GRB 081203A: Swift UVOT captures the earliest ultraviolet spectrum of a gamma-ray burst

Abstract

We present the earliest ultraviolet (UV) spectrum of a gamma-ray burst (GRB) as observedwith the Swift Ultra-Violet/Optical Telescope (UVOT). The GRB 081203A spectrum wasobserved for 50 s with the UV-grism starting 251 s after the Swift-Burst-Alert-Telescope(BAT) trigger. During this time, the GRB was ˜13.4 mag (u filter) and was still rising to itspeak optical brightness. In the UV-grism spectrum, we find a damped Lya line, Lyß and theLyman continuum break at a redshift z = 2.05 ± 0.01. A model fit to the Lyman absorptionimplies a gas column density of log NHI = 22.0 ± 0.1 cm-2, which is typical of GRB hostgalaxies with damped Lya absorbers. This observation of GRB 081203A demonstrates thatfor brighter GRBs (v ˜ 14 mag) with moderate redshift (0.5 < z < 3.5) the UVOT is able toprovide redshifts, and probe for damped Lya absorbers within 4–6 min from the time of theSwift-BAT trigger.