The nature of the X-Ray Flash of August 24 2005

Aims. Our aim is to investigate the nature of the X-Ray Flash (XRF) of August 24, 2005. Methods. We present comprehensive photometric R-band observations of the fading optical afterglow of XRF 050824, from 11 min to 104 days after the burst. In addition we present observations taken during the first day in the BRIK bands and two epochs of spectroscopy. We also analyse available X-ray data. Results. The R-band lightcurve of the afterglow resembles the lightcurves of long duration Gamma-Ray Bursts (GRBs), i.e., a power-law, albeit with a rather shallow slope of alpha = 0.6 (F-v proportional to t(-alpha)). Our late R-band images reveal the host galaxy. The rest-frame B- band luminosity is similar to 0.5 L*. The star-formation rate as determined from the [O II] emission line is similar to 1.8 M-circle dot yr(-1). When accounting for the host contribution, the slope is alpha = 0.65 +/- 0.01 and a break in the lightcurve is suggested. A potential lightcurve bump at 2 weeks can be interpreted as a supernova only if this is a supernova with a fast rise and a fast decay. However, the overall fit still shows excess scatter in the lightcurve in the form of wiggles and bumps. The flat lightcurves in the optical and X-rays could be explained by a continuous energy injection scenario, with an on-axis viewing angle and a wide jet opening angle (theta(j) greater than or similar to 10 degrees). If the energy injections are episodic this could potentially help explain the bumps and wiggles. Spectroscopy of the afterglow gives a redshift of z = 0.828 +/- 0.005 from both absorption and emission lines. The spectral energy distribution ( SED) of the afterglow has a power-law (F-v proportional to v(-beta)) shape with slope beta = 0.56 +/- 0.04. This can be compared to the X-ray spectral index which is beta(X) = 1.0 +/- 0.1. The curvature of the SED constrains the dust reddening towards the burst to A(v) < 0.5 mag.