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~2.0 (Fnu~t-alpha). Furthermore, the spectral energy distribution (SED) has a power-law (Fnu~nu-beta) shape with slope beta~1.0. However, the decay slope at t