A multiband study of the optically dark GRB 051028

Observations were made of the optical afterglow of GRB 051028 with the Lulin observatory 1.0 m telescope and the Wide-Field Telescope for GRB Early Timing (WIDGET) robotic telescope system. R-band photometric data were obtained on 2005 October 28 (UT), or 0.095-0.180 d after the burst. There is a possible plateau in the optical light curve around 0.1 d after the burst; the afterglow of GRB 051028 resembles optically bright afterglows (e.g., GRB 041006, GRB 050319, GRB 060605) in shape of the light curve, but not in brightness. The brightness of the GRB 051028 afterglow is 3 mag fainter than that of one of the dark events, GRB 020124. Optically dark GRBs have been attributed to dust extinction within the host galaxy or a high redshift. However, a spectrum analysis of the X-rays implies that there is no significant absorption by the host galaxy. Furthermore, according to a theoretical calculation of the Lyα absorption to find the limit of the GRB 051028's redshift, the expected R-band absorption is not high enough to explain the darkness of the afterglow. The present results disfavor either the high-redshift hypothesis or the high-extinction scenario for optically dark bursts; rather, they are consistent with the possibility that the brightness of the optical afterglow is intrinsically dark.