254 research outputs found
GRB Fermi-LAT Afterglows: Explaining Flares, Breaks, and Energetic Photons
The Fermi-LAT collaboration presented the second gamma-ray burst (GRB) catalog covering its first 10 years of operations. A significant fraction of afterglow-phase light curves in this catalog cannot be explained by the closure relations of the standard synchrotron forward-shock model, suggesting that there could be an important contribution from another process. In view of the above, we derive the synchrotron self-Compton (SSC) light curves from the reverse shock in the thick- and thin-shell regime for a uniform-density medium. We show that this emission could explain the GeV flares exhibited in some LAT light curves. Additionally, we demonstrate that the passage of the forward shock synchrotron cooling break through the LAT band from jets expanding in a uniform-density environment may be responsible for the late time (â10ÂČ s) steepening of LAT GRB afterglow light curves. As a particular case, we model the LAT light curve of GRB 160509A that exhibited a GeV flare together with a break in the long-lasting emission, and also two very high energy photons with energies of 51.9 and 41.5 GeV observed 76.5 and 242 s after the onset of the burst, respectively. Constraining the microphysical parameters and the circumburst density from the afterglow observations, we show that the GeV flare is consistent with an SSC reverse-shock model, the break in the long-lasting emission with the passage of the synchrotron cooling break through the Fermi-LAT band, and the very energetic photons with SSC emission from the forward shock, when the outflow carries a significant magnetic field (R_B â 30) and it decelerates in a uniform-density medium with a very low density (n = 4.554_(-1.121)^(+1.128) x 10â»âŽ cmâ»Âł
Radio data challenge the broadband modelling of GRB160131A afterglow
Context. Gamma-ray burst (GRB) afterglows originate from the interaction
between the relativistic ejecta and the surrounding medium. Consequently, their
properties depend on several aspects: radiation mechanisms, relativistic shock
micro-physics, circumburst environment, and the structure and geometry of the
relativistic jet. While the standard afterglow model accounts for the overall
spectral and temporal evolution for a number of GRBs, its validity limits
emerge when the data set is particularly rich and constraining, especially in
the radio band.
Aims. We aimed to model the afterglow of the long GRB160131A (redshift ), for which we collected a rich, broadband, and accurate data set,
spanning from to Hz in frequency, and from 330
s to 160 days post burst in time.
Methods. We modelled the spectral and temporal evolution of this GRB
afterglow through two approaches: the adoption of empirical functions to model
optical/X-rays data set, later assessing their compatibility with the radio
domain; the inclusion of the entire multi-frequency data set simultaneously
through the Python package named sAGa (Software for AfterGlow Analysis), to
come up with an exhaustive and self-consistent description of the
micro-physics, geometry, and dynamics of the afterglow.
Results. From deep broadband analysis (from radio to X-ray frequencies) of
the afterglow light curves, GRB160131A outflow shows evidence of jetted
emission. Moreover, we observe dust extinction in the optical spectra, and
energy injection in the optical/X-ray data. Radio spectra are characterised by
several peaks, that could be due to either interstellar scintillation (ISS)
effects or a multi-component structure.
Conclusions. The inclusion of radio data in the broadband set of GRB160131A
makes a self-consistent modelling hardly attainable within the standard model
of GRB afterglows.Comment: 36 pages, 16 figures, 8 tables, accepted by A&A; v2: updated
Acknowledgement
Reverse Shock Emission Revealed in Early Photometry in the Candidate Short GRB 180418A
We present observations of the possible short GRB 180418A in -rays,
X-rays, and in the optical. Early optical photometry with the TAROT and RATIR
instruments show a bright peak ( 14.2 AB mag) between and
seconds that we interpret as the signature of a reversal shock. Later
observations can be modeled by a standard forward shock model and show no
evidence of jet break, allowing us to constrain the jet collimation to
. Using deep late-time optical observations we place an
upper limit of AB mag on any underlying host galaxy. The detection of
the afterglow in the \textit{Swift} UV filters constrains the GRB redshift to
and places an upper bound on the -ray isotropic equivalent
energy erg.
The properties of this GRB (e.g. duration, hardness ratio, energetic, and
environment) lie at the intersection between short and long bursts, and we can
not conclusively identify its type. We estimate that the probability that it is
drawn from the population of short GRBs is 10\%-30\%.Comment: Accepted por publication in Ap
Evidence of extended emission in GRB 181123B and other high-redshift short GRBs
We study the high-energy properties of GRB 181123B, a short gamma-ray burst
(sGRB) at redshift z1.75. We show that, despite its nominal short
duration with T90 < 2 s, this burst display evidence of a temporally extended
emission (EE) at high energies and that the same trend is observed in the
majority of sGRBs at z > 1. We discuss the impact of instrumental selection
effects on the GRB classification, stressing that the measured T90 is not an
unambiguous indicator of the burst physical origin. By examining their
environment (e.g. stellar mass, star formation, offset distribution), we find
that these high-z sGRBs share many properties of long GRBs at a similar
distance and are consistent with a short-lived progenitor system. If produced
by compact binary mergers, these sGRBs with EE may herald a larger population
of sGRBs in the early universe.Comment: 10 pages, 6 figures, 2 tables. Submitted to ApJ
Polarization From A Radially Stratified Off-Axis GRB Outflow
While the dominant radiation mechanism gamma-ray bursts (GRBs) remains a
question of debate, synchrotron emission is one of the foremost candidates to
describe the multi-wavelength afterglow observations. As such, it is expected
that GRBs should present some degree of polarization across their evolution -
presenting a feasible means of probing these bursts' energetic and angular
properties. Although obtaining polarization data is difficult due to the
inherent complexities regarding GRB observations, advances are being made, and
theoretical modeling of synchrotron polarization is now more relevant than
ever. In this manuscript, we present the polarization for a fiduciary model
where the synchrotron forward-shock emission evolving in the
radiative-adiabatic regime is described by a radially stratified off-axis
outflow. This is parameterized with a power-law velocity distribution and
decelerated in a constant-density and wind-like external environment. We apply
this theoretical polarization model for selected bursts presenting evidence of
off-axis afterglow emission, including the nearest orphan GRB candidates
observed by the Neil Gehrels Swift Observatory and a few Gravitational Wave
(GWs) events that could generate electromagnetic emission. In the case of GRB
170817A, we require the available polarimetric upper limits in radio
wavelengths to constrain its magnetic field geometry.Comment: In submission. 18 pages, 7 figures, 3 table
Afterglow Polarization from Off-Axis GRB Jets
As we further our studies on Gamma-ray bursts (GRBs), both on theoretical
models and observational tools, more and more options begin to open for
exploration of its physical properties. As transient events primarily dominated
by synchrotron radiation, it is expected that the synchrotron photons emitted
by GRBs should present some degree of polarization throughout the evolution of
the burst. Whereas observing this polarization can still be challenging due to
the constraints on observational tools, especially for short GRBs, it is
paramount that the groundwork is laid for the day we have abundant data. In
this work, we present a polarization model linked with an off-axis spreading
top-hat jet synchrotron scenario in a stratified environment with a density
profile . We present this model's expected temporal
polarization evolution for a realistic set of afterglow parameters constrained
within the values observed in the GRB literature for four degrees of
stratification and two magnetic field
configurations with high extreme anisotropy. We apply this model and predict
polarization from a set of GRBs exhibiting off-axis afterglow emission. In
particular, for GRB 170817A, we use the available polarimetric upper limits to
rule out the possibility of a extremely anisotropic configuration for the
magnetic field.Comment: 17 pages, 3 tables, 6 figures. Accepted for publication in the
Astrophysical Journal (ApJ
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