12 research outputs found
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
Modeling Gamma-ray burst Afterglow observations with an Off-axis Jet emission
Gamma-ray bursts (GRBs) are fascinating extragalactic objects. They represent
a fantastic opportunity to investigate unique properties not exhibited in other
sources. Multi-wavelength afterglow observations from some short- and
long-duration GRBs reveal an atypical long-lasting emission that evolves
differently from the canonical afterglow light curves favoring the off-axis
emission. We present an analytical synchrotron afterglow scenario, and the
hydrodynamical evolution of an off-axis top-hat jet decelerated in a stratified
surrounding environment. The analytical synchrotron afterglow model is shown
during the coasting, deceleration (off- and on-axis emission), and the
post-jet-break decay phases, and the hydrodynamical evolution is computed by
numerical simulations showing the time evolution of the Doppler factor, the
half-opening angle, the bulk Lorentz factor, and the deceleration radius. We
show that numerical simulations are in good agreement with those derived with
our analytical approach. We apply the current synchrotron model and describe
successfully the delayed non-thermal emission observed in a sample of long and
short GRBs with evidence of off-axis emission. Furthermore, we provide
constraints on the possible afterglow emission by requiring the
multi-wavelength upper limits derived for the closest Swift-detected GRBs and
promising gravitational-wave events.Comment: 36 pages, 16 figures, accepted for publication in Ap