Constraining the dark GRB mystery with HETE-2 SXC localizations

Over 85% of the long GRBs fully localized by HETEs soft X-ray camera (SXC) have an associated fading optical counterpart, suggesting that we are close to solving the mystery of “dark” bursts. SXC localizations are characterized by small error boxes (∼1 arcmin) sent out within a few hours after the burst. Are there selection effects that may account for this favorable statistic? Examining and comparing the list of SXC detections with the overall HETE sample, we find no spectral differences between the two sets, e.g., the SXC sample spans both X-ray bright and dim bursts similar to the overall HETE sample. SXC localizations, however, have prompted fast, and deep counterpart searches over highly constrained error boxes. A large fraction of optically dark bursts are probably classified as such due to inadequate follow-up observations, and most GRBs should have a detectable optical counterpart.