350 research outputs found
Thermal to Nonthermal Energy Partition at the Early Rise Phase of Solar Flares
In some flares the thermal component appears much earlier than the nonthermal
component in X-ray range. Using sensitive microwave observations we revisit
this finding made by Battaglia et al. (2009) based on RHESSI data analysis. We
have found that nonthermal microwave emission produced by accelerated electrons
with energy of at least several hundred keV, appears as early as the thermal
soft X-ray emission indicative that the electron acceleration takes place at
the very early flare phase. The non-detection of the hard X-rays at that early
stage of the flares is, thus, an artifact of a limited RHESSI sensitivity. In
all considered events, the microwave emission intensity increases at the early
flare phase. We found that either thermal or nonthermal gyrosynchrotron
emission can dominate the low-frequency part of the microwave spectrum below
the spectral peak occurring at 3-10 GHz. In contrast, the high-frequency
optically thin part of the spectrum is always formed by the nonthermal,
accelerated electron component, whose power-law energy spectrum can extend up
to a few MeV at this early flare stage. This means that even though the total
number of accelerated electrons is small at this stage, their nonthermal
spectrum is fully developed. This implies that an acceleration process of
available seed particles is fully operational. While, creation of this seed
population (the process commonly called `injection' of the particles from the
thermal pool into acceleration) has a rather low efficiency at this stage,
although, the plasma heating efficiency is high. This imbalance between the
heating and acceleration (in favor of the heating) is difficult to reconcile
within most of available flare energization models. Being reminiscent of the
tradeoff between the Joule heating and runaway electron acceleration, it puts
additional constraints on the electron injection into the acceleration process.Comment: 11 pages, 12 figures, accepted for Ap
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Electrophysiological Guidance of Epidural Electrode Array Implantation over the Human Lumbosacral Spinal Cord to Enable Motor Function after Chronic Paralysis.
Epidural electrical stimulation (EES) of the spinal cord has been shown to restore function after spinal cord injury (SCI). Characterization of EES-evoked motor responses has provided a basic understanding of spinal sensorimotor network activity related to EES-enabled motor activity of the lower extremities. However, the use of EES-evoked motor responses to guide EES system implantation over the spinal cord and their relation to post-operative EES-enabled function in humans with chronic paralysis attributed to SCI has yet to be described. Herein, we describe the surgical and intraoperative electrophysiological approach used, followed by initial EES-enabled results observed in 2 human subjects with motor complete paralysis who were enrolled in a clinical trial investigating the use of EES to enable motor functions after SCI. The 16-contact electrode array was initially positioned under fluoroscopic guidance. Then, EES-evoked motor responses were recorded from select leg muscles and displayed in real time to determine electrode array proximity to spinal cord regions associated with motor activity of the lower extremities. Acceptable array positioning was determined based on achievement of selective proximal or distal leg muscle activity, as well as bilateral muscle activation. Motor response latencies were not significantly different between intraoperative recordings and post-operative recordings, indicating that array positioning remained stable. Additionally, EES enabled intentional control of step-like activity in both subjects within the first 5 days of testing. These results suggest that the use of EES-evoked motor responses may guide intraoperative positioning of epidural electrodes to target spinal cord circuitry to enable motor functions after SCI
The Interplanetary Network Supplement to the Fermi GBM Catalog of Cosmic Gamma-Ray Bursts
We present Interplanetary Network (IPN) data for the gamma-ray bursts in the
first Fermi Gamma-Ray Burst Monitor (GBM) catalog. Of the 491 bursts in that
catalog, covering 2008 July 12 to 2010 July 11, 427 were observed by at least
one other instrument in the 9-spacecraft IPN. Of the 427, the localizations of
149 could be improved by arrival time analysis (or triangulation). For any
given burst observed by the GBM and one other distant spacecraft, triangulation
gives an annulus of possible arrival directions whose half-width varies between
about 0.4' and 32 degrees, depending on the intensity, time history, and
arrival direction of the burst, as well as the distance between the spacecraft.
We find that the IPN localizations intersect the 1 sigma GBM error circles in
only 52% of the cases, if no systematic uncertainty is assumed for the latter.
