56 research outputs found
The Signature of Refreshed Shocks in the of Afterglow of GRB030329
GRB030329 displays one clear and, possibly, multiple less intense fast-rising
() jumps in its optical afterglow light curve. The decay
rate of the optical light curve remains the same before and after the photon
flux jumps. This may be the signature of energy injection into the forward and
reverse shocked material at the front of the jet. In this study, we model the
Gamma-Ray Burst (GRB) ejecta as a series of shells of material. We follow the
dynamical evolution of the ejecta as it interacts with itself (i.e., internal
shocks) and with the circumburst medium (i.e., external forward and reverse
shocks), and we calculate the emission from each shock event assuming
synchrotron emission. We confirm the viability of the model proposed by
\citet{2003Natur.426..138G} in which the jumps in the optical afterglow light
curve of GRB030329 are produced via refreshed shocks. The refreshed shocks may
be the signatures of the collisions between earlier ejected material with an
average Lorentz factor and later ejected material
with once the early material has decelerated due to
interaction with the circumburst medium. We show that even if the late material
is ejected with a spread of Lorentz factors, internal shocks naturally produce
a narrow distribution of Lorentz factors (),
which is a necessary condition to produce the observed quick rise times of the
jumps. These results imply a phase of internal shocks at some point in the
dynamical evolution of the ejecta, which requires a low magnetization in the
outflow.Comment: 11 pages, 6 figure
Hadronic Models for the Extra Spectral Component in the short GRB 090510
A short gamma-ray burst GRB 090510 detected by {\it Fermi} shows an extra
spectral component between 10 MeV and 30 GeV, an addition to a more usual
low-energy ( MeV) Band component. In general, such an extra component
could originate from accelerated protons. In particular, inverse Compton
emission from secondary electron-positron pairs and proton synchrotron emission
are competitive models for reproducing the hard spectrum of the extra component
in GRB 090510. Here, using Monte Carlo simulations, we test the hadronic
scenarios against the observed properties. To reproduce the extra component
around GeV with these models, the proton injection isotropic-equivalent
luminosity is required to be larger than erg/s. Such large proton
luminosities are a challenge for the hadronic models.Comment: 12pages, 4 figures. Accepted for publication in ApJ
Temporal and Spectral Evolution of Gamma-ray Burst Broad Pulses: Identification of High Latitude Emission in the Prompt Emission
We perform a detailed analysis on broad pulses in bright Gamma-ray bursts
(GRBs) to understand the evolution of GRB broad pulses. Using the temporal and
spectral properties, we test the high latitude emission (HLE) scenario in the
decaying phase of broad pulses. The HLE originates from the curvature effect of
a relativistic spherical jet, where higher latitude photons are delayed and
softer than the observer's line-of-sight emission. The signature of HLE has not
yet been identified undisputedly during the prompt emission of GRBs. The HLE
theory predicts a specific relation, F E_{p}\!^{2},
between the peak energy in F spectra and the spectral flux
F measured at , F. We search for evidence of this
relation in 2157 GRBs detected by the Gamma-ray Burst Monitor (GBM) on board
the Fermi Gamma-ray Space Telescope (Fermi) from the years 2008 to 2017. After
imposing unbiased selection criteria in order to minimize contamination in a
signal by background and overlaps of pulses, we build a sample of 32 broad
pulses in 32 GRBs. We perform a time-resolved spectral analysis on each of
these 32 broad pulses and find that the evolution of 18 pulses (56%) is clearly
consistent with the HLE relation. For the 18 broad pulses, the exponent
in the relation of F E_{p}\!^{\delta} is
distributed as a Gaussian function with median and width of 1.99 and 0.34,
respectively. This result provides constraint on the emission radius of GRBs
with the HLE signature.Comment: 30 pages, 36 figures, accepted to Ap
Supermassive black holes at high redshifts
MeV blazars are the most luminous persistent sources in the Universe and emit
most of their energy in the MeV band. These objects display very large jet
powers and accretion luminosities and are known to host black holes with a mass
often exceeding . An MeV survey, performed by a new generation
MeV telescope which will bridge the entire energy and sensitivity gap between
the current generation of hard X-ray and gamma-ray instruments, will detect
1000 MeV blazars up to a redshift of . Here we show that this would
allow us: 1) to probe the formation and growth mechanisms of supermassive black
holes at high redshifts, 2) to pinpoint the location of the emission region in
powerful blazars, 3) to determine how accretion and black hole spin interplay
to power the jet.Comment: 7 pages, 4 figure. Submitted to the Astro2020 call for Science White
Paper
Anomalies in low-energy Gamma-Ray Burst spectra with the Fermi Gamma-Ray Burst Monitor
A Band function has become the standard spectral function used to describe
the prompt emission spectra of gamma-ray bursts (GRBs). However, deviations
from this function have previously been observed in GRBs detected by BATSE and
in individual GRBs from the \textit{Fermi} era. We present a systematic and
