127 research outputs found
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
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
Shedding light on the prompt high efficiency paradox - self consistent modeling of GRB afterglows
We examine GRBs with both Fermi-LAT and X-ray afterglow data. Assuming that
the 100MeV (LAT) emission is radiation from cooled electrons accelerated by
external shocks, we show that the kinetic energy of the blast wave estimated
from the 100MeV flux is 50 times larger than the one estimated from the X-ray
flux. This can be explained if either: i) electrons radiating at X-rays are
significantly cooled by SSC (suppressing the synchrotron flux above the cooling
frequency) or ii) if the X-ray emitting electrons, unlike those emitting at
100MeV energies, are in the slow cooling regime. In both cases the X-ray flux
is no longer an immediate proxy of the blast wave kinetic energy. We model the
LAT, X-ray and optical data and show that in general these possibilities are
consistent with the data, and explain the apparent disagreement between X-ray
and LAT observations. All possible solutions require weak magnetic fields:
(where is the fraction of shocked
plasma energy in magnetic fields). Using the LAT emission as a proxy for the
blast wave kinetic energy we find that the derived prompt efficiencies are of
order 15%. This is considerably lower compared with previous estimates (87% and
higher for the same bursts). This provides at least a partial solution to the
"prompt high efficiency paradox".Comment: 8 pages, 2 figures, proceedings of "Swift: 10 Years of Discovery
Ready, Set, Launch: Time Interval between a Binary Neutron Star Merger and Short Gamma-Ray Burst Jet Formation
The joint detection of GW170817/GRB 170817 confirmed the long-standing theory that binary neutron star mergers produce short gamma-ray burst (sGRB) jets that can successfully break out of the surrounding ejecta. At the same time, the association with a kilonova provided unprecedented information regarding the physical properties (such as masses and velocities) of the different ejecta constituents. Combining this knowledge with the observed luminosities and durations of cosmological sGRBs detected by the Burst Alert Telescope onboard the Neil Gehrels Swift Observatory, we revisit the breakout conditions of sGRB jets. Assuming self-collimation of sGRB jets does not play a critical role, we find that the time interval between the binary merger and the launch of a typical sGRB jet is ≾0.1 s. We also show that for a fraction of at least ~30% of sGRBs, the usually adopted assumption of static ejecta is inconsistent with observations, even if the polar ejecta mass is an order of magnitude smaller than that in GRB 170817. Our results disfavor magnetar central engines for powering cosmological sGRBs, limit the amount of energy deposited in the cocoon prior to breakout, and suggest that the observed delay of ~1.7 s in GW170817/GRB 170817 between the gravitational wave and gamma-ray signals is likely dominated by the propagation time of the jet to the gamma-ray production site
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