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We present ground-based and Hubble Space Telescope optical observations ofthe X-ray flash (XRF) 020903, covering 300 days. The afterglow showed a veryrapid rise in the first day, followed by a relatively slow decay in the nextfew days. There was a clear bump in the light curve after ~25 days, accompaniedby a drastic change in the spectral energy distribution. The light curve andthe spectral energy distribution are naturally interpreted as the emergence --and subsequent decay -- of a supernova (SN), similar to SN 1998bw. At peakluminosity, the SN is estimated to be 0.8 +/- 0.1 mag fainter than SN1998bw.This argues in favor of the existence of a supernova associated with this X-rayflash. A spectrum obtained 35 days after the burst shows emission lines fromthe host galaxy. We use this spectrum to put an upper limit on the oxygenabundance of the host at [O/H] < -0.6 dex. We also discuss a possible trendbetween the softness of several bursts and the early behavior of the opticalafterglow, in the sense that XRFs and X-ray rich GRBs seem to have a plateauphase or even a rising light curve. This can be naturally explained in modelswhere XRFs are similar to GRBs but seen off the jet axis.

Evidence for a Supernova Associated with the X‐Ray Flash 020903