An Hst Search for Supernovae Accompanying X-ray Flashes

We present the results from an Hubble Space Telescope/ACS search for supernovae associated with Xray flashes 020903, 040701, 040812 and 040916. We find strong evidence that XRF020903 (z = 0.25) was associated with a SN 1998bw-like supernova and confirm this using optical spectroscopy at t ∼ 25 days. We find no evidence, however, for SN1998bw-like supernovae associated with the other three events. In the case of XRF040701 (z = 0.21), we rule out even a faint supernova similar to SN2002ap, using template light-curves for several local Type Ic supernovae. For the two cases in which the redshift is not known, XRFs 040812 and 040916, we derive robust redshift limits assuming they were accompanied by supernovae similar to SN1998bw and compare these limits with photometric redshift constraints provided by their host galaxies. We supplement this analysis with results for three additional events (XRFs 011030, 020427 and 030723) and discuss the observed diversity of supernovae associated with X-ray flashes and gamma-ray bursts. We conclude that XRF-SNe exist, but can be significantly fainter than SN1998bw, possibly consistent with the observed spread in local Type Ibc supernovae. Subject headings: gamma rays: bursts radiation mechanisms: nonthermal supernova: individual