95 research outputs found
Background estimation in a wide-field background-limited instrument such as Fermi GBM
The supporting instrument on board the Fermi Gamma-ray Space Telescope, the
Gamma-ray Burst Monitor (GBM) is a wide-field gamma-ray monitor composed of 14
individual scintillation detectors, with a field of view which encompasses the
entire unocculted sky. Primarily designed as transient monitors, the
conventional method for background determination with GBM-like instruments is
to time interpolate intervals before and after the source as a polynomial. This
is generally sufficient for sharp impulsive phenomena such as Gamma-Ray Bursts
(GRBs) which are characterised by impulsive peaks with sharp rises, often
highly structured, and easily distinguishable against instrumental backgrounds.
However, smoother long lived emission, such as observed in solar flares and
some GRBs, would be difficult to detect in a background-limited instrument
using this method. We present here a description of a technique which uses the
rates from adjacent days when the satellite has approximately the same
geographical footprint to distinguish low-level emission from the instrumental
background. We present results from the application of this technique to GBM
data and discuss the implementation of it in a generalised background limited
detector in a non-equatorial orbit.Comment: Proceedings of SPIE, Vo. 8443, Paper No. 8443-3
BurstCube: A CubeSat for Gravitational Wave Counterparts
BurstCube will detect long GRBs, attributed to the collapse of massive stars,
short GRBs (sGRBs), resulting from binary neutron star mergers, as well as
other gamma-ray transients in the energy range 10-1000 keV. sGRBs are of
particular interest because they are predicted to be the counterparts of
gravitational wave (GW) sources soon to be detectable by LIGO/Virgo. BurstCube
contains 4 CsI scintillators coupled with arrays of compact low-power Silicon
photomultipliers (SiPMs) on a 6U Dellingr bus, a flagship modular platform that
is easily modifiable for a variety of 6U CubeSat architectures. BurstCube will
complement existing facilities such as Swift and Fermi in the short term, and
provide a means for GRB detection, localization, and characterization in the
interim time before the next generation future gamma-ray mission flies, as well
as space-qualify SiPMs and test technologies for future use on larger gamma-ray
missions. The ultimate configuration of BurstCube is to have a set of
BurstCubes to provide all-sky coverage to GRBs for substantially lower cost
than a full-scale mission.Comment: In the 35th International Cosmic Ray Conference, Busan, Kore
Anomalies in low-energy Gamma-Ray Burst spectra with the Fermi Gamma-Ray Burst Monitor
A Band function has become the standard spectral function used to describe
the prompt emission spectra of gamma-ray bursts (GRBs). However, deviations
from this function have previously been observed in GRBs detected by BATSE and
in individual GRBs from the \textit{Fermi} era. We present a systematic and
rigorous search for spectral deviations from a Band function at low energies in
a sample of the first two years of high fluence, long bursts detected by the
\textit{Fermi} Gamma-Ray Burst Monitor (GBM). The sample contains 45 bursts
with a fluence greater than 2 erg / cm (10 - 1000 keV). An
extrapolated fit method is used to search for low-energy spectral anomalies,
whereby a Band function is fit above a variable low-energy threshold and then
the best fit function is extrapolated to lower energy data. Deviations are
quantified by examining residuals derived from the extrapolated function and
the data and their significance is determined via comprehensive simulations
which account for the instrument response. This method was employed for both
time-integrated burst spectra and time-resolved bins defined by a signal to
noise ratio of 25 and 50 . Significant deviations are evident
in 3 bursts (GRB\,081215A, GRB\,090424 and GRB\,090902B) in the time-integrated
sample ( 7%) and 5 bursts (GRB\,090323, GRB\,090424, GRB\,090820,
GRB\,090902B and GRB\,090926A) in the time-resolved sample ( 11%).} The
advantage of the systematic, blind search analysis is that it can demonstrate
the requirement for an additional spectral component without any prior
knowledge of the nature of that extra component. Deviations are found in a
large fraction of high fluence GRBs; fainter GRBs may not have sufficient
statistics for deviations to be found using this method
Longitudinal evaluation of peritoneal macrophage function and activation during CAPD: Maturity, cytokine synthesis and arachidonic acid metabolism
Longitudinal evaluation of peritoneal macrophage function and activation during CAPD: Maturity, cytokine synthesis and arachidonic acid metabolism. The release of cytokines and prostaglandins (PG) by peritoneal macrophages (PMĂ) may influence the cytokine network controlling peritoneal inflammation and in the long-term the function of the peritoneum as a dialysis membrane. In the present study, an evaluation of the long-term effects of peritoneal dialysis on the release of cytokines and prostaglandins, and the expression of surface markers of cellular maturation on blood and mononuclear cells has been performed in patients during their first year on CAPD. Spontaneous release of tumour necrosis factor α (TNFα) and interleukin 6 (IL-6) by PMĂ, after 4 or 24 hours in culture, increased significantly with time on CAPD, while