232 research outputs found
Impulsive and Varying Injection in GRB Afterglows
The standard model of Gamma-Ray Bursts afterglows is based on synchrotron
radiation from a blast wave produced when the relativistic ejecta encounters
the surrounding medium. We reanalyze the refreshed shock scenario, in which
slower material catches up with the decelerating ejecta and reenergizes it.
This energization can be done either continuously or in discrete episodes. We
show that such scenario has two important implications. First there is an
additional component coming from the reverse shock that goes into the
energizing ejecta. This persists for as long as the re-energization itself,
which could extend for up to days or longer. We find that during this time the
overall spectral peak is found at the characteristic frequency of the reverse
shock. Second, if the injection is continuous, the dynamics will be different
from that in constant energy evolution, and will cause a slower decline of the
observed fluxes. A simple test of the continuously refreshed scenario is that
it predicts a spectral maximum in the far IR or mm range after a few days.Comment: 12 page
Gamma-ray burst beaming: a universal configuration with a standard energy reservoir?
We consider a gamma-ray burst (GRB) model based on an anisotropic fireball
with an axisymmetric power-law distribution of the energy per solid angle with
index -k, and allow for the observer's viewing direction being at an arbitrary
angle with respect to the jet axis. This model can reproduce the key features
expected from the conventional on-axis uniform jet models, with the novelty
that the achromatic break time in the broadband afterglow lightcurves
corresponds to the epoch when the relativistic beaming angle is equal to the
viewing angle rather than to the jet half opening angle. If all the GRB
fireballs have such a similar energy distribution form with 1.5 < k < (or \sim)
2, GRBs may be modeled by a quasi-universal beaming configuration, and an
approximately standard energy reservoir. The conclusion also holds for some
other forms of angular energy distributions, such as the Gaussian function.Comment: Slightly expanded version accepted for publication in Ap
- …