362 research outputs found
Constraining the Cosmic Star Formation Rate with the MeV Background
The Cosmic Gamma-ray Background (CGB) in the MeV regime has been measured
with COMPTEL and SMM. The origin of the CGB in this energy regime is believed
to be dominated by gamma-rays from Type Ia supernovae. We calculate the CGB
spectrum within the framework of FRW cosmology as a function of the cosmic star
formation rate, SFR(z). Several estimates of the SFR(z) have been reported
since the pioneering work of Madau et al. Here we discuss observational
constraints on SFR(z) derived from models of the CGB. In particular, we
consider the SFR obtained from Gamma-Ray Burst observations, which increases
dramatically with redshift beyond z ~ 1 in contrast to most estimates which
saturate or show a mild increase with redshift. Gamma-ray bursts may be the
most powerful tracers of star formation in the early universe and thus provide
signposts of the initial epoch of element synthesis. The star formation rate
implied by GRB statistics results in a gamma-ray background that matches the
observations more closely than that inferred from other tracers of star
formation. This may provide some support for the GRB/SFR-paradigm, which in
turn promises a powerful diagnostic of star formation, and thus cosmic chemical
evolution, from the era of Population III stars to the present
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Legislative Actions to Repeal, Defund, or Delay the Affordable Care Act
This report summarizes legislative and other actions taken to repeal, defund, or delay the Affordable Care Act (ACA), since the law's enactment. The information is presented in four appendixes
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Legislative Actions to Repeal, Defund, or Delay the Affordable Care Act
This report summarizes legislative and other actions taken to repeal, defund, or delay the Affordable Care Act (ACA), since the law's enactment. The information is presented in four appendixes
Advanced Compton Telescope Designs and SN Science
The Advanced Compton Telescope (ACT) has been suggested to be the optimal
next-generation instrument to study nuclear gamma-ray lines. In this work, we
investigate the potential of three hypothetical designs of the ACT to perform
SN science. We provide estimates of 1) the SN detection rate, 2) the SN Ia
discrimination rate, and 3) which gamma-ray lines would be detected from
specific supernova remnants. We find that the prompt emission from a SN Ia is
such that it is unlikely that one would be within the range that an
INTERMEDIATE ACT would be able to distinguish between explosion scenarios,
although such an instrument would detect a handful of SNRs.
We further find that the SUPERIOR ACT design would be a truly breakthrough
instrument for SN science. By supplying these estimates, we intend to assist
the gamma-ray astrophysics community in deciding the course of the next decade
of gamma-ray SN science.Comment: 10 pages, accepted for publication in New astronomy Reviews
(Astronomy with Radioactivities III
OSSE observations of galactic 511 keV annihilation radiation
The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory has performed several observations of the galactic plane and galactic center region to measure the distribution of galactic 511 keV positron annihilation radiation. Preliminary analysis of data collected during the observation of the galactic center region over the period 13-24 Jun. 1991, indicates the presence of a 511 keV line and positronium continuum superimposed on a power-law continuum. The line of flux was found to be (2.7 +/- 0.5) x 10(exp -4) gamma/sq cm sec, with a positronium fraction of (0.9 +/- 0.2). The 3(sigma) upper limit to daily variations in the 511 keV line flux from the mean during the observation interval is 3 x 10(exp -4) gamma/sq cm sec. If all of the observed annihilation radiation is assumed to originate from the x-ray source 1E 1740.7-2942, the corresponding 511 keV line flux would be (3.0 +/- 0.6) x 10(exp -4) gamma/sq cm sec. The 3(sigma) upper limit for 511 keV line emission from the x-ray binary GX1+4 is 6 x 10(exp -4) gamma/sq cm sec. Results from the galactic plane observations at galactic longitudes of 25 degrees (16-21 Aug. 1991) and 339 degrees (6-11 Sep. 1991) suggest that the emission is concentrated near the galactic center. The observations and the preliminary results are described
Hypernovae/GRB in the Galactic Center as possible sources of Galactic Positrons
The observation of a strong and extended positron-electron line annihilation
emission in the central regions of the Galaxy by INTEGRAL-SPI, consistent with
the Galactic bulge geometry, without any counterpart in the gamma-ray range,
