617 research outputs found
The Fermi Gamma-ray Burst Monitor: Results from the first two years
In the first two years since the launch of the Fermi Observatory, the
Gamma-ray Burst Monitor (GBM) has detected over 500 Gamma-Ray Bursts (GRBs), of
which 18 were confidently detected by the Large Area Telescope (LAT) above 100
MeV. Besides GRBs, GBM has triggered on other transient sources, such as Soft
Gamma Repeaters (SGRs), Terrestrial Gamma-ray Flashes (TGFs) and solar flares.
Here we present the science highlights of the GBM observations.Comment: 4 pages, 1 figure, Proceedings of the 8th Workshop on Science with
the New Generation of High Energy Gamma-ray Experiments (SciNeGHE 2010),
Nuovo Cimento C, in pres
Magnetar giant flare high-energy emission
High energy ( keV) emission has been detected persisting for several
tens of seconds after the initial spike of magnetar giant flares. It has been
conjectured that this emission might arise via inverse Compton scattering in a
highly extended corona generated by super-Eddington outflows high up in the
magnetosphere. In this paper we undertake a detailed examination of this model.
We investigate the properties of the required scatterers, and whether the
mechanism is consistent with the degree of pulsed emission observed in the tail
of the giant flare. We conclude that the mechanism is consistent with current
data, although the origin of the scattering population remains an open
question. We propose an alternative picture in which the emission is closer to
that star and is dominated by synchrotron radiation. The observations
of the December 2004 flare modestly favor this latter picture. We assess the
prospects for the Fermi Gamma-Ray Space Telescope to detect and characterize a
similar high energy component in a future giant flare. Such a detection should
help to resolve some of the outstanding issues.Comment: 20 pages, 14 figure
Comparison of Fermi-LAT and CTA in the region between 10-100 GeV
The past decade has seen a dramatic improvement in the quality of data
available at both high (HE: 100 MeV to 100 GeV) and very high (VHE: 100 GeV to
100 TeV) gamma-ray energies. With three years of data from the Fermi Large Area
Telescope (LAT) and deep pointed observations with arrays of Cherenkov
telescope, continuous spectral coverage from 100 MeV to TeV exists for
the first time for the brightest gamma-ray sources. The Fermi-LAT is likely to
continue for several years, resulting in significant improvements in high
energy sensitivity. On the same timescale, the Cherenkov Telescope Array (CTA)
will be constructed providing unprecedented VHE capabilities. The optimisation
of CTA must take into account competition and complementarity with Fermi, in
particularly in the overlapping energy range 10100 GeV. Here we compare the
performance of Fermi-LAT and the current baseline CTA design for steady and
transient, point-like and extended sources.Comment: Accepted for Publication in Astroparticle Physic
The connection between Gamma-ray bursts and Supernovae Ib/c
It has been established that Gamma-Ray Bursts (GRBs) are connected to Supernovae (SNe) explosions of Type Ib/c. We intend to test whether the hypothesis of Type Ib/c SNe from different massive progenitors can reproduce the local GRB rate as well as the GRB rate as a function of redshift. We aim to predict the GRB rate at very high redshift under different assumptions about galaxy formation and star formation histories in galaxies. We assume different star formation histories in galaxies of different morphological type: ellipticals, spirals and irregulars. We explore different hypotheses concerning the progenitors of Type Ib/c SNe. We find an excellent agreement between the observed GRB local rate and the predicted Type Ib/c SN rate in irregular galaxies, when a range for single Wolf-Rayet stars of 40-100 M_sun is adopted. We also predict the cosmic Type Ib/c SN rate by taking into account all the galaxy types in an unitary volume of the Universe and we compare it with the observed cosmic GRB rate as a function of redshift. By assuming the formation of spheroids at high redshift, we predict a cosmic Type Ib/c SN rate, which is always higher than the GRB rate, suggesting that only a small fraction (0.1-1 %) of Type Ib/c SNe become GRBs. In particular, we find a ratio between the cosmic GRB rate and the cosmic Type Ib/c rate in the range 0.001-0.01, in agreement with previous estimates. Finally, due to the high star formation in spheroids at high redshift, which is our preferred scenario for galaxy formation, we predict more GRBs at high redshift than in the hierarchical scenario for galaxy formation, a prediction which awaits to be proven by future observations
Searching for long faint astronomical high energy transients: a data driven approach
HERMES Pathfinder is an in-orbit demonstration consisting of a constellation of six 3U nano-satellites hosting simple but innovative detectors for the monitoring of cosmic high-energy transients. The main objective of HERMES Pathfinder is to prove that accurate position of high-energy cosmic transients can be obtained using miniaturized hardware. The transient position is obtained by studying the delay time of arrival of the signal to different detectors hosted by nano-satellites on low-Earth orbits. In this context, we need to develop novel tools to fully exploit the future scientific data output of HERMES Pathfinder. In this paper, we introduce a new framework to assess the background count rate of a spaceborne, high energy detector; a key step towards the identification of faint astrophysical transients. We employ a neural network to estimate the background lightcurves on different timescales. Subsequently, we employ a fast change-point and anomaly detection technique called Poisson-FOCuS to identify observation segments where statistically significant excesses in the observed count rate relative to the background estimate exist. We test the new software on archival data from the NASA Fermi Gamma-ray Burst Monitor (GBM), which has a collecting area and background level of the same order of magnitude to those of HERMES Pathfinder. The neural network performances are discussed and analyzed over period of both high and low solar activity. We were able to confirm events in the Fermi-GBM catalog, both solar flares and gamma-ray bursts, and found events, not present in Fermi-GBM database, that could be attributed to solar flares, terrestrial gamma-ray flashes, gamma-ray bursts and galactic X-ray flashes. Seven of these are selected and further analyzed, providing an estimate of localisation and a tentative classification
