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

We present the earliest ultraviolet (UV) spectrum of a gamma-ray burst (GRB) as observed with the Swift Ultra-Violet/Optical Telescope (UVOT). The GRB 081203A spectrum was observed for 50 s with the UV-grism starting 251 s after the Swift-Burst-Alert-Telescope (BAT) trigger. During this time, the GRB was approximate to 13.4 mag (u filter) and was still rising to its peak optical brightness. In the UV-grism spectrum, we find a damped Ly alpha line, Ly beta and the Lyman continuum break at a redshift z = 2.05 +/- 0.01. A model fit to the Lyman absorption implies a gas column density of log N-HI = 22.0 +/- 0.1 cm(-2), which is typical of GRB host galaxies with damped Ly alpha absorbers. This observation of GRB 081203A demonstrates that for brighter GRBs (v approximate to 14 mag) with moderate redshift (0.5 < z < 3.5) the UVOT is able to provide redshifts, and probe for damped Ly alpha absorbers within 4-6 min from the time of the Swift-BAT trigger.