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 Rlim ≈ 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 ≈1'', 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 ≈1.6 × 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 NH = (5 ± 1) × 1021 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 ≳ 109 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 ≳ 5 (and therefore that the optical flux is extinguished by Lyα 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.

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