Anatomy of a dark burst – the afterglow of GRB 060108

We present a multiwavelength study of GRB 060108 - the 100th Gamma Ray Burst discovered by Swift. The X-ray flux and light curve (3-segments plus a flare) detected with the XRT are typical of Swift long bursts. We report the discovery of a faint optical afterglow detected in deep BV Riband imaging obtained with the Faulkes Telescope North (FTN) beginning 2.75 minutes after the burst. The afterglow is below the detection limit of the UVOT within 100s of the burst, while is evident in K-band images taken with the United Kingdom Infrared Telescope (UKIRT) 45 minutes after the burst. The optical light curve is sparsely sampled. Observations taken in the R and ibands can either be fit with a single power law decay in flux, F(t)/ t −� where α = 0.43 ± 0.08, or a 2-segment light curve with an initial steep decay α1<0.88±0.2, flattening to a slope α2�0.31±0.12. A marginal evidence for rebrightening is seen in the iband. Deep R-band imaging obtained � 12 days post burst with the VLT 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 slow decay with index α2, 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 the maximum photometric redshift z < 3.2 for GRB 060108. We find that the Spectral Energy Distribution at 1000 s after the burst, from the optical to the X-ray range, is best fit by a simple power law, F� / ν −� , with βOX = 0.54 and a small amount of extinction. The optical to X-ray spectral index (βOX) confirm GRB 060108 to be one of the optically darkest bursts detected. Our observations rule out a high redshift as the reason for the optical faintness of GRB 060108. We conclude that a more likely explanation is a combination of an intrinsic optical faintness of the burst, an hard optical to X-ray spectrum and a moderate amount of extinction in the host galaxy.