2006MNRAS.372..327O

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) ∝ t^–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 R-band imaging obtained ∼12 d post-burst with the Very Large Telescope reveals a faint, extended object (R ∼ 23.5mag) 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, F_ν∝ ν^–β, with β_OX = 0.54 and a small amount of extinction. The optical to X-ray spectral index (β_OX) 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.