On the afterglow of the X-ray flash of 2003 July 23:: Photometric evidence for an off-axis gamma-ray burst with an associated supernova?

We present optical and near-infrared follow-up observations of the X-ray flash (XRF) of 2003 July 23. Our observations in the R band cover the temporal range from 4.2 hr to 64 days after the high-energy event. We also present the results of multicolor imaging extending to the K band on three epochs. The light curve of the R-band afterglow the first week after the burst is similar to the light curve for long-duration gamma-ray bursts (GRBs), i.e., a broken power law with a late time slope of alpha approximate to 2.0 (F-nu proportional to t(-alpha)). Furthermore, the spectral energy distribution (SED) has a power-law (F-nu proportional to nu(-beta)) shape with slope beta approximate to 1.0. However, the decay slope at t < 1 day is shallow, consistent with zero. This is in qualitative agreement with the prediction that XRFs are off-axis classical GRBs. After the first week there is a strong bump in the light curve, which peaks at around 16 days. The SED after the peak becomes significantly redder. We discuss the possible interpretations of this bump and conclude that an underlying supernova is the most likely explanation since no other model appears consistent with the evolution of the SED. Finally, we present deep spectroscopy of the burst both in the afterglow and in the bump phase. A firm upper limit of z = 2.3 is placed on the redshift of XRF 030723 from the lack of Ly alpha forest lines in the spectrum of the afterglow. The lack of significant absorption and emission lines in either of the two spectra excludes a spectroscopic redshift determination.