The bright gamma-ray burst of 2000 February 10: A case study of an optically dark gamma-ray burst

The gamma-ray burst GRB 000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet, but it did not have a detected optical afterglow, despite prompt and deep searches down to R(lim) approximate to 23.5. It is therefore one of the events recently classified as dark GRBs, whose origin is still unclear. Chandra observations allowed us to localize the X-ray afterglow of GRB 000210 to within approximate to1', and a radio transient was detected with the Very Large Array. The precise X-ray and radio positions allowed us to identify the likely host galaxy of this burst and to measure its redshift, z = 0.846. The probability that this galaxy is a field object is approximate to1.6 x 10(-2). The X-ray spectrum of the afterglow shows significant absorption in excess of the Galactic one corresponding, at the redshift of the galaxy, to N(H) = (5 +/- 1) x 10(21) cm(-2). The amount of dust needed to absorb the optical flux of this object is consistent with the above H I column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a giant molecular cloud. We therefore conclude that either the gas is local to the GRB but is condensed in small-scale high-density (n greater than or similar to 10(9) cm(-3)) clouds, or the GRB is located in a dusty, gas-rich region of the Galaxy. Finally, we examine the hypothesis that GRB 000210 lies at z greater than or similar to 5 (and therefore that the optical flux is extinguished by Lyalpha forest clouds), but we conclude that the X-ray absorbing medium would have to be substantially thicker from that observed in GRBs with optical afterglows.