691 research outputs found
A War Fleet Built for Peace: British Naval Rearmament in the 1930s and the Dilemma of Deterrence versus Defence
The first point to make about British naval rearmament in the 1930s is that the Royal Navy was preparing not for war, but to deter war, at least until after March 1939
Real-time Likelihood Methods for Improved Gamma-ray Transient Detection and Localization
We present a maximum likelihood (ML) algorithm that is fast enough to detect
gamma-ray transients in real time on low-performance processors often used for
space applications. We validate the routine with simulations and find that,
relative to algorithms based on excess counts, the ML method is nearly twice as
sensitive, allowing detection of 240-280% more short gamma-ray bursts. We
characterize a reference implementation of the code, estimating its
computational complexity and benchmarking it on a range of processors. We
exercise the reference implementation on archival data from the Fermi Gamma-ray
Burst Monitor (GBM), verifying the sensitivity improvements. In particular, we
show that the ML algorithm would have detected GRB 170817A even if it had been
nearly four times fainter. We present an ad hoc but effective scheme for
discriminating transients associated with background variations. We show that
the on-board localizations generated by ML are accurate, but that refined
off-line localizations require a detector response matrix with about ten times
finer resolution than is current practice. Increasing the resolution of the GBM
response matrix could substantially reduce the few-degree systematic
uncertainty observed in the localizations of bright bursts.Comment: 18 pages, 11 figure
Using an A-10 Aircraft for Airborne measurements of TGFs
Plans are underway to convert an A-10 combat attack aircraft into a research aircraft for thunderstorm research. This aircraft would be configured and instrumented for flights into large, convective thunderstorms. It would have the capabilities of higher altitude performance and protection for thunderstorm conditions that exceed those of aircraft now in use for this research. One area of investigation for this aircraft would be terrestrial gamma ]ray flashes (TGFs), building on the pioneering observations made by the Airborne Detector for Energetic Lightning Emissions (ADELE) project several years ago. A new and important component of the planned investigations are the continuous, detailed correlations of TGFs with the electric fields near the aircraft, as well as detailed measurements of nearby lightning discharges. Together, the x-and gamma-radiation environments, the electric field measurements, and the lightning observations (all measured on microsecond timescales) should provide new insights into this TGF production mechanism. The A -10 aircraft is currently being modified for thunderstorm research. It is anticipated that the initial test flights for this role will begin next year
X-ray and gamma-ray spectra and variability of the black-hole candidate GX 339-4
We analyse five observations of the X-ray binary GX 339-4 by the soft
gamma-ray OSSE detector on board CGRO simultaneous with either Ginga or RXTE
observations. The source was bright during four of them, with the luminosity of
L ~ 10^{37} erg/s and the spectrum typical for hard states of accreting black
holes, and it was in an off state during the fifth one, with L ~ 10^{35} erg/s.
Our broad-band spectral fits show the mean electron energy of electrons in the
Comptonizing plasma decreasing with increasing luminosity within the hard
(bright) state. For the observation with the best statistics at soft
gamma-rays, approximately 1/4 of energy in the Comptonizing plasma is probably
carried by non-thermal electrons. Then, considering the efficiency of
Comptonized hybrid synchrotron emission allows us to obtain an upper limit on
the strength of the magnetic field in the X-ray source. Furthermore, this
synchrotron emission is capable of producing the optical spectrum observed in
an optically-high state of GX 339-4. In the off state, the hard X-ray spectrum
is consistent with being dominated by bremsstrahlung. The unusually strong Fe K
alpha line observed by the PCA during that state is found not to be intrinsic
to the source but to originate mostly in the Galactic diffuse emission.Comment: 12 pages, 7 figures (2 in colour). Accepted for publication in MNRA
Development of Dual-Gain SiPM Boards for Extending the Energy Dynamic Range
Astronomical observations with gamma rays in the range of several hundred keV
to hundreds of MeV currently represent the least explored energy range. To
address this so-called MeV gap, we designed and built a prototype CsI:Tl
calorimeter instrument using a commercial off-the-shelf (COTS) SiPMs and
