1,855 research outputs found
Evidence of an Internal Dissipation Origin for the High-energy Prompt Emission of GRB 170214A
The origin of the prompt high-energy (MeV) emission of Gamma-ray Bursts
(GRBs), detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray
Space Telescope, is still under debate, for which both the external shock
origin and internal dissipation origin have been suggested. In the internal
dissipation scenario, the high energy emission is expected to exhibit
significant temporal variability, tracking the keV/MeV fast variable behavior.
Here, we report a detailed analysis on the Fermi data of GRB~170214A, which is
sufficiently bright in the high energy to enable a quantitative analysis of the
correlation between high-energy emission and keV/MeV emission with high
statistics. Our result shows a clear temporal correlation between high-energy
and keV/MeV emission in the whole prompt emission phase as well as in two
decomposed short time intervals. Such correlation behavior is also found in
some other bright LAT GRBs, i.e., GRB 080916C, 090902B and 090926A. For these
GRBs as well as GRB 090510, we also find the rapid temporal variability in the
high-energy emission. We thus conclude that the prompt high-energy emission in
these bright LAT GRBs should be due to internal origin.Comment: 12 pages, 4 figures, Accepted for publication in Ap
Context-Aware Single-Shot Detector
SSD is one of the state-of-the-art object detection algorithms, and it
combines high detection accuracy with real-time speed. However, it is widely
recognized that SSD is less accurate in detecting small objects compared to
large objects, because it ignores the context from outside the proposal boxes.
In this paper, we present CSSD--a shorthand for context-aware single-shot
multibox object detector. CSSD is built on top of SSD, with additional layers
modeling multi-scale contexts. We describe two variants of CSSD, which differ
in their context layers, using dilated convolution layers (DiCSSD) and
deconvolution layers (DeCSSD) respectively. The experimental results show that
the multi-scale context modeling significantly improves the detection accuracy.
In addition, we study the relationship between effective receptive fields
(ERFs) and the theoretical receptive fields (TRFs), particularly on a VGGNet.
The empirical results further strengthen our conclusion that SSD coupled with
context layers achieves better detection results especially for small objects
( on MS-COCO compared to the newest SSD), while
maintaining comparable runtime performance
First detection of GeV emission from an ultraluminous infrared galaxy: Arp 220 as seen with the Fermi Large Area Telescope
Cosmic rays (CRs) in starburst galaxies produce high energy gamma-rays by
colliding with the dense interstellar medium (ISM). Arp 220 is the nearest
ultra luminous infrared galaxy (ULIRG) that has star-formation at extreme
levels, so it has long been predicted to emit high-energy gamma-rays. However,
no evidence of gamma-ray emission was found despite intense efforts of search.
Here we report the discovery of high-energy gamma-ray emission above 200 MeV
from Arp 220 at a confidence level of using 7.5 years of
\textsl {Fermi} Large Area Telescope observations. The gamma-ray emission shows
no significant variability over the observation period and it is consistent
with the quasi-linear scaling relation between the gamma-ray luminosity and
total infrared luminosity for star-forming galaxies, suggesting that these
gamma-rays arise from CR interactions. As the high density medium of Arp 220
makes it an ideal CR calorimeter, the gamma-ray luminosity can be used to
measure the efficiency of powering CRs by supernova (SN) remnants given a known
supernova rate in Arp 220. We find that this efficiency is about
for CRs above 1 GeV.Comment: Accepted by ApJL, 6 pages, 3 figure
Discovery of an extra hard spectral component in the high-energy afterglow emission of GRB 130427A
The extended high-energy gamma-ray (>100 MeV) emission occurred after the
prompt gamma-ray bursts (GRBs) is usually characterized by a single power-law
spectrum, which has been explained as the afterglow synchrotron radiation. The
afterglow inverse-Compton emission has long been predicted to be able to
produce a high-energy component as well, but previous observations have not
revealed such a signature clearly, probably due to the small number of >10 GeV
photons even for the brightest GRBs known so far. In this Letter, we report on
the Fermi Large Area Telescope (LAT) observations of the >100 MeV emission from
the very bright and nearby GRB 130427A. We characterize the time-resolved
spectra of the GeV emission from the GRB onset to the afterglow phase. By
performing time-resolved spectral fits of GRB 130427A, we found a strong
evidence of an extra hard spectral component that exists in the extended
high-energy emission of this GRB. We argue that this hard component may arise
from the afterglow inverse Compton emission.Comment: 5 pages, 2 figures, 2 tables, ApJL, in pres
Evidence of a spectral break in the gamma-ray emission of the disk component of Large Magellanic Cloud: a hadronic origin?
It has been suggested that high-energy gamma-ray emission ()
of nearby star-forming galaxies may be produced predominantly by cosmic rays
colliding with the interstellar medium through neutral pion decay. The
pion-decay mechanism predicts a unique spectral signature in the gamma-ray
spectrum, characterized by a fast rising spectrum and a spectral break below a
few hundreds of MeV. We here report the evidence of a spectral break around 500
MeV in the disk emission of Large Magellanic Cloud (LMC), which is found in the
analysis of the gamma-ray data extending down to 60 MeV observed by {\it
Fermi}-Large Area Telescope. The break is well consistent with the pion-decay
model for the gamma-ray emission, although leptonic models, such as the
electron bremsstrahlung emission, cannot be ruled out completely.Comment: 11 pages, 4 figures, Accepted by Ap
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