The afterglow of the short/intermediate-duration gamma-ray burst GRB 000301C: A reddened jet at z=2.04

Abstract
We present Ulysses and NEAR data from the detection of the short or intermediate duration (2 s) gamma-ray burst GRB000301C. The GRB was localized by IPN and RXTE to an area of 50 arcmin^2. A fading optical counterpart was subsequently discovered with the Nordic Optical Telescope (NOT) about 36h after the burst. The GRB lies at the border between the long-soft and the short-hard classes of GRBs. If GRB000301C belongs to the latter class, this would be the first detection of an afterglow to a short-hard burst. We present UBRI photometry from 2 until 11 days after the burst. We also present JHK photometry from 3.1 days after the burst and spectroscopic observations obtained with the ESO VLT Antu telescope 4 and 5 days after the burst. The optical light curve is achromatic from 2 to 11 days after the burst and exhibits a break. A broken power-law fit yields a shallow pre-break decay power-law slope of alpha_1=-0.72+-0.34, a break time of t_break=4.39+-1.52 days after the burst, and a post-break slope of alpha_2=-2.29+-1.00. These properties of the light curve are best explained by a sideways expanding jet in an ambient medium of constant mean density. In the optical spectrum we find absorption features consistent with FeII, CIV, CII, SiII and Ly-alpha at a redshift of 2.0404+-0.0008. We find evidence for a curved shape of the spectral energy distribution of the observed afterglow. It is best fitted with a power-law spectral distribution with index beta=-0.5 reddened by a SMC-like extinction law with A_V=0.2 mag. Based on the Ly-alpha absorption line we estimate the HI column density to be logN(HI)=21.2+-0.5. This is the first direct indication of a connection between a GRB host galaxy and a Damped Ly-alpha Absorbers.

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