P. Ferrero, S. Klose, A. Kann, A. Zeh, and B. Stecklum,
Thueringer Landessternwarte Tautenburg,
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.
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
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