3,017 research outputs found
On the Non-existence of a Sharp Cooling Break in GRB Afterglow Spectra
Although the widely-used analytical afterglow model of gamma-ray bursts
(GRBs) predicts a sharp cooling break in its afterglow spectrum, the
GRB observations so far rarely show clear evidence for a cooling break in their
spectra or its corresponding temporal break in their light curves. Employing a
Lagrangian description of the blast wave, we conduct a sophisticated
calculation of the afterglow emission. We precisely follow the cooling history
of non-thermal electrons accelerated into each Lagrangian shell. We show that a
detailed calculation of afterglow spectra does not in fact give rise to a sharp
cooling break at . Instead, it displays a very mild and smooth
transition, which occurs gradually over a few orders of magnitude in energy or
frequency. The main source of this slow transition is that different
mini-shells have different evolution histories of the comoving magnetic field
strength , so that deriving the current value of of each mini-shell
requires an integration of its cooling rate over the time elapsed since its
creation. We present the time evolution of optical and X-ray spectral indices
to demonstrate the slow transition of spectral regimes, and discuss the
implications of our result in interpreting GRB afterglow data.Comment: Accepted for publication in ApJ, 17 pages, 5 figures; significantly
expanded to address the referee's reports, new section (2.2) and three more
figures added, conclusion unchange
A Statistical Study of GRB X-ray Flares: Evidence of Ubiquitous Bulk Acceleration in the Emission Region
When emission in a conical relativistic jet ceases abruptly (or decays
sharply), the observed decay light curve is controlled by the high-latitude
"curvature effect". Recently, Uhm & Zhang found that the decay slopes of three
GRB X-ray flares are steeper than what the standard model predicts. This
requires bulk acceleration of the emission region, which is consistent with a
Poynting-flux-dominated outflow. In this paper, we systematically analyze a
sample of 85 bright X-ray flares detected in 63 Swift GRBs, and investigate the
relationship between the temporal decay index and spectral index
during the steep decay phase of these flares. The value
depends on the choice of the zero time point . We adopt two methods.
"Method I" takes as the first rising data point of each flare, and is
the most conservative approach. We find that at 99.9% condifence level 56/85
flares have decay slopes steeper than the simplest curvature effect prediction,
and therefore, are in the acceleration regime. "Method II" extrapolates the
rising light curve of each flare backwards until the flux density is three
orders of magnitude lower than the peak flux density, and defines the
corresponding time as the time zero point (t_0^II). We find that 74/85 flares
fall into the acceleration regime at 99.9% condifence level. This suggests that
bulk acceleration is common, may be even ubiquitous among X-ray flares,
pointing towards a Poynting-flux-dominated jet composition for these events.Comment: 68 pages, 6 figures, 2 tables, ApJS, in pres
Gamma-ray burst early optical afterglows: implications for the initial Lorentz factor and the central engine
Early optical afterglows have been observed from GRB 990123, GRB 021004, and
GRB 021211, which reveal rich emission features attributed to reverse shocks.
It is expected that Swift will discover many more early afterglows. Here we
investigate in a unified manner both the forward and the reverse external shock
emission components, and introduce a straightforward recipe for directly
constraining the initial Lorentz factor of the fireball using early optical
afterglow data. The scheme is largely independent of the shock microphysics. We
identify two types of combinations of the reverse and forward shock emission,
and explore their parameter regimes. We also discuss a possible diagnostic for
magnetized ejecta. There is evidence that the central engine of GRB 990123 is
strongly magnetized.Comment: emulateapj style, 6 pages, 1 figure. Expanded version accepted for
publication in ApJ Part
Early photon-shock interaction in stellar wind: sub-GeV photon flash and high energy neutrino emission from long GRBs
For gamma-ray bursts (GRBs) born in a stellar wind, as the reverse shock
crosses the ejecta, usually the shocked regions are still precipitated by the
prompt MeV \gamma-ray emission. Because of the tight overlapping of the MeV
photon flow with the shocked regions, the optical depth for the GeV photons
produced in the shocks is very large. These high energy photons are absorbed by
the MeV photon flow and generate relativistic e^\pm pairs. These pairs
re-scatter the soft X-ray photons from the forward shock as well as the prompt
\gamma-ray photons and power detectable high energy emission, significant part
of which is in the sub-GeV energy range. Since the total energy contained in
the forward shock region and the reverse shock region are comparable, the
predicted sub-GeV emission is independent on whether the GRB ejecta are
magnetized (in which case the reverse shock IC and synchrotron self-Compton
emission is suppressed). As a result, a sub-GeV flash is a generic signature
for the GRB wind model, and it should be typically detectable by the future
{\em Gamma-Ray Large Area Telescope} (GLAST). Overlapping also influence
neutrino emission. Besides the 10^{15} \sim 10^{17} eV neutrino emission
powered by the interaction of the shock accelerated protons with the
synchrotron photons in both the forward and reverse shock regions, there comes
another eV neutrino emission component powered by protons interacting
with the MeV photon flow. This last component has a similar spectrum to the one
generated in the internal shock phase, but the typical energy is slightly
lower.Comment: 7 pages, accepted for publication in Ap
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