188 research outputs found
AMRVAC and Relativistic Hydrodynamic simulations for GRB afterglow phases
We apply a novel adaptive mesh refinement code, AMRVAC, to numerically
investigate the various evolutionary phases in the interaction of a
relativistic shell with its surrounding cold Interstellar Medium (ISM). We do
this for both 1D isotropic as well as full 2D jetlike fireball models. This is
relevant for Gamma Ray Bursts, and we demonstrate that, thanks to the AMR
strategy, we resolve the internal structure of the shocked shell-ISM matter,
which will leave its imprint on the GRB afterglow. We determine the
deceleration from an initial Lorentz factor up to the almost
Newtonian phase of the flow. We present axisymmetric 2D
shell evolutions, with the 2D extent characterized by their initial opening
angle. In such jetlike GRB models, we discuss the differences with the 1D
isotropic GRB equivalents. These are mainly due to thermally induced sideways
expansions of both the shocked shell and shocked ISM regions. We found that the
propagating 2D ultrarelativistic shell does not accrete all the surrounding
medium located within its initial opening angle. Part of this ISM matter gets
pushed away laterally and forms a wide bow-shock configuration with swirling
flow patterns trailing the thin shell. The resulting shell deceleration is
quite different from that found in isotropic GRB models. As long as the lateral
shell expansion is merely due to ballistic spreading of the shell, isotropic
and 2D models agree perfectly. As thermally induced expansions eventually lead
to significantly higher lateral speeds, the 2D shell interacts with comparably
more ISM matter and decelerates earlier than its isotropic counterpart.Comment: 12 pages, accepted in MNRAS, 12/01/200
Flares in GRB afterglows from delayed magnetic dissipation
One of the most intriguing discoveries made by the Swift satellite is the
flaring activity in about half of the afterglow lightcurves. Flares have been
observed on both long and short duration GRBs and on time scales that range
from minutes to ~1 day after the prompt emission. The rapid evolution of some
flares led to the suggestion that they are caused by late central engine
activity. Here, I propose an alternative explanation that does not need
reviving of the central engine. Flares can be powered by delayed magnetic
dissipation in strongly magnetized (i.e. with initial Poynting to kinetic flux
ratio \simmore 1) ejecta during its deceleration due to interaction with the
external medium. A closer look at the length scales of the dissipation regions
shows that magnetic dissipation can give rise to fast evolving and energetic
flares. Multiple flares are also expected in the context of the model.Comment: 5 pages, accepted for publication in A&A Letter
Multiwavelength afterglow light curves from magnetized GRB flows
We use high-resolution relativistic MHD simulations coupled with a radiative
transfer code to compute multiwavelength afterglow light curves of magnetized
ejecta of gamma-ray bursts interacting with a uniform circumburst medium. The
aim of our study is to determine how the magnetization of the ejecta at large
distance from the central engine influences the afterglow emission, and to
assess whether observations can be reliably used to infer the strength of the
magnetic field. We find that, for typical parameters of the ejecta, the
emission from the reverse shock peaks for magnetization of the flow, and that it is greatly suppressed for higher . The
emission from the forward shock shows an achromatic break shortly after the end
of the burst marking the onset of the self-similar evolution of the blast wave.
Fitting the early afterglow of GRB 990123 and 090102 with our numerical models
we infer respective magnetizations of and for these bursts. We argue that the lack of observed reverse shock
emission from the majority of the bursts can be understood if \sigma_0
\simmore 0.1, since we obtain that the luminosity of the reverse shock
decreases significantly for . For ejecta with \sigma_0
\simmore 0.1 our models predict that there is sufficient energy left in the
magnetic field, at least during an interval of ~10 times the burst duration, to
produce a substantial emission if the magnetic energy can be dissipated (for
instance, due to resistive effects) and radiated away.Comment: 9 pages, 9 figures. Submitted to MNRAS
Energy and time-lag spectra of black-hole systems in the low state
Most, probably all, accreting binaries that are believed to contain a
black-hole emit radio waves when they are in the low/hard state. Whenever this
radio emission has been resolved, a jet-like structure has become apparent. We
propose that Compton upscattering of low-energy photons in the jet can explain
both the energy spectra and the time lags versus Fourier frequency observed in
the low/hard state of black-hole systems. The soft photons originate in the
inner part of the accretion disk. We have performed Monte Carlo simulations of
Compton upscattering in a jet and have found that for a rather wide range of
values of the parameters we can obtain power-law high-energy X-ray spectra with
photon-number index in the range 1.5 - 2 and power-law time lags versus Fourier
frequency with index ~ 0.7. The black-hole source Cyg X-1 in the low/hard state
is well described by our model.Comment: 4 pages 3 figures, to be published in A&A Letter
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