36 research outputs found
Magnetic Fields In Relativistic Collisionless Shocks
We present a systematic study on magnetic fields in Gamma-Ray Burst (GRB)
external forward shocks (FSs). There are 60 (35) GRBs in our X-ray (optical)
sample, mostly from Swift. We use two methods to study epsilon_B (fraction of
energy in magnetic field in the FS). 1. For the X-ray sample, we use the
constraint that the observed flux at the end of the steep decline is the
X-ray FS flux. 2. For the optical sample, we use the condition that the
observed flux arises from the FS (optical sample light curves decline as ~t^-1,
as expected for the FS). Making a reasonable assumption on E (jet isotropic
equivalent kinetic energy), we converted these conditions into an upper limit
(measurement) on epsilon_B n^{2/(p+1)} for our X-ray (optical) sample, where n
is the circumburst density and p is the electron index. Taking n=1 cm^-3, the
distribution of epsilon_B measurements (upper limits) for our optical (X-ray)
sample has a range of ~10^-8 -10^-3 (~10^-6 -10^-3) and median of ~few x 10^-5
(~few x 10^-5). To characterize how much amplification is needed, beyond shock
compression of a seed magnetic field ~10 muG, we expressed our results in terms
of an amplification factor, AF, which is very weakly dependent on n (AF propto
n^0.21 ). The range of AF measurements (upper limits) for our optical (X-ray)
sample is ~ 1-1000 (~10-300) with a median of ~50 (~50). These results suggest
that some amplification, in addition to shock compression, is needed to explain
the afterglow observations.Comment: Accepted to ApJ. Minor changes after Referee Report. 22 Pages, 7
Figure
Off-axis short GRBs from structured jets as counterparts to GW events
Binary neutron star mergers are considered to be the most favorable sources
that produce electromagnetic (EM) signals associated with gravitational waves
(GWs). These mergers are the likely progenitors of short duration gamma-ray
bursts (GRBs). The brief gamma-ray emission (the "prompt GRB" emission) is
produced by ultra-relativistic jets, as a result, this emission is strongly
beamed over a small solid angle along the jet. It is estimated to be a decade
or more before a short GRB jet within the LIGO volume points along our line of
sight. For this reason, the study of the prompt signal as an EM counterpart to
GW events has been sparse. We argue that for a realistic jet model, one whose
luminosity and Lorentz factor vary smoothly with angle, the prompt signal can
be detected for a significantly broader range of viewing angles. This can lead
to a new type of EM counterpart, an "off-axis" short GRB. Our estimates and
simulations show that it is feasible to detect these signals with the aid of
the temporal coincidence from a LIGO trigger, even if the observer is
substantially misaligned with respect to the jet.Comment: 6 pages, 4 figures, accepted to MNRAS Letter