there was a small but significant decrease in release of prostaglandin E2 (PGE2). Production of TNFα and IL-6 was enhanced following incubation of the cells with lipopolysaccharide (LPS), but the effect of LPS was proportionally greater on blood monocytes than on PMĂ. There was a significant increase in the concentrations of PGE2 and 6-keto-prostaglandin F1α in overnight dwell peritoneal dialysis effluent with time on CAPD. The levels of TNFα and IL-6 in uninfected PDE were below the detection limit of the immunoassay over the whole time period studied. Expression of CD15, which correlates with immaturity, by PMĂ and blood monocytes increased with time on CAPD, while expression of CD11c, a marker of maturation, decreased on blood monocytes, but did not change significantly on PMĂ. There was also a slight increase in expression of transferrin receptor in both PMĂ and monocytes, but this did not reach statistical significance. These findings suggest that peritoneal macrophages and blood monocytes isolated from CAPD patients over a one year period become increasingly immature with time, and this is accompanied by a significant modulation of their ability to secrete inflammatory cytokines. Dysregulation of macrophage function may have important consequences with respect to inflammatory processes and the long-term function of the peritoneal membrane in CAPD patients
Detection of a Thermal Spectral Component in the Prompt Emission of GRB 100724B
Observations of GRB 100724B with the Fermi Gamma-Ray Burst Monitor (GBM) find
that the spectrum is dominated by the typical Band functional form, which is
usually taken to represent a non-thermal emission component, but also includes
a statistically highly significant thermal spectral contribution. The
simultaneous observation of the thermal and non-thermal components allows us to
confidently identify the two emission components. The fact that these seem to
vary independently favors the idea that the thermal component is of
photospheric origin while the dominant non-thermal emission occurs at larger
radii. Our results imply either a very high efficiency for the non-thermal
process, or a very small size of the region at the base of the flow, both quite
challenging for the standard fireball model. These problems are resolved if the
jet is initially highly magnetized and has a substantial Poynting flux.Comment: 6 pages, 3 figures, 1 table, Accepted for publication in the
Astrophysical Journal Letters November, 23 2010 (Submitted October, 20 2010
The 3rd Fermi GBM Gamma-Ray Burst Catalog: The First Six Years
Since its launch in 2008, the Fermi Gamma-ray Burst Monitor (GBM) has
triggered and located on average approximately two gamma-ray bursts (GRB) every
three days. Here we present the third of a series of catalogs of GRBs detected
by GBM, extending the second catalog by two more years, through the middle of
July 2014. The resulting list includes 1405 triggers identified as GRBs. The
intention of the GBM GRB catalog is to provide information to the community on
the most important observables of the GBM detected GRBs. For each GRB the
location and main characteristics of the prompt emission, the duration, peak
flux and fluence are derived. The latter two quantities are calculated for the
50-300~keV energy band, where the maximum energy release of GRBs in the
instrument reference system is observed, and also for a broader energy band
from 10-1000 keV, exploiting the full energy range of GBM's low-energy NaI(Tl)
detectors. Using statistical methods to assess clustering, we find that the
hardness and duration of GRBs are better fitted by a two-component model with
short-hard and long-soft bursts, than by a model with three components.
Furthermore, information is provided on the settings and modifications of the
triggering criteria and exceptional operational conditions during years five
and six in the mission. This third catalog is an official product of the Fermi
GBM science team, and the data files containing the complete results are
available from the High-Energy Astrophysics Science Archive Research Center
(HEASARC).Comment: 225 pages, 13 figures and 8 tables. Accepted for publication in
Astrophysical Journal Supplement 201
Temporal Deconvolution study of Long and Short Gamma-Ray Burst Light curves
The light curves of Gamma-Ray Bursts (GRBs) are believed to result from
internal shocks reflecting the activity of the GRB central engine. Their
temporal deconvolution can reveal potential differences in the properties of
the central engines in the two populations of GRBs which are believed to
originate from the deaths of massive stars (long) and from mergers of compact
objects (short). We present here the results of the temporal analysis of 42
GRBs detected with the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray
Space Telescope. We deconvolved the profiles into pulses, which we fit with
lognormal functions. The distributions of the pulse shape parameters and
intervals between neighboring pulses are distinct for both burst types and also
fit with lognormal functions. We have studied the evolution of these parameters
in different energy bands and found that they differ between long and short
bursts. We discuss the implications of the differences in the temporal
properties of long and short bursts within the framework of the internal shock
model for GRB prompt emission.Comment: 38 pages, 11 figure
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