Anatomy of A Dark Burst - The Afterglow of GRB 060108

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Abstract
We report the first detection of an optical afterglow of a GRB (060108) that would have been classified as 'dark' in the absence of deep, rapid ground-based optical imaging with the 2-m robotic Faulkes Telesscope (FTN). Our multiwavelength analysis reveals an X-ray light curve typical of many Swift long GRBs (3-segments plus flare). Its optical afterglow, however, was already fainter than the detection limit of the UVOT within 100s of the burst. Optical imaging in BVRi' filters with the FTN began 2.75 minutes after the burst and resulted in the detection of the optical afterglow at 5.3 minutes, with a UKIRT K-band identification at ~45 mins. R and i'-band light curves are consistent with a single power law decay in flux, F(t) prop t^-a where a=0.43+/-0.08, or a 2-segment light curve with a steep decay a_1 <0.88, flattening to a_2 ~ 0.31, with evidence for rebrightening at i' band. Deep VLT R-band imaging at ~12 days reveals a faint, extended object (R ~23.5 mag) at the location of the afterglow. Although the brightness is compatible with the extrapolation of the a_2 slow decay, significant flux is likely due to a host galaxy. This implies that the optical light curve had a break before 12 days, akin to what observed in the X-rays. We derive a maximum photometric redshift z<3.2 for GRB 060108 and a best-fitting optical-to-X-ray SED at 1000 s after the burst consistent with a power law with index beta_OX = 0.54 and a small amount of extinction. The unambiguous detection at B-band and the derived photometric redshift rule out a high redshift as the reason for the optical faintness of GRB 060108. Instead, the hard opt/X-ray spectral index confirms it as one of the optically-darkest bursts detected and with modest host extinction explains the UVOT non-detection (abridged).

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