Correlated optical and X-ray flares in the afterglow of XRF 071031
  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. Simultaneous and
continuous observations in seven photometric bands from g' to K with GROND at
the 2.2 m MPI/ESO telescope on LaSilla were performed between 4 minutes and 7
hours after the burst. The light curve consists of 547 individual points which
allows us to study the early evolution of the optical transient associated with
XRF 071031 in great detail. 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 emission. 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 from early to late times. The availability of
high quality, 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 bands
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.