rigorous search for spectral deviations from a Band function at low energies in
a sample of the first two years of high fluence, long bursts detected by the
\textit{Fermi} Gamma-Ray Burst Monitor (GBM). The sample contains 45 bursts
with a fluence greater than 2 erg / cm (10 - 1000 keV). An
extrapolated fit method is used to search for low-energy spectral anomalies,
whereby a Band function is fit above a variable low-energy threshold and then
the best fit function is extrapolated to lower energy data. Deviations are
quantified by examining residuals derived from the extrapolated function and
the data and their significance is determined via comprehensive simulations
which account for the instrument response. This method was employed for both
time-integrated burst spectra and time-resolved bins defined by a signal to
noise ratio of 25 and 50 . Significant deviations are evident
in 3 bursts (GRB\,081215A, GRB\,090424 and GRB\,090902B) in the time-integrated
sample ( 7%) and 5 bursts (GRB\,090323, GRB\,090424, GRB\,090820,
GRB\,090902B and GRB\,090926A) in the time-resolved sample ( 11%).} The
advantage of the systematic, blind search analysis is that it can demonstrate
the requirement for an additional spectral component without any prior
knowledge of the nature of that extra component. Deviations are found in a
large fraction of high fluence GRBs; fainter GRBs may not have sufficient
statistics for deviations to be found using this method
First-year Results of Broadband Spectroscopy of the Brightest Fermi-GBM Gamma-Ray Bursts
We present here our results of the temporal and spectral analysis of a sample
of 52 bright and hard gamma-ray bursts (GRBs) observed with the Fermi Gamma-ray
Burst Monitor (GBM) during its first year of operation (July 2008-July 2009).
Our sample was selected from a total of 253 GBM GRBs based on each event peak
count rate measured between 0.2 and 40MeV. The final sample comprised 34 long
and 18 short GRBs. These numbers show that the GBM sample contains a much
larger fraction of short GRBs, than the CGRO/BATSE data set, which we explain
as the result of our (different) selection criteria and the improved GBM
trigger algorithms, which favor collection of short, bright GRBs over BATSE. A
first by-product of our selection methodology is the determination of a
detection threshold from the GBM data alone, above which GRBs most likely will
be detected in the MeV/GeV range with the Large Area Telescope (LAT) onboard
Fermi. This predictor will be very useful for future multiwavelength GRB follow
ups with ground and space based observatories. Further we have estimated the
burst durations up to 10MeV and for the first time expanded the duration-energy
relationship in the GRB light curves to high energies. We confirm that GRB
durations decline with energy as a power law with index approximately -0.4, as
was found earlier with the BATSE data and we also notice evidence of a possible
cutoff or break at higher energies. Finally, we performed time-integrated
spectral analysis of all 52 bursts and compared their spectral parameters with
those obtained with the larger data sample of the BATSE data. We find that the
two parameter data sets are similar and confirm that short GRBs are in general
harder than longer ones.Comment: 40 pages, 11 figures, 3 tables, Submitted to Ap
Time-Resolved Spectroscopy of the 3 Brightest and Hardest Short Gamma-Ray Bursts Observed with the FGST Gamma-Ray Burst Monitor
From July 2008 to October 2009, the Gamma-ray Burst Monitor (GBM) on board
the Fermi Gamma-ray Space Telescope (FGST) has detected 320 Gamma-Ray Bursts
(GRBs). About 20% of these events are classified as short based on their T90
duration below 2 s. We present here for the first time time-resolved
spectroscopy at timescales as short as 2 ms for the three brightest short GRBs
observed with GBM. The time-integrated spectra of the events deviate from the
Band function, indicating the existence of an additional spectral component,
which can be fit by a power-law with index ~-1.5. The time-integrated Epeak
values exceed 2 MeV for two of the bursts, and are well above the values
observed in the brightest long GRBs. Their Epeak values and their low-energy
power-law indices ({\alpha}) confirm that short GRBs are harder than long ones.
We find that short GRBs are very similar to long ones, but with light curves
contracted in time and with harder spectra stretched towards higher energies.
In our time-resolved spectroscopy analysis, we find that the Epeak values range
from a few tens of keV up to more than 6 MeV. In general, the hardness
evolutions during the bursts follows their flux/intensity variations, similar
to long bursts. However, we do not always see the Epeak leading the light-curve
rises, and we confirm the zero/short average light-curve spectral lag below 1
MeV, already established for short GRBs. We also find that the time-resolved
low-energy power-law indices of the Band function mostly violate the limits
imposed by the synchrotron models for both slow and fast electron cooling and
may require additional emission processes to explain the data. Finally, we
interpreted these observations in the context of the current existing models
and emission mechanisms for the prompt emission of GRBs.Comment: 14 pages, 10 figures, 9 tables, Accepted for publication in the
Astrophysical Journal September, 23 2010 (Submitted May, 16 2010)
Corrections: 1 reference updated, figure 10 captio
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