2006/12/18 by A. Panaitescu, P. Kumar · 1 citation
Physics and Astronomy · #astro-ph
paper · pdf · doi:10.1111/j.1365-2966.2007.11398.x
published as Mon.Not.Roy.Astron.Soc.376:1065-1072,2007 · 9 pages, accepted for publication by MNRAS
arxiv created 2006/12/18 · arxiv updated 2009/12/01
The prompt optical emission of GRB 990123 was uncorrelated to the gamma-ray light-curve and exhibited temporal properties similar to those of the steeply-decaying, early X-ray emission observed by Swift at the end of many bursts. These facts suggest that the optical counterpart of GRB 990123 was the large-angle emission released during (the second pulse of) the burst. If the optical and gamma-ray emissions of GRB 990123 have, indeed, the same origin then their properties require that (i) the optical counterpart was synchrotron emission and the gamma-rays arose from inverse-Compton scatterings ("synchrotron self-Compton model"), (ii) the peak-energy of the optical-synchrotron component was at ~20 eV, and (iii) the burst emission was produced by a relativistic outflow moving at Lorentz factor > 450 and at a radius > 1015 cm, which is comparable to the outflow deceleration radius. Because the spectrum of GRB 990123 was optically thin above 2 keV, the magnetic field behind the shock must have decayed on a length-scale of <1% of the thickness of the shocked gas, which corresponds to 106-107 plasma skin-depths. Consistency of the optical counterpart decay rate and its spectral slope (or that of the burst, if they represent different spectral components) with the expectations for the large-angle burst emission represents the most direct test of the unifying picture proposed here for GRB 990123.