553 research outputs found
Studying the High-Energy Gamma-Ray Sky with Glast
Building on the success of the Energetic Gamma Ray Experiment Telescope
(EGRET) on the Compton Gamma Ray Observatory, the Gamma-ray Large Area Space
Telescope (GLAST) will make a major step in the study of such subjects as
blazars, gamma-ray bursts, the search for dark matter, supernova remnants,
pulsars, diffuse radiation, and unidentified high-energy sources. The
instrument will be built on new and mature detector technologies such as
silicon strip detectors, low-power low-noise LSI, and a multilevel data
acquisition system. GLAST is in the research and development phase, and one
full tower (of 25 total) is now being built in collaborating institutes. The
prototype tower will be tested thoroughly at SLAC in the fall of 1999.Comment: 6 pages with 2 figures, to appear in the proceedings of the COSPAR 98
Symposium E 1.1, postscript file also available at
http://glast.gsfc.nasa.gov/COSPAR
Gamma-Ray Bursts: Progress, Problems & Prospects
The cosmological gamma-ray burst (GRB) phenomenon is reviewed. The broad
observational facts and empirical phenomenological relations of the GRB prompt
emission and afterglow are outlined. A well-tested, successful fireball shock
model is introduced in a pedagogical manner. Several important uncertainties in
the current understanding of the phenomenon are reviewed, and prospects of how
future experiments and extensive observational and theoretical efforts may
address these problems are discussed.Comment: 86 pages, 17 figures, 566 references, an invited review for
International Journal of Modern Physics A, in pres
GeV Photons from Ultra High Energy Cosmic Rays accelerated in Gamma Ray Bursts
Gamma-ray bursts are produced by the dissipation of the kinetic energy of a
highly relativistic fireball, via the formation of a collisionless shock. When
this happens, Ultra High Energy Cosmic Rays up to 10^20 eV are produced. I show
in this paper that these particles produce, via synchrotron emission as they
cross the acceleration region, photons up to 300 GeV which carry away a small,
~0.01, but non-negligible fraction of the total burst energy. I show that, when
the shock occurs with the interstellar medium, the optical depth to
photon-photon scattering, which might cause energy degradation of the photons,
is small. The burst thusly produced would be detected at Earth simultaneoulsy
with the parent gamma-ray burst, although its duration may differ significantly
from that of the lower energy photons. The expected fluences, ~10^{-5}-10^{-6}
erg/cm^2 are well within the range of planned detectors. A new explanation for
the exceptional burst GRB 940217 is discussed.Comment: Accepted for publication in The Physical Review Letters. 4 pages,
RevTeX needed, no figure
Section on Prospects for Dark Matter Detection of the White Paper on the Status and Future of Ground-Based TeV Gamma-Ray Astronomy
This is a report on the findings of the dark matter science working group for
the white paper on the status and future of TeV gamma-ray astronomy. The white
paper was commissioned by the American Physical Society, and the full white
paper can be found on astro-ph (arXiv:0810.0444). This detailed section
discusses the prospects for dark matter detection with future gamma-ray
experiments, and the complementarity of gamma-ray measurements with other
indirect, direct or accelerator-based searches. We conclude that any
comprehensive search for dark matter should include gamma-ray observations,
both to identify the dark matter particle (through the charac- teristics of the
gamma-ray spectrum) and to measure the distribution of dark matter in galactic
halos.Comment: Report from the Dark Matter Science Working group of the APS
commissioned White paper on ground-based TeV gamma ray astronomy (19 pages, 9
figures
Can Fireball or Firecone Models Explain Gamma Ray Bursts?
The observed afterglows of gamma ray bursts, in particular that of GRB 970228
six months later, seem to rule out relativistic fireballs and relativistic
firecones driven by merger or accretion induced collapse of compact stellar
objects in galaxies as the origin of GRBs. GRBs can be produced by superluminal
jets from such events.Comment: A short summary of the main properties of GRBs which are produced by
relativistic jets from merger/AIC of compact stellar objects is included.
Additional references to very recent publication
The Study of TeV Variability and Duty Cycle of Mrk 421 from 3 Years of Observations with the Milagro Observatory
TeV flaring activity with time scales as short as tens of minutes and an
orphan TeV flare have been observed from the blazar Markarian 421 (Mrk 421).