The H.E.S.S. II GRB Program
Gamma-ray bursts (GRBs) are some of the most energetic and exotic events in
the Universe, however their behaviour at the highest energies (>10 GeV) is
largely unknown. Although the Fermi-LAT space telescope has detected several
GRBs in this energy range, it is limited by the relatively small collection
area of the instrument. The H.E.S.S. experiment has now entered its second
phase by adding a fifth telescope of 600 m mirror area to the centre of
the array. This new telescope increases the energy range of the array, allowing
it to probe the sub-100 GeV range while maintaining the large collection area
of ground based gamma-ray observatories, essential to probing short-term
variability at these energies. We will present a description of the GRB
observation scheme used by the H.E.S.S. experiment, summarising the behaviour
and performance of the rapid GRB repointing system, the conditions under which
potential GRB repointings are made and the data analysis scheme used for these
observations.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherland
Limits on the GeV Emission from Gamma-Ray Bursts
The Large Area Telescope (LAT) on board of the Fermi satellite detected
emission above 30 MeV only in a small fraction of the long gamma-ray bursts
(GRBs) detected by the Fermi Gamma-ray Burst Monitor (GBM) at 8 keV - 10 MeV.
Those bursts that were detected by the LAT were among the brightest GBM bursts.
We examine a sample of the most luminous GBM bursts with no LAT detection and
obtain upper limits on their high energy fluence. We find an average upper
limit of LAT/GBM fluence ratio of 0.13 for GeV fluence during and an
average upper limit ratio of 0.45 for GeV fluence during the first 600 seconds
after the trigger. These ratios strongly constrain various emission models and
in particular rule out SSC models for the prompt emission. In about a third of
both LAT detected and LAT non-detected bursts, we find that the extrapolation
of the MeV range Band spectrum to the GeV range is larger than the observed GeV
fluence (or its upper limit). While this excess is not highly significant for
any specific burst, the overall excess in a large fraction of the bursts
suggests a decline in the high energy spectral slope in at least some of these
bursts. Possibly an evidence for the long sought after pair creation limit.Comment: Accepted for publication in MNRA
SiPM and front-end electronics development for Cherenkov light detection
The Italian Institute of Nuclear Physics (INFN) is involved in the
development of a demonstrator for a SiPM-based camera for the Cherenkov
Telescope Array (CTA) experiment, with a pixel size of 66 mm. The
camera houses about two thousands electronics channels and is both light and
compact. In this framework, a R&D program for the development of SiPMs suitable
for Cherenkov light detection (so called NUV SiPMs) is ongoing. Different
photosensors have been produced at Fondazione Bruno Kessler (FBK), with
different micro-cell dimensions and fill factors, in different geometrical
arrangements. At the same time, INFN is developing front-end electronics based
on the waveform sampling technique optimized for the new NUV SiPM. Measurements
on 11 mm, 33 mm, and 66 mm NUV SiPMs
coupled to the front-end electronics are presentedComment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
Analysis of Sub-threshold Short Gamma-ray Bursts in Fermi GBM Data
The Fermi Gamma-ray Burst Monitor (GBM) is currently the most prolific
detector of Gamma-Ray Bursts (GRBs). Recently the detection rate of short GRBs
(SGRBs) has been dramatically increased through the use of ground-based
searches that analyze GBM continuous time tagged event (CTTE) data. Here we
examine the efficiency of a method developed to search CTTE data for
sub-threshold transient events in temporal coincidence with LIGO/Virgo compact
binary coalescence triggers. This targeted search operates by coherently
combining data from all 14 GBM detectors by taking into account the complex
spatial and energy dependent response of each detector. We use the method to
examine a sample of SGRBs that were independently detected by the Burst Alert
Telescope on board the Neil Gehrels Swift Observatory, but which were too
intrinsically weak or viewed with unfavorable instrument geometry to initiate
an on-board trigger of GBM. We find that the search can successfully recover a
majority of the BAT detected sample in the CTTE data. We show that the targeted
search of CTTE data will be crucial in increasing the GBM sensitivity, and
hence the gamma-ray horizon, to weak events such as GRB 170817A. We also
examine the properties of the GBM signal possibly associated with the LIGO
detection of GW150914 and show that it is consistent with the observed
properties of other sub-threshold SGRBs in our sample. We find that the
targeted search is capable of recovering true astrophysical signals as weak as
the signal associated with GW150914 in the untriggered data.Comment: 10 pages, 9 figures, 1 table, submitted to Ap
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