TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4085 SUBJECT: GRB 051008 - a low redshift low-luminosity burst? DATE: 05/10/11 11:06:37 GMT FROM: Patrizia Ferrero at TLS Tautenburg P. Ferrero, S. Klose, A. Kann, A. Zeh, and B. Stecklum, Thueringer Landessternwarte Tautenburg, report: We have finally analysed the I-band images of the error box of GRB 051008 obtained with the Tautenburg Schmidt telescope. Photometric calibration was performed using the USNO-B1.0 catalog (Monet et al. 2003). Two sets of images were taken. The first one consists of six 20 sec frames (mean time 1.00 hrs after the burst), the second set consists of six 120 sec frames (mean time 1.25 hrs after the burst). We do not detect any souce in the XRT error circle (Perri et al., GCN 4073) down to about I=18.0 and I=18.8, respectively. Given these I-band upper limits and the deep R upper limit reported by Rumyantsev et al. (GCN 4081), the afterglow of GRB 051008 is another example for the general faintness of Swift afterglows compared to those discovered prior to Swift (cf. Berger et al. 2005, ApJ 629, 328). According to our data archive (Kann et al. 2006, in preparation), there are 20 afterglows with reliable I-band data (known light curve parameters alpha_1, alpha_2, and t_break) so that one can at least extrapolate the light curve to a time period of 1 hr after the corresponding burst. Among these 20 afterglows at least 18 were brighter than the one of GRB 051008, after correction for Galactic extinction (GRB 051008 has E(B-V) = 0.01 mag; Schlegel et al. 1998, ApJ 500, 525). We note that the very early break time of 0.19 days found in the X-ray light curve (Perri et al. 2005, GCN 4080) favors a relatively near-by event when compared with the afterglow sample in the pre-Swift era. Assuming that this is a jet break, it is earlier than most of the bursts in the sample of Zeh et al. (2005, astro-ph/0509299). To equal the earliest jet break in the Zeh et al. sample (GRB 041006: t_break = 0.14 days in the host frame), the redshift of GRB 051008 must be z < 0.36. On the other hand, if we use the burst data (fluence & peak energy from Golenetskii et al., jet break time from Perri et al.) to place the GRB on the Ghirlanda relation (Ghirlanda et al. 2004, ApJ, 616, 331) using the fit from Zeh et al. and the typical density parameter n=10 cm^-3 of Friedman & Bloom 2005 (ApJ, 627, 1), low redshifts are highly disfavored, as the high peak energy is in contrast with the early break and thus a large collimation correction. A redshift around one is favored. If the burst was near-by, which could also be indicated by its high gamma-ray flux and fluence (Golenetskii et al. 2005, GCN 4078), given the deduced NH column density (Perri et al. 2005, GCN 4080) host extinction alone cannot explain the faintness of the afterglow for a gas-to-dust ratio that is close to that of the SMC. This implies a relatively low-luminosity afterglow. It also implies the possible appearance of a detectable supernova component with a peak magnitude brighter than R=21.9 (assuming a SN 1998bw event). This message may be quoted.