The TeV emission from Mrk 421 is believed to be produced by leptonic
synchrotron self-Compton (SSC) emission. In this scenario, correlations between
the X-ray and the TeV fluxes are expected, TeV orphan flares are hardly
explained and the activity (measured as duty cycle) of the source at TeV
energies is expected to be equal or less than that observed in X-rays if only
SSC is considered. To estimate the TeV duty cycle of Mrk 421 and to establish
limits on its variability at different time scales, we continuously observed
Mrk 421 with the Milagro observatory. Mrk 421 was detected by Milagro with a
statistical significance of 7.1 standard deviations between 2005 September 21
and 2008 March 15. The observed spectrum is consistent with previous
observations by VERITAS. We estimate the duty cycle of Mrk 421 for energies
above 1 TeV for different hypothesis of the baseline flux and for different
flare selections and we compare our results with the X-ray duty cycle estimated
by Resconi et al. 2009. The robustness of the results is discussed.Comment: 27 pages, 6 figures, ApJ accepte
Spectrum and Morphology of the Two Brightest Milagro Sources in the Cygnus Region: MGRO J2019+37 and MGRO J2031+41
The Cygnus region is a very bright and complex portion of the TeV sky, host
to unidentified sources and a diffuse excess with respect to conventional
cosmic-ray propagation models. Two of the brightest TeV sources, MGRO J2019+37
and MGRO J2031+41, are analyzed using Milagro data with a new technique, and
their emission is tested under two different spectral assumptions: a power law
and a power law with an exponential cutoff. The new analysis technique is based
on an energy estimator that uses the fraction of photomultiplier tubes in the
observatory that detect the extensive air shower. The photon spectrum is
measured in the range 1 to 200 TeV using the last 3 years of Milagro data
(2005-2008), with the detector in its final configuration. MGRO J2019+37 is
detected with a significance of 12.3 standard deviations (), and is
better fit by a power law with an exponential cutoff than by a simple power
law, with a probability % (F-test). The best-fitting parameters for the
power law with exponential cutoff model are a normalization at 10 TeV of
, a spectral
index of and a cutoff energy of TeV. MGRO
J2031+41 is detected with a significance of 7.3, with no evidence of a
cutoff. The best-fitting parameters for a power law are a normalization of
and a
spectral index of . The overall flux is subject to an
30% systematic uncertainty. The systematic uncertainty on the power law
indices is 0.1. A comparison with previous results from TeV J2032+4130,
MGRO J2031+41 and MGRO J2019+37 is also presented.Comment: 11 pages, 10 figure
The Commensal Real-time ASKAP Fast Transients (CRAFT) survey
We are developing a purely commensal survey experiment for fast (<5s)
transient radio sources. Short-timescale transients are associated with the
most energetic and brightest single events in the Universe. Our objective is to
cover the enormous volume of transients parameter space made available by
ASKAP, with an unprecedented combination of sensitivity and field of view. Fast
timescale transients open new vistas on the physics of high brightness
temperature emission, extreme states of matter and the physics of strong
gravitational fields. In addition, the detection of extragalactic objects
affords us an entirely new and extremely sensitive probe on the huge reservoir
of baryons present in the IGM. We outline here our approach to the considerable
challenge involved in detecting fast transients, particularly the development
of hardware fast enough to dedisperse and search the ASKAP data stream at or
near real-time rates. Through CRAFT, ASKAP will provide the testbed of many of
the key technologies and survey modes proposed for high time resolution science
with the SKA.Comment: accepted for publication in PAS
A Search for Ultra-High Energy Counterparts to Gamma-Ray Bursts
A small air shower array operating over many years has been used to search
for ultra-high energy (UHE) gamma radiation ( TeV) associated with
gamma-ray bursts (GRBs) detected by the BATSE instrument on the Compton
Gamma-Ray Observatory (CGRO). Upper limits for a one minute interval after each
burst are presented for seven GRBs located with zenith angles . A excess over background was observed between 10 and
20 minutes following the onset of a GRB on 11 May 1991. The confidence level
that this is due to a real effect and not a background fluctuation is 99.8\%.
If this effect is real then cosmological models are excluded for this burst
because of absorption of UHE gamma rays by the intergalactic radiation fields.Comment: 4 pages LaTeX with one postscript figure. This version does not use
kluwer.sty and will allow automatic postscript generatio
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