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

Abstract

We present comprehensive photometric R-band observations of the fading optical afterglow of the X-Ray Flash XRF050824, from 11 minutes to 104 days after the burst. 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. Our late R-band images reveal the host galaxy with a rest-frame B-band luminosity corresponding to roughly 0.5 Lstar. The star-formation rate as determined from the [O II] emission line luminosity is about 1.8 Msun per year. When accounting for the host contribution, the slope is alpha=0.65+-0.01 and a break in the lightcurve is also 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 show excess scatter in the lightcurve due to 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>10 deg). If the energy injections are episodic, this could potentially help explain the bumps and wiggles. Spectroscopy of the afterglow give 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 shape with slope beta=0.56+-0.04. This can be compared to the X-ray spectral index which is betaX=1.0+-0.1. The curvature of the SED constrain the dust reddening towards the burst to Av<0.5 mag. (abridged)