{
  "bibcode": "2005GCN..4085....1F",
  "body": "P. Ferrero, S. Klose, A. Kann, A. Zeh, and B. Stecklum,\n   Thueringer Landessternwarte Tautenburg,\n\nreport:\n\n\nWe have finally analysed the I-band images of the error box of GRB\n051008 obtained with the Tautenburg Schmidt telescope. Photometric\ncalibration was performed using the USNO-B1.0 catalog (Monet et al.\n2003).\n\nTwo sets of images were taken. The first one consists of six 20 sec\nframes (mean time 1.00 hrs after the burst), the second set consists\nof six 120 sec frames (mean time 1.25 hrs after the burst). We do not\ndetect any souce in the XRT error circle (Perri et al., GCN 4073) down\nto about I=18.0 and I=18.8, respectively.\n\nGiven these I-band upper limits and the deep R upper limit reported by \nRumyantsev et al. (GCN 4081), the afterglow of GRB 051008 is another\nexample for the general faintness of Swift afterglows compared to\nthose discovered prior to Swift (cf. Berger et al. 2005, ApJ 629,\n328). According to our data archive (Kann et al. 2006, in preparation), there \nare 20 afterglows with reliable I-band data\n(known light curve parameters alpha_1, alpha_2, and t_break) so that\none can at least extrapolate the light curve to a time period of 1 hr\nafter the corresponding burst. Among these 20 afterglows at least 18\nwere brighter than the one of GRB 051008, after correction\nfor Galactic extinction (GRB 051008 has E(B-V) = 0.01 mag; Schlegel\net al. 1998, ApJ 500, 525).\n\nWe note that the very early break time of 0.19 days found in  the\nX-ray light curve (Perri et al. 2005, GCN 4080) favors a relatively\nnear-by event when compared with the afterglow sample in the pre-Swift\nera. Assuming that this is a jet break, it is earlier than most of\nthe bursts in the sample of  Zeh et al. (2005, astro-ph/0509299). To\nequal the earliest jet break in the Zeh et al. sample (GRB  041006:\nt_break = 0.14 days in the host frame), the redshift of GRB 051008\nmust be z < 0.36.\n\nOn the other hand, if we use the burst data (fluence & peak energy from \nGolenetskii et al., jet break time from Perri et al.) to place the GRB on the \nGhirlanda relation (Ghirlanda et al. 2004, ApJ, 616, 331) using the fit from \nZeh et al. and the typical density parameter n=10 cm^-3 of Friedman & Bloom \n2005 (ApJ, 627, 1), low redshifts are highly disfavored, as the high peak \nenergy is in contrast with the early break and thus a large collimation \ncorrection. A redshift around one is favored.\n\nIf the burst was near-by, which could also be indicated by its\nhigh gamma-ray flux and fluence (Golenetskii et al. 2005, GCN 4078),\ngiven the deduced NH column density (Perri et al. 2005, GCN 4080)\nhost extinction alone cannot explain the faintness of the afterglow\nfor a gas-to-dust ratio that is close to that of the SMC.\nThis implies a relatively low-luminosity afterglow. It also implies\nthe possible appearance of a detectable supernova component\nwith a peak magnitude brighter than R=21.9 (assuming a SN 1998bw\nevent).\n\nThis message may be quoted.",
  "circularId": 4085,
  "createdOn": 1129028797000,
  "email": "ferrero@tls-tautenburg.de",
  "subject": "GRB 051008 - a low redshift low-luminosity burst?",
  "submitter": "Patrizia Ferrero at TLS Tautenburg  <ferrero@tls-tautenburg.de>",
  "eventId": "GRB 051008"
}