If a 6 degree systematic uncertainty is assumed and added in quadrature, the
two localization samples agree about 87% of the time, as would be expected. If
we then multiply the resulting error radii by a factor of 3, the two samples
agree in slightly over 98% of the cases, providing a good estimate of the GBM 3
sigma error radius. The IPN 3 sigma error boxes have areas between about 1
square arcminute and 110 square degrees, and are, on the average, a factor of
180 smaller than the corresponding GBM localizations. We identify two bursts in
the IPN/GBM sample that did not appear in the GBM catalog. In one case, the GBM
triggered on a terrestrial gamma flash, and in the other, its origin was given
as uncertain. We also discuss the sensitivity and calibration of the IPN.Comment: 52 pages, 12 figures, 4 tables. Revised version, resubmitted to the
Astrophysical Journal Supplement Series following refereeing. Figures of the
localizations in Table 3 may be found on the IPN website, at
ssl.berkeley.edu/ipn3/YYMMDD, where YY, MM, and DD are the year, month, and
day of the burst, sometimes with suffixes A or
Implications of the Visible and X-Ray Counterparts to GRB970228
The gamma-ray burst source GRB970228 has been observed after a delay of 8--12
hours in X-rays and after one day in visible and near infrared light. This
marks the first detection of emission at lower frequencies following the
gamma-ray observation of a GRB and the first detection of any visible
counterpart to a GRB. We consider possible delayed visible and X-ray emission
mechanisms, and conclude that the intrinsic gamma-ray activity continued at a
much reduced intensity for at least a day. There are hints of such continued
activity in other GRB, and future observations can decide if this is true of
GRB in general. The observed multi-band spectrum of GRB970228 agrees with the
predictions of relativistic shock theory when the flux is integrated over a
time longer than that required for a radiating electron to lose its energy.Comment: 5 pp., tex, 1 figur
Quasi-Periodic Pulsations in Solar Flares: new clues from the Fermi Gamma-Ray Burst Monitor
In the last four decades it has been observed that solar flares show
quasi-periodic pulsations (QPPs) from the lowest, i.e. radio, to the highest,
i.e. gamma-ray, part of the electromagnetic spectrum. To this day, it is still
unclear which mechanism creates such QPPs. In this paper, we analyze four
bright solar flares which show compelling signatures of quasi-periodic behavior
and were observed with the Gamma-Ray Burst Monitor (\gbm) onboard the Fermi
satellite. Because GBM covers over 3 decades in energy (8 keV to 40 MeV) it can
be a key instrument to understand the physical processes which drive solar
flares. We tested for periodicity in the time series of the solar flares
observed by GBM by applying a classical periodogram analysis. However, contrary
to previous authors, we did not detrend the raw light curve before creating the
power spectral density spectrum (PSD). To assess the significance of the
frequencies we made use of a method which is commonly applied for X-ray
binaries and Seyfert galaxies. This technique takes into account the underlying
continuum of the PSD which for all of these sources has a P(f) ~ f^{-\alpha}
dependence and is typically labeled red-noise. We checked the reliability of
this technique by applying it to a solar flare which was observed by the Reuven
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) which contains, besides
any potential periodicity from the Sun, a 4 s rotational period due to the
rotation of the spacecraft around its axis. While we do not find an intrinsic
solar quasi-periodic pulsation we do reproduce the instrumental periodicity.