front-ends which may serve as a subsystem for a larger gamma-ray mission
concept. During development, we observed significant non-linearity in the
energy response. Additionally, using the COTS readout, the calorimeter could
not cover the four orders of magnitude in energy range required for the
telescope. We, therefore, developed dual-gain silicon photomultiplier (SiPM)
boards that make use of two SiPM species that are read out separately to
increase the dynamic energy range of the readout. In this work, we investigate
the SiPM's response with regards to active area ( and
) and various microcell sizes (, , and ). We read out CsI:Tl chunks
using dual-gain SiPMs that utilize microcells for both
SiPM species and demonstrate the concept when tested with high-energy gamma-ray
and proton beams. We also studied the response of $17 \times 17 \times 100 \
\mathrm{mm}^30.25-400 \ \mathrm{MeV}2.5-30 \ \mathrm{MeV}$. This development aims to demonstrate
the concept for future scintillator-based high-energy calorimeters with
applications in gamma-ray astrophysics
Development of a CsI Calorimeter for the Compton-Pair (ComPair) Balloon-Borne Gamma-Ray Telescope
There is a growing interest in astrophysics to fill in the observational
gamma-ray MeV gap. We, therefore, developed a CsI:Tl calorimeter prototype as a
subsystem to a balloon-based Compton and Pair-production telescope known as
ComPair. ComPair is a technology demonstrator for a gamma-ray telescope in the
MeV range that is comprised of 4 subsystems: the double-sided silicon detector,
virtual Frisch grid CdZnTe, CsI calorimeter, and a plastic-based
anti-coincidence detector. The prototype CsI calorimeter is composed of thirty
CsI logs, each with a geometry of .
The logs are arranged in a hodoscopic fashion with 6 in a row that alternate
directions in each layer. Each log has a resolution of around
full-width-at-half-maximum (FWHM) at with a dynamic energy
range of around . A array of
SensL J-series SiPMs read out each end of the log to estimate the depth of
interaction and energy deposition with signals read out with an IDEAS ROSSPAD.
We also utilize an Arduino to synchronize with the other ComPair subsystems
that comprise the full telescope. This work presents the development and
performance of the calorimeter, its testing in thermal and vacuum conditions,
and results from irradiation by monoenergetic gamma-ray
beams. The CsI calorimeter will fly onboard ComPair as a balloon experiment in
the summer of 2023
Gamma Ray Glow Observations at 20-km Altitude
In the spring of 2017 an ER‐2 aircraft campaign was undertaken over continental United States to observe energetic radiation from thunderstorms and lightning. The payload consisted of a suite of instruments designed to detect optical signals, electric fields, and gamma rays from lightning. Starting from Georgia, USA, 16 flights were performed, for a total of about 70 flight hours at a cruise altitude of 20 km. Of these, 45 flight hours were over thunderstorm regions. An analysis of two gamma ray glow events that were observed over Colorado at 21:47 UT on 8 May 2017 is presented. We explore the charge structure of the cloud system, as well as possible mechanisms that can produce the gamma ray glows. The thundercloud system we passed during the gamma ray glow observation had strong convection in the core of the cloud system. Electric field measurements combined with radar and radio measurements suggest an inverted charge structure, with an upper negative charge layer and a lower positive charge layer. Based on modeling results, we were not able to unambiguously determine the production mechanism. Possible mechanisms are either an enhancement of cosmic background locally (above or below 20 km) by an electric field below the local threshold or an enhancement of the cosmic background inside the cloud but then with normal polarity and an electric field well above the Relativistic Runaway Electron Avalanche threshold.publishedVersio
OSSE and RXTE Observations of GRS 1915+105: Evidence for Non-thermal Comptonization
GRS 1915+105 was observed by the CGRO/OSSE 9 times in 1995-2000, and 8 of
those observations were simultaneous with those by RXTE. We present an analysis
of all of the OSSE data and of two RXTE-OSSE spectra with the lowest and
highest X-ray fluxes. The OSSE data show a power-law--like spectrum extending
up to ~600 keV without any break. We interpret this emission as strong evidence
for the presence of non-thermal electrons in the source. The broad-band spectra
cannot be described by either thermal or bulk-motion Comptonization, whereas
they are well described by Comptonization in hybrid thermal/ non-thermal
plasmas.Comment: 4 pages in emulateapj5 style; Slightly modified version accepted to
ApJ Letter
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