Correlated optical and X-ray flares in the afterglow of XRF 071031

TL;DR

This study presents a detailed, multi-band optical/NIR light curve of XRF 071031's afterglow, revealing correlated optical and X-ray flares and spectral evolution, indicating late central engine activity.

Contribution

It provides the first densely sampled early optical/NIR light curve of XRF 071031 and demonstrates the correlation between optical and X-ray flares with spectral changes.

Findings

Optical/NIR light curve shows early rise, bumps, and decay.

Optical and X-ray flares are correlated, suggesting common origin.

Spectral hardening occurs during optical bumps.

Abstract

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.