114,197 research outputs found
Expected neutrino fluence from short Gamma-Ray Burst 170817A and off-axis angle constraints
We compute the expected neutrino fluence from SGRB 170817A, associated with
the gravitational wave event GW 170817, directly based on Fermi observations in
two scenarios: structured jet and off-axis (observed) top-hat jet. While the
expected neutrino fluence for the structured jet case is very small, large
off-axis angles imply high radiation densities in the jet, which can enhance
the neutrino production efficiency. In the most optimistic allowed scenario,
the neutrino fluence can reach only of the sensitivity of the
neutrino telescopes. We furthermore demonstrate that the fact that gamma-rays
can escape limits the baryonic loading (energy in protons versus photons) and
the off-axis angle for the internal shock scenario. In particular, for a
baryonic loading of ten, the off-axis angle is more strongly constrained by the
baryonic loading than by the time delay between the gravitational wave event
and the onset of the gamma-ray emission.Comment: 9 pages, 6 figure
"Orphan" afterglows in the Universal Structured Jet Model for gamma-ray bursts
The paucity of reliable achromatic breaks in Gamma-Ray Burst afterglow light
curves motivates independent measurements of the jet aperture. Serendipitous
searches of afterglows, especially at radio wavelengths, have long been the
classic alternative. These survey data have been interpreted assuming a
uniformly emitting jet with sharp edges (``top-hat'' jet), in which case the
ratio of weakly relativistically beamed afterglows to GRBs scales with the jet
solid angle. In this paper, we consider, instead, a very wide outflow with a
luminosity that decreases across the emitting surface. In particular, we adopt
the universal structured jet (USJ) model, that is an alternative to the top-hat
model for the structure of the jet. However, the interpretation of the survey
data is very different: in the USJ model we only observe the emission within
the jet aperture and the observed ratio of prompt emission rate to afterglow
rate should solely depend on selection effects. We compute the number and rate
of afterglows expected in all-sky snapshot observations as a function of the
survey sensitivity. We find that the current (negative) results for OA searches
are in agreement with our expectations. In radio and X-ray bands this was
mainly due to the low sensitivity of the surveys, while in the optical band the
sky-coverage was not sufficient. In general we find that X-ray surveys are poor
tools for OA searches, if the jet is structured. On the other hand, the FIRST
radio survey and future instruments like the Allen Telescope Array (in the
radio band) and especially GAIA, Pan-Starrs and LSST (in the optical band) will
have chances to detect afterglows.Comment: 10 pages, 8 figures. MNRAS accepted. Moderate revision
VERITAS Upper Limit on the VHE Emission from the Radio Galaxy NGC 1275
The recent detection by the Fermi gamma-ray space telescope of high-energy
gamma-rays from the radio galaxy NGC 1275 makes the observation of the very
high energy (VHE: E > 100 GeV) part of its broadband spectrum particularly
interesting, especially for the understanding of active galactic nuclei (AGN)
with misaligned multi-structured jets. The radio galaxy NGC 1275 was recently
observed by VERITAS at energies above 100 GeV for about 8 hours. No VHE
gamma-ray emission was detected by VERITAS from NGC 1275. A 99% confidence
level upper limit of 2.1% of the Crab Nebula flux level is obtained at the
decorrelation energy of approximately 340 GeV, corresponding to 19% of the
power-law extrapolation of the Fermi Large Area Telescope (LAT) result.Comment: Accepted for publication in ApJ Letter
VERITAS Upper Limit on the VHE Emission from the Radio Galaxy NGC 1275
The recent detection by the Fermi gamma-ray space telescope of high-energy
gamma-rays from the radio galaxy NGC 1275 makes the observation of the very
high energy (VHE: E > 100 GeV) part of its broadband spectrum particularly
interesting, especially for the understanding of active galactic nuclei (AGN)
with misaligned multi-structured jets. The radio galaxy NGC 1275 was recently
observed by VERITAS at energies above 100 GeV for about 8 hours. No VHE
gamma-ray emission was detected by VERITAS from NGC 1275. A 99% confidence
level upper limit of 2.1% of the Crab Nebula flux level is obtained at the
decorrelation energy of approximately 340 GeV, corresponding to 19% of the
power-law extrapolation of the Fermi Large Area Telescope (LAT) result.Comment: Accepted for publication in ApJ Letter
A stacking method to study the gamma-ray emission of source samples based on the co-adding of Fermi LAT count maps
We present a stacking method that makes use of co-added maps of gamma-ray
counts produced from data taken with the Fermi Large Area Telescope. Sources
with low integrated gamma-ray fluxes that are not detected individually may
become detectable when their corresponding count maps are added. The combined
data set is analyzed with a maximum likelihood method taking into account the
contribution from point-like and diffuse background sources. For both simulated
and real data, detection significance and integrated gamma-ray flux are
investigated for different numbers of stacked sources using the public Fermi
Science Tools for analysis and data preparation. The co-adding is done such
that potential source signals add constructively, in contrast to the signals
from background sources, which allows the stacked data to be described with
simply structured models. We show, for different scenarios, that the stacking
method can be used to increase the cumulative significance of a sample of
sources and to characterize the corresponding gamma-ray emission. The method
can, for instance, help to search for gamma-ray emission from galaxy clusters.Comment: accepted for publication in Astronomy & Astrophysics, 10 pages, 12
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