Constraining the dark GRB mystery with HETE-2 SXC localizations J. Villasenor * , R. Vanderspek, G. Ricker, N. Butler, G. Crew, A. Dullighan, J.G. Jernigan Center for Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Cambridge, MA 02139, USA Received 2 November 2004; received in revised form 5 January 2005; accepted 3 February 2005 Abstract 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, how- ever, 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. Ó 2006 Published by Elsevier Ltd on behalf of COSPAR. Keywords: GRB; Optically dark; SXC; HETE 1. The mystery of dark bursts The discovery of optical afterglows with gamma-ray bursts (GRB) by Beppo-SAX firmly established the cos- mic origin of these tremendous explosions. In follow up observations made within several hours after a GRB detection in the X-ray and c-ray bands, 35% of Beppo-SAX localized GRBs revealed an optical coun- terpart; the same ratio was also found for HETEs wide-field X-ray monitor (WXM) localizations. ‘‘Dark’’ GRBs without any optical counterpart thus characterize the bulk of GRB localizations (Fynbo et al., 2001). Is the large proportion of dark GRBs truly a physical condition (e.g., dust obscuration) where no optical flux is truly emitted in these events, or is this a selection effect limited by our ability to point instruments fast and deep enough to catch a rapidly decaying afterglow? HETEÕs Soft X-ray Camera (SXC) list of localized bursts indicates that this is indeed a selection effect. Since its commissioning in the middle half of 2002, the SXC has issued out 19 localizations, of which 14% or 78% had identifiable visible or IR counterparts. If we consider only detections limiting both x and y directions (two-dimensional (2D), with error boxes <15 arcmin 2 ), this ratio increases to 85%. The combination of faster alert notices and smaller error boxes have encouraged observers to carry out aggressive searches for optical afterglows. For this list of SXC localizations, we address the question of: is there anything special about SXC bursts that allow them to be detected more easily within the HETE subset, and GRBs in general? Are they brighter as a subclass, or is the SXC locking on to a particular type of GRB? 2. SXC burst catalog Table 1 lists all the SXC localizations sent out to the GRB Coordinates Network (GCN). The SXC is com- posed of two separate units, each localizing along one 0273-1177/$30 Ó 2006 Published by Elsevier Ltd on behalf of COSPAR. doi:10.1016/j.asr.2005.02.006 * Corresponding author. Tel.: +1 617 252 1667. E-mail address: jsvilla@space.mit.edu (J. Villasenor). www.elsevier.com/locate/asr Advances in Space Research 38 (2006) 1320–1324