A Reverse Shock in GRB 181201A

LASKAR, TANMOY; LEVATO, H. AUTOR 29

The Astrophysical Journal

American Astronomical Society

Año: 2019 vol. 884 p. 121 - 138

We present comprehensive multiwavelength radio to X-ray observations of GRB 181201A spanningfrom ≈ 150 s to ≈ 163 days after the burst, comprising the first joint ALMA-VLA-GMRT observationsof a gamma-ray burst (GRB) afterglow. The radio and mm-band data reveal a distinct signature at≈ 3.9 days, which we interpret as reverse shock (RS) emission. Our observations present the firsttime that a single radio-frequency spectral energy distribution can be decomposed directly into RSand forward shock (FS) components. We perform detailed modeling of the full multiwavelength dataset, using Markov Chain Monte Carlo sampling to construct the joint posterior density function ofthe underlying physical parameters describing the RS and FS synchrotron emission. We uncover andaccount for all degeneracies in the model parameters. The joint RS-FS modeling reveals a weaklymagnetized (σ ≈ 3×10−3), mildly relativistic RS, from which we derive an initial bulk Lorentz factorof 􀀀0 ≈ 103 for the GRB jet. Our results support the hypothesis that low-density environments areconducive to the observability of RS emission. We compare our observations to other events withstrong RS detections, and find a likely observational bias selecting for longer lasting, non-relativisticreverse shocks. We present and begin to address new challenges in modeling posed by the presentgeneration of comprehensive, multi-frequency data sets.Keywords: gamma-ray burst: general ? gamma-ray burst: individual (GRB181201