neither at high energy nor in the 1809 keV Al decay line, is
challenging. Leaving aside the geometrical question, we address the problem of
the adequate positron sources, showing the potentiality of a new category of SN
Ic, exemplified by SN2003dh, which is associated to a gamma-ray burst. This
kind of supernova/hypernova/GRB event is interpreted as the result of a bipolar
Wolf-Rayet explosion, which produces a large amount of Ni and ejects it
at high velocity along the rotation axis. The bulk of positrons resulting from
Co decay escapes in the surrounding medium due to the rapid thinning of
the ejecta in the polar direction. We show that a rate of about 0.02
SN2003dh-like events per century in the central region of the Galaxy is
sufficient to explain the positron flux detected by INTEGRAL-SPI. In order to
explain this flux by SN Ia events alone, a rate of 0.5 per century is
necessary, much higher than indicated by Galactic evolutionary models applied
to the bulge. Further observations of late light curves of SNe Ia and SNe Ic in
the bulge of spiral galaxies, together with 3D hydrodynamic calculations of
anisotropic ejections of Ni in SN Ic/GRB events, will allow to estimate
the separate contributions of SNe Ia and SNe Ic to positron injection.Comment: 7 pages, 0 figures, accepted for publication in Astrophysical Journal
Letters, 2003 12 0
OSSE spectral analysis techniques
Analysis of the spectra from the Oriented Scintillation Spectrometer Experiment (OSSE) is complicated because of the typically low signal to noise (approx. 0.1 percent) and the large background variability. The OSSE instrument was designed to address these difficulties by periodically offset-pointing the detectors from the source to perform background measurements. These background measurements are used to estimate the background during each of the source observations. The resulting background-subtracted spectra can then be accumulated and fitted for spectral lines and/or continua. Data selection based on various environmental parameters can be performed at various stages during the analysis procedure. In order to achieve the instrument's statistical sensitivity, however, it will be necessary for investigators to develop a detailed understanding of the instrument operation, data collection, and the background spectrum and its variability. A brief description of the major steps in the OSSE spectral analysis process is described, including a discussion of the OSSE background spectrum and examples of several observational strategies
Operation and performance of the OSSE instrument
The Oriented Scintillation Spectrometer Experiment (OSSE) on the Arthur Holly Compton Gamma Ray Observatory is described. An overview of the operation and control of the instrument is given, together with a discussion of typical observing strategies used with OSSE and basic data types produced by the instrument. Some performance measures for the instrument are presented that were obtained from pre-launch and in-flight data. These include observing statistics, continuum and line sensitivity, and detector effective area and gain stability
Boston University Medical Center: Perspectives on Health Policy
Report of a symposium held at the Boston University Medical Center
Performance Analysis of DNN Inference/Training with Convolution and non-Convolution Operations
Today's performance analysis frameworks for deep learning accelerators suffer
from two significant limitations. First, although modern convolutional neural
network (CNNs) consist of many types of layers other than convolution,
especially during training, these frameworks largely focus on convolution
layers only. Second, these frameworks are generally targeted towards inference,
and lack support for training operations. This work proposes a novel
performance analysis framework, SimDIT, for general ASIC-based systolic
hardware accelerator platforms. The modeling effort of SimDIT comprehensively
covers convolution and non-convolution operations of both CNN inference and
training on a highly parameterizable hardware substrate. SimDIT is integrated
with a backend silicon implementation flow and provides detailed end-to-end
performance statistics (i.e., data access cost, cycle counts, energy, and
power) for executing CNN inference and training workloads. SimDIT-enabled
performance analysis reveals that on a 64X64 processing array, non-convolution
operations constitute 59.5% of total runtime for ResNet-50 training workload.
In addition, by optimally distributing available off-chip DRAM bandwidth and
on-chip SRAM resources, SimDIT achieves 18X performance improvement over a
generic static resource allocation for ResNet-50 inference
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