A Deep Search with theHubble Space Telescopefor Late‐Time Supernova Signatures in the Hosts of XRF 011030 and XRF 020427

X-ray Flashes (XRFs) are, like Gamma-Ray Bursts (GRBs), thought to signal thecollapse of massive stars in distant galaxies. Many models posit that theisotropic equivalent energies of XRFs are lower than those for GRBs, such thatthey are visible from a reduced range of distances when compared with GRBs.Here we present the results of two epoch Hubble Space Telescope imaging of twoXRFs. These images taken approximately 45 and 200 days post burst reveal noevidence for an associated supernova in either case. Supernovae such as SN1998bw would have been visible out to z ~1.5 in each case, while faintersupernovae such as SN 2002ap would have been visible to z ~ 1. If the XRFs lieat such large distances, their energies would not fit the observed correlationbetween the GRB peak energy and isotropic energy release, in which soft burstsare less energetic. We conclude that, should these XRFs reside at low redshifts($z<0.6$), either their line of sight is heavily extinguished, or they areassociated with extremely faint supernovae, or, unlike GRBs, these XRFs do nothave temporally coincident supernovae.Comment: 12 Pages, 4 Figures, accepted for publication in Ap