910 research outputs found
Synthesis and characterization of model polymers for use in the investigation of char forming heat shields status report no. 1
Model polymer synthesis for use in char forming heat shield
Spectral Hardness Decay with Respect to Fluence in BATSE Gamma-Ray Bursts
We have analyzed the evolution of the spectral hardness parameter Epk as a
function of fluence in gamma-ray bursts. We fit 41 pulses within 26 bursts with
the trend reported by Liang & Kargatis (1996) which found that Epk decays
exponentially with respect to photon fluence. We also fit these pulses with a
slight modification of this trend, where Epk decays linearly with energy
fluence. In both cases, we found the set of 41 pulses to be consistent with the
trend. For the latter trend, which we believe to be more physical, the
distribution of the decay constant is roughly log-normal, with a mean of 1.75
+/- 0.07 and a FWHM of 1.0 +/- 0.1. Regarding an earlier reported invariance in
the decay constant among different pulses in a single burst, we found
probabilities of 0.49 to 0.84 (depending on the test used) that such invariance
would occur by coincidence, most likely due to the narrow distribution of decay
constant values among pulses.Comment: 17 pages, 7 figure pages, 2 table pages, submitted to The
Astrophysical Journa
Evolution of the Low-Energy Photon Spectra in Gamma-Ray Bursts
We report evidence that the asymptotic low-energy power law slope alpha
(below the spectral break) of BATSE gamma-ray burst photon spectra evolves with
time rather than remaining constant. We find a high degree of positive
correlation exists between the time-resolved spectral break energy E_pk and
alpha. In samples of 18 "hard-to-soft" and 12 "tracking" pulses, evolution of
alpha was found to correlate with that of the spectral break energy E_pk at the
99.7% and 98% confidence levels respectively. We also find that in the flux
rise phase of "hard-to-soft" pulses, the mean value of alpha is often positive
and in some bursts the maximum value of alpha is consistent with a value > +1.
BATSE burst 3B 910927, for example, has a alpha_max equal to 1.6 +/- 0.3. These
findings challenge GRB spectral models in which alpha must be negative of
remain constant.Comment: 12 pages (including 6 figures), accepted to Ap
Seismic stratigraphic framework of the continental shelf offshore Delmarva, USA: implications for Mid-Atlantic Bight evolution since the Pliocene
© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Brothers, L. L., Foster, D. S., Pendleton, E. A., & Baldwin, W. E. Seismic stratigraphic framework of the continental shelf offshore Delmarva, USA: implications for Mid-Atlantic Bight evolution since the Pliocene. Marine Geology, 428, : (2020)106287, doi:10.1016/j.margeo.2020.106287.Understanding how past coastal systems have evolved is critical to predicting future coastal change. Using over 12,000 trackline kilometers of recently collected, co-located multi-channel boomer, sparker and chirp seismic reflection profile data integrated with previously collected borehole and vibracore data, we define the upper (< 115 m below mean lower low water) seismic stratigraphic framework offshore of the Delmarva Peninsula, USA. Twelve seismic units and 11 regionally extensive unconformities (U1-U11) were mapped over 5900 km2 of North America's Mid-Atlantic continental shelf. We interpret U3, U7, U9, U11 as transgressive ravinement surfaces, while U1,2,4,5,6,8,10 are subaerial unconformities illustrating distinct periods of lower sea-level. Based on areal distribution, stratigraphic relationships and dating results (Carbon 14 and amino acid racemization estimates) from earlier vibracore and borehole studies, we interpret the infilled channels as late Neogene and Quaternary courses of the Susquehanna, Potomac, Rappahannock, York, James rivers and tributaries, and a broad flood plain. These findings indicate that the region's geologic framework is more complex than previously thought and that Pleistocene paleochannels are abundant in the Mid-Atlantic. This study synthesizes and correlates the findings of other Atlantic Margin studies and establishes a large-scale Quaternary framework that enables more detailed stratigraphic analysis in the future. Such work has implications for inner continental shelf systems tract evolution, the relationship between antecedent geology and modern coastal systems, assessments of eustacy, glacial isostatic adjustment, and other processes and forcings that play a role in passive margin evolution.This work was supported by the U.S. Department of the Interior's Response to Hurricane Sandy
H2 reformation in post-shock regions
H2 formation is an important process in post-shock regions, since H2 is an
active participant in the cooling and shielding of the environment. The onset
of H2 formation therefore has a strong effect on the temperature and chemical
evolution in the post shock regions. We recently developed a model for H2
formation on a graphite surface in warm conditions. The graphite surface acts
