Anatomy of a dark burst - The afterglow of GRB 060108

Abstract

We present a multiwavelength study of GRB 060108 - the 100th gamma-ray burst discovered by Swift. The X-ray flux and light curve (three segments plus a flare) detected with the X-ray Telescope are typical of Swift long bursts. We report the discovery of a faint optical afterglow detected in deep BVRi′-band imaging obtained with the Faulkes Telescope North beginning 2.75 min after the burst. The afterglow is below the detection limit of the Ultraviolet/Optical Telescope within 100 s of the burst, while is evident in K-band images taken with the United Kingdom Infrared Telescope 45 min after the burst. The optical light curve is sparsely sampled. Observations taken in the R and i′ bands can be fitted either with a single power-law decay in flux, F(t) ∝ r-α where α = 0.43 ± 0.08, or with a two-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 i′ band. Deep A-band imaging obtained ∼ 12 d post-burst with the Very Large Telescope 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 d, 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 fitted by a simple power law, Fv ∝ v-β, with β ox = 0.54 and a small amount of extinction. The optical to X-ray spectral index (βox) confirms 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, a hard optical to X-ray spectrum and a moderate amount of extinction in the host galaxy. © 2006 RAS.