The afterglow of XRF 071031: Evidence for correlated optical and X-ray flares

We present a densely sampled early light curve of the optical/near-infrared (NIR) afterglow of the X-Ray Flash (XRF) 071031 at z 2.692. Continuous observations in seven photometric bands from g' to KS simultaneously with the Gamma Ray Burst Optical Near-Infrared Detector (GROND) at the 2.2 m MPI/ESO telescope on LaSilla were performed between 4 minutes and 7 hours after the burst. The optical/NIR light curve is dominated by an early increase in brightness which can be attributed to the apparent onset of the forward shock (FS) emission. In addition, there are several bumps which are superimposed onto the overall rise and decay. Significant flaring is also visible in the Swift X-Ray Telescope (XRT) light curve at early and late times. The broadband data enables detailed studies of the connection between the X-ray and optical/NIR afterglow and its colour evolution during the first night post burst. We find evidence of spectral hardening in the optical spectral energy distribution contemporaneous with the emergence of the bumps from an underlying afterglow component. The bumps in the optical/NIR light curve can be associated with flares in the X-ray regime suggesting late central engine activity as the common origin.