Rodolfo Barniol Duran

External forward shock origin of high energy emission for three GRBs detected by Fermi

We analyze the >100MeV data for 3 GRBs detected by Fermi (GRBs 080916C, 090510, 090902B) and find that these photons were generated via synchrotron emission in the external forward shock. We arrive at this conclusion by four different methods as follows. (1) We check the light curve and spectral behavior of the >100MeV data, and late time X-ray and optical data, and find them consistent with the closure relations for the external forward shock radiation. (2) We calculate the expected external forward shock synchrotron flux at 100MeV, and it matches the observed flux value. (3) We determine the external forward shock model parameters using the >100MeV data, and with these we calculate the expected X-ray and optical fluxes at late times (hours to days after the burst) and find these to be in good agreement with the observed data. (4) We calculate the external forward shock model parameters using only the late time X-ray, optical and radio data and from these estimate the expected flux at >100 MeV at the end of the sub-MeV burst (and at subsequent times) and find that to be entirely consistent with the high energy data obtained by Fermi/LAT. The ability of a simple external forward shock, to fit the entire data from the end of the burst (1-50s) to about a week, covering more than eight-decades in photon frequency provides compelling confirmation of the external forward shock synchrotron origin of the >100MeV radiation from these Fermi GRBs. Moreover, the parameters determined in points (3) and (4) show that the magnetic field required in these GRBs is consistent with shock-compressed magnetic field in the circum-stellar medium with pre-shocked values of a few tens of micro-Gauss.

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 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 ≥ 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 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 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 × 10–5 (~few × 10–5). To characterize how much amplification is needed, beyond shock compression of a seed magnetic field ~10 μG, we expressed our results in terms of an amplification factor, AF, which is very weakly dependent on n (AF∝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.

Energies of GRB blast waves and prompt efficiencies as implied by modelling of X-ray and GeV afterglows

We consider a sample of ten GRBs with long lasting (

ON THE ORIGIN OF THE RADIO EMISSION OF Sw 1644+57

\gtrsim10^2\rm\,sec

Simulations of AGN jets: magnetic kink instability versus conical shocks

) emission detected by Fermi/LAT and for which X-ray data around

TDE fallback cut-off due to a pre-existing accretion disc

1\,

Radio SNRs in the Magellanic Clouds as probes of shock microphysics

day are also available. We assume that both the X-rays and the GeV emission are produced by electrons accelerated at the external forward shock, and show that the X-ray and the GeV fluxes lead to very different estimates of the initial kinetic energy of the blast wave. The energy estimated from GeV is on average

First ALMA Light Curve Constrains Refreshed Reverse Shocks and Jet Magnetization in GRB 161219B

\sim50

Collapsar γ-ray bursts: how the luminosity function dictates the duration distribution

times larger than the one estimated from X-rays. We model the data (accounting also for optical detections around

Off-axis short GRBs from structured jets as counterparts to GW events

1\,