A unified picture for the γ-ray and prompt optical emissions of GRB 990123

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 gamma-rays arose from inverse-Compton scatterings ( the 'synchrotron self-Compton model'), (ii) the peak energy of the optical-synchrotron component was at similar to 20 eV and (iii) the burst emission was produced by a relativistic outflow moving at Lorentz factor greater than or similar to 450 and at a radius greater than or similar to 10(15) 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 less than or similar to 1 per cent of the thickness of the shocked gas, which corresponds to 10(6) - 10(7) 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.