78 research outputs found
Economia da produção de pimenta vermelha no municĂpio de Turuçu-RS.
bitstream/item/30264/1/boletim-19.pd
EGRET Spectral Index and the Low-Energy Peak Position in the Spectral Energy Distribution of EGRET-Detected Blazars
In current theoretical models of the blazar subclass of active galaxies, the
broadband emission consists of two components: a low-frequency synchrotron
component with a peak in the IR to X-ray band, and a high-frequency inverse
Compton component with a peak in the gamma-ray band. In such models, the
gamma-ray spectral index should be correlated with the location of the
low-energy peak, with flatter gamma-ray spectra expected for blazars with
synchrotron peaks at higher photon energies and vice versa. Using the
EGRET-detected blazars as a sample, we examine this correlation and possible
uncertainties in its construction.Comment: 17 pages including 1 figure, accepted for publication in The
Astrophysical Journa
The high-energy gamma-ray fluence and energy spectrum of GRB 970417a from observations with Milagrito
Evidence of TeV emission from GRB970417a has been previously reported using
data from the Milagrito detector. Constraints on the TeV fluence and the energy
spectrum are now derived using additional data from a scaler system that
recorded the rate of signals from the Milagrito photomultipliers. This analysis
shows that if emission from GRB970417a has been observed, it must contain
photons with energies above 650 GeV. Some consequences of this observation are
discussed.Comment: Submitted to the Astrophysical Journa
EGRET Observations of the Extragalactic Gamma Ray Emission
The all-sky survey in high-energy gamma rays (E30 MeV) carried out by the
Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma-Ray
Observatory provides a unique opportunity to examine in detail the diffuse
gamma-ray emission. The observed diffuse emission has a Galactic component
arising from cosmic-ray interactions with the local interstellar gas and
radiation as well an almost uniformly distributed component that is generally
believed to originate outside the Galaxy. Through a careful study and removal
of the Galactic diffuse emission, the flux, spectrum and uniformity of the
extragalactic emission is deduced. The analysis indicates that the
extragalactic emission is well described by a power law photon spectrum with an
index of -(2.10+-0.03) in the 30 MeV to 100 GeV energy range. No large scale
spatial anisotropy or changes in the energy spectrum are observed in the
deduced extragalactic emission. The most likely explanation for the origin of
this extragalactic high-energy gamma-ray emission is that it arises primarily
from unresolved gamma-ray-emitting blazars.Comment: 19 pages latex, 10 figures, accepted for publication in Ap
In-Flight Calibration of the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory
The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory has been operating for over seven years since its launch in 1991 April. This span of time far exceeds the design lifetime of two years. As the instrument has aged, several changes have occurred due to spark chamber gas exchanges as well as some hardware degradation and failures, all of which have an influence on the instrument sensitivity. This paper describes post-launch measurements and analysis that are done to calibrate the instrument response functions. The updated instrument characteristics are incorporated into the analysis software
Search for Gamma Ray Bursts with the Argo-YBJ Detector in Scaler Mode
We report on the search for Gamma Ray Bursts (GRBs) in the energy range 1-100
GeV in coincidence with the prompt emission detected by satellites using the
Astrophysical Radiation with Ground-based Observatory at YangBaJing (ARGO-YBJ)
air shower detector. Thanks to its mountain location (Yangbajing, Tibet, P.R.
China, 4300 m a.s.l.), active surface (about 6700 m**2 of Resistive Plate
Chambers), and large field of view (about 2 sr, limited only by the atmospheric
absorption), the ARGO-YBJ air shower detector is particularly suitable for the
detection of unpredictable and short duration events such as GRBs. The search
is carried out using the "single particle technique", i.e. counting all the
particles hitting the detector without measurement of the energy and arrival
direction of the primary gamma rays.
Between 2004 December 17 and 2009 April 7, 81 GRBs detected by satellites
occurred within the field of view of ARGO-YBJ (zenith angle < 45 deg). It was
possible to examine 62 of these for >1 GeV counterpart in the ARGO-YBJ data
finding no statistically significant emission. With a lack of detected spectra
in this energy range fluence upper limits are profitable, especially when the
redshift is known and the correction for the extragalactic absorption can be
considered. The obtained fluence upper limits reach values as low as 10**{-5}
erg cm**{-2} in the 1-100 GeV energy region.
Besides this individual search for a higher energy counterpart, a statistical
study of the stack of all the GRBs both in time and in phase was made, looking
for a common feature in the GRB high energy emission. No significant signal has
been detected.Comment: accepted for publication in Ap
The Escape of High-Energy Photons from Gamma-Ray Bursts
Eleven bright gamma-ray bursts (GRBs) detected by BATSE have also been seen
at much higher energies by EGRET, six at energies above 10 MeV. Such
observations imply that these bursts are optically thin to photon-photon pair
production at all observed energies. For bursts more than about 30pc away,
internal transparency can be achieved only if the source is moving with a
relativistic bulk Lorentz factor , or if the radiation is highly
beamed. Early calculations of considerations for GRBs
were limited to cases of a beam with opening half-angle \Thetab\sim 1/\Gamma,
or expansions of infinitely thin spherical shells. This paper presents our
extension of pair production optical depth calculations in relativistically
expanding sources to more general geometries, including shells of finite
thickness and arbitrary opening angle. The problem is reduced analytically to a
single integral in the broadly applicable case of observing photons along the
axis of the expansion. We find that the minimum bulk Lorentz factor for the
EGRET sources to be optically thin is only moderately dependent on the shell
thickness and virtually independent of its opening solid angle if
\Thetab\gtrsim 1/\Gamma. This insensitivity to \Thetab relieves the
commonly-perceived number problem for non-repeating sources at cosmological
distances, i.e. it is not necessary to invoke small \Thetab to effect photon
escape.Comment: 51 pages, including 8 eps figures, to appear in ApJ, December 20 199
Gamma ray astrophysics: the EGRET results
Cosmic gamma rays provide insight into some of the most dynamic processes in
the Universe. At the dawn of a new generation of gamma-ray telescopes, this
review summarizes results from the Energetic Gamma Ray Experiment Telescope
(EGRET) on the Compton Gamma Ray Observatory, the principal predecessor mission
studying high-energy photons in the 100 MeV energy range. EGRET viewed a
gamma-ray sky dominated by prominent emission from the Milky Way, but featuring
an array of other sources, including quasars, pulsars, gamma-ray bursts, and
many sources that remain unidentified. A central feature of the EGRET results
was the high degree of variability seen in many gamma-ray sources, indicative
of the powerful forces at work in objects visible to gamma-ray telescopes.Comment: 23 pages, 24 figure
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