ApJ Letter
RHESSI Spectral Fits of Swift GRBs
One of the challenges of the Swift era has been accurately determining Epeak
for the prompt GRB emission. RHESSI, which is sensitive from 30 keV to 17 MeV,
can extend spectral coverage above the Swift-BAT bandpass. Using the public
Swift data, we present results of joint spectral fits for 26 bursts co-observed
by RHESSI and Swift-BAT through May 2007. We compare these fits to estimates of
Epeak which rely on BAT data alone. A Bayesian Epeak estimator gives better
correspondence with our measured results than an estimator relying on
correlations with the Swift power law indices.Comment: 4 pages, 1 figure. To appear in the proceedings of Gamma Ray Bursts
2007, Santa Fe, New Mexico, November 5-9 200
Hard Burst Emission from the Soft Gamma Repeater SGR 1900+14
We present evidence for burst emission from SGR 1900+14 with a power-law high
energy spectrum extending beyond 500 keV. Unlike previous detections of high
energy photons during bursts from SGRs, these emissions are not associated with
high-luminosity burst intervals. Not only is the emission hard, but the spectra
are better fit by Band's GRB function rather than by the traditional
optically-thin thermal bremsstrahlung model. We find that the spectral
evolution within these hard events obeys a hardness/intensity anti-correlation.
Temporally, these events are distinct from typical SGR burst emissions in that
they are longer (~ 1 s) and have relatively smooth profiles. Despite a
difference in peak luminosity of > 1E+11 between these bursts from SGR 1900+14
and cosmological GRBs, there are striking temporal and spectral similarities
between the two kinds of bursts, aside from spectral evolution. We outline an
interpretation of these events in the context of the magnetar model.Comment: 11 pages (text and figures), submitted to ApJ Letters, corrected
erroneous hardness ratio
OTS 1809+314 and the Gamma-Ray Burst GB 790325b
We present deep CCD images of the field containing the recurring Optical Transient Source (OTS) 1809+314 recently discovered on the archival plates of the Sonneberg Observatory. At the position of OTS 1809+314 we find no optical counterpart brighter tha V=24.0 24.5, rgunn = 22.0-22.5 and I = 22.0 22.5, and I = 22.0-22.5. In the zero proper motion limit our observations place severe constraints on possible quiescent counterparts. There is no compelling evidence to suggest an association between the historical optical transient source and the gamma-ray burst GB 790325b located ~ 5\u27 to the east of OTS 1809+314
Potential of known and short prokaryotic protein motifs as a basis for novel peptide-based antibacterial therapeutics: a computational survey
Short linear motifs (SLiMs) are functional stretches of protein sequence that are of crucial importance for numerous biological processes by mediating protein–protein interactions. These motifs often comprise peptides of less than 10 amino acids that modulate protein–protein interactions. While well-characterized in eukaryotic intracellular signaling, their role in prokaryotic signaling is less well-understood. We surveyed the distribution of known motifs in prokaryotic extracellular and virulence proteins across a range of bacterial species and conducted searches for novel motifs in virulence proteins. Many known motifs in virulence effector proteins mimic eukaryotic motifs and enable the pathogen to control the intracellular processes of their hosts. Novel motifs were detected by finding those that had evolved independently in three or more unrelated virulence proteins. The search returned several significantly over-represented linear motifs of which some were known motifs and others are novel candidates with potential roles in bacterial pathogenesis. A putative C-terminal G[AG]. motif that has been previously identified in a plasminogen-binding protein, was demonstrated to be enriched across a number of adhesion and lipoproteins. While there is some potential to develop peptide drugs against bacterial infection based on bacterial peptides that mimic host components, this could have unwanted effects on host signaling. Thus, novel SLiMs in virulence factors that do not mimic host components but are crucial for bacterial pathogenesis, such as the type IV secretion system, may be more useful to develop as leads for anti-microbial peptides or drugs
Large Torque Variations in Two Soft Gamma Repeaters
We have monitored the pulse frequencies of the two soft gamma repeaters SGR
1806-20 and SGR 1900+14 through the beginning of year 2001 using primarily
Rossi X-ray Timing Explorer Proportional Counter Array observations. In both
sources, we observe large changes in the spin-down torque up to a factor of ~4,
which persist for several months. Using long baseline phase-connected timing
solutions as well as the overall frequency histories, we construct torque noise
power spectra for each SGR. The power spectrum of each source is very red
(power-law slope ~-3.5). The torque noise power levels are consistent with some
accreting systems on time scales of ~1 year, yet the full power spectrum is
much steeper in frequency than any known accreting source. To the best of our
knowledge, torque noise power spectra with a comparably steep frequency
dependence have only been seen in young, glitching radio pulsars (e.g. Vela).
The observed changes in spin-down rate do not correlate with burst activity,
therefore, the physical mechanisms behind each phenomenon are also likely
unrelated. Within the context of the magnetar model, seismic activity cannot
account for both the bursts and the long-term torque changes unless the
seismically active regions are decoupled from one another.Comment: 26 pages, 11 figures included, accepted for publication in ApJ,
analysis of torque noise power density spectra is revised from previous
version and minor text changes were mad
Discovery of a New Soft Gamma Repeater, SGR 1627-41
We report the discovery of a new soft gamma repeater (SGR), SGR 1627-41, and
present BATSE observations of the burst emission and BeppoSAX NFI observations
of the probable persistent X-ray counterpart to this SGR. All but one burst
spectrum are well fit by an optically thin thermal bremsstrahlung (OTTB) model
with kT values between 25 and 35 keV. The spectrum of the X-ray counterpart,
SAX J1635.8-4736, is similar to that of other persistent SGR X-ray
counterparts. We find weak evidence for a periodic signal at 6.41 s in the
light curve for this source. Like other SGRs, this source appears to be
associated with a young supernova remnant G337.0-0.1. Based upon the peak
luminosities of bursts observed from this SGR, we find a lower limit on the
dipole magnetic field of the neutron star B_dipole > 5 * 10^14 Gauss.Comment: 5 pages, 4 figures, submitted to ApJ Letter
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