Moreover, with the method adopted here, we do not detect significant QPPs in
the four bright solar flares observed by GBM. We stress that for the purpose of
such kind of analyses it is of uttermost importance to appropriately account
for the red-noise component in the PSD of these astrophysical sources.Comment: accepted by A&
Supernovae - Optical Precursors of Short Gamma-Ray Bursts
The probability of observing "supernova - gamma-ray burst" (GRB) pair events
and recurrent GRBs from one galaxy in a time interval of several years has been
estimated. Supernova explosions in binary systems accompanied by the formation
of a short-lived pair of compact objects can be the sources of such events. If
a short GRB is generated during the collision of a pair, then approximately
each of ~300 short GRBs with redshift z must have an optical precursor - a
supernova in the observer's time interval <2(1+z)yr. If the supernova explosion
has the pattern of a hypernova, then a successive observation of long and short
GRBs is possible. The scenario for the generation of multiple GRBs in
collapsing galactic nuclei is also discussed.Comment: 12 pages, 1 figure; this paper has the e-precursor arXiv:1101.3298
[astro-ph.HE
The Fourth BATSE Gamma-Ray Burst Catalog (Revised)
The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray
Observatory (CGRO) has triggered on 1637 cosmic gamma-ray bursts between 1991
April 19 and 1996 August 29. These events constitute the Fourth BATSE burst
catalog. The current version (4Br) has been revised from the version first
circulated on CD-ROM in September 1997 (4B) to include improved locations for a
subset of bursts that have been reprocssed using additional data. A significant
difference from previous BATSE catalogs is the inclusion of bursts from periods
when the trigger energy range differed from the nominal 50-300 keV. We present
tables of the burst occurrence times, locations, peak fluxes, fluences, and
durations. In general, results from previous BATSE catalogs are confirmed here
with greater statistical significance.Comment: 45 pages, 12 Postscript figures, accepted for publication in Ap. J.
Supp
Rest-frame properties of 32 gamma-ray bursts observed by the Fermi Gamma-Ray Burst Monitor
Aims: In this paper we study the main spectral and temporal properties of
gamma-ray bursts (GRBs) observed by Fermi/GBM. We investigate these key
properties of GRBs in the rest-frame of the progenitor and test for possible
intra-parameter correlations to better understand the intrinsic nature of these
events. Methods: Our sample comprises 32 GRBs with measured redshift that were
observed by GBM until August 2010. 28 of them belong to the long-duration
population and 4 events were classified as short/hard bursts. For all of these
events we derive, where possible, the intrinsic peak energy in the spectrum (\eprest), the duration in the rest-frame, defined as the
time in which 90% of the burst fluence was observed (\tninetyrest) and the
isotropic equivalent bolometric energy (\eiso). Results: The distribution of
\eprest has mean and median values of 1.1 MeV and 750 keV, respectively. A
log-normal fit to the sample of long bursts peaks at ~800 keV. No high-\ep
population is found but the distribution is biased against low \ep values. We
find the lowest possible \ep that GBM can recover to be ~ 15 keV. The
\tninetyrest distribution of long GRBs peaks at ~10 s. The distribution of
\eiso has mean and median values of erg and erg, respectively. We confirm the tight correlation between \eprest
and \eiso (Amati relation) and the one between \eprest and the 1-s peak
luminosity () (Yonetoku relation). Additionally, we observe a parameter
reconstruction effect, i.e. the low-energy power law index gets softer
when \ep is located at the lower end of the detector energy range. Moreover, we
do not find any significant cosmic evolution of neither \eprest nor
\tninetyrest.Comment: accepted by A&
Integrating the Fermi Gamma-Ray Burst Monitor into the 3rd Interplanetary Network
We are integrating the Fermi Gamma-Ray Burst Monitor (GBM) into the
Interplanetary Network (IPN) of Gamma-Ray Burst (GRB) detectors. With the GBM,
the IPN will comprise 9 experiments. This will 1) assist the Fermi team in
understanding and reducing their systematic localization uncertainties, 2)
reduce the sizes of the GBM and Large Area Telescope (LAT) error circles by 1
to 4 orders of magnitude, 3) facilitate the identification of GRB sources with
objects found by ground- and space-based observatories at other wavelengths,
from the radio to very high energy gamma-rays, 4) reduce the uncertainties in
associating some LAT detections of high energy photons with GBM bursts, and 5)
facilitate searches for non-electromagnetic GRB counterparts, particularly
neutrinos and gravitational radiation. We present examples and demonstrate the
synergy between Fermi and the IPN. This is a Fermi Cycle 2 Guest Investigator
project.Comment: 5 pages, 11 figures. 2009 Fermi Symposium. eConf Proceedings C09112
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