as a model system for grains containing large areas of polycyclic aromatic
hydrocarbon structures. Here this model is used to obtain a new description of
the H2 formation rate as a function of gas temperature that can be implemented
in molecular shock models. The H2 formation rate is substantially higher at
high gas temperatures as compared to the original implementation of this rate
in shock models, because of the introduction of H atoms which are chemically
bonded to the grain (chemisorption). Since H2 plays such a key role in the
cooling, the increased rate is found to have a substantial effect on the
predicted line fluxes of an important coolant in dissociative shocks [O I] at
63.2 and 145.5 micron. With the new model a better agreement between model and
observations is obtained. Since one of the goals of Herschel/PACS will be to
observe these lines with higher spatial resolution and sensitivity than the
former observations by ISO-LWS, this more accurate model is very timely to help
with the interpretation of these future results.Comment: 12 pages, 3 figures, 1 table, accepted in MNRAS Letter
The BATSE experiment on the Gamma Ray Observatory: Solar flare hard x ray and gamma-ray capabilities
The Burst and Transient Source Experiment (BATSE) for the Gamma Ray Observatory (GRO) consists of eight detector modules that provide full-sky coverage for gamma-ray bursts and other transient phenomena such as solar flares. Each detector module has a thin, large-area scintillation detector (2025 sq cm) for high time-resolution studies, and a thicker spectroscopy detector (125 sq cm) to extend the energy range and provide better spectral resolution. The total energy range of the system is 15 keV to 100 MeV. These 16 detectors and the associated onboard data system should provide unprecedented capabilities for observing rapid spectral changes and gamma-ray lines from solar flares. The presence of a solar flare can be detected in real-time by BATSE; a trigger signal is sent to two other experiments on the GRO. The launch of the GRO is scheduled for June 1990, so that BATSE can be an important component of the Max '91 campaign
Secondary gamma-ray production in a coded aperture mask
The application of the coded aperture mask to high energy gamma-ray astronomy will provide the capability of locating a cosmic gamma-ray point source with a precision of a few arc-minutes above 20 MeV. Recent tests using a mask in conjunction with drift chamber detectors have shown that the expected point spread function is achieved over an acceptance cone of 25 deg. A telescope employing this technique differs from a conventional telescope only in that the presence of the mask modifies the radiation field in the vicinity of the detection plane. In addition to reducing the primary photon flux incident on the detector by absorption in the mask elements, the mask will also be a secondary radiator of gamma-rays. The various background components in a CAMTRAC (Coded Aperture Mask Track Chamber) telescope are considered. Monte-Carlo calculations are compared with recent measurements obtained using a prototype instrument in a tagged photon beam line
Operating characteristics of a prototype high energy gamma-ray telescope
The field of gamma-ray astronomy in the energy range from ten to several hundred MeV is severely limited by the angular resolution that can be achieved by present instruments. The identification of some of the point sources found by the COS-B mission and the resolution of detailed structure existing in those sources may depend on the development of a new class of instrument. The coded aperture mask telescope, used successfully at X-ray energies hold the promise of being such an instrument. A prototype coded aperture telescope was operated in a tagged photon beam ranging in energy from 23 to 123 MeV. The purpose of the experiment was to demonstrate the feasibility of operating a coded aperture mask telescope in this energy region. Some preliminary results and conclusions drawn from some of the data resulting from this experiment are presented
BATSE Gamma-Ray Burst Line Search: III. Line Detectability
We evaluate the ability of the BATSE spectroscopy detectors to detect the
absorption lines observed by Ginga in gamma ray burst spectra. We find that
BATSE can detect the 20.6 keV line in the S1 segment of GB870303 with a
detection probability of ~1/4 in nearly normally incident bursts, with the
probability dropping off to nearly 0 at a burst angle of 50 degrees; the lines
at 19.4 and 38.8 keV lines in GB880205 have a high detection probability in
BATSE up to burst angles of 75 degrees. In addition, we calculate detection
probabilities for these two line types as a function of signal-to-noise ratio
and burst angle for use in detailed comparisons between BATSE and Ginga.
Finally, we consider the probability averaged over the sky of detecting a line
feature with the actual array of BATSE detectors on CGRO.Comment: 21 pages including tables, AASTeX, 8 figures on request from
[email protected]
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