0 ----- Attempting Submission 2 (Wed Jul 30 07:42:37 GMT 2014) ----- The nature of the low redshift ultra-long GRB130925A: core collapse or tidal disruption? In the past three years several cosmological high-energy transients have been found by Swift and other satellites that have durations of at least several hours, well above the typical few minute durations of classical long-GRBs. The origin of these so-called "ultra-long" bursts remains very uncertain due to the small number found and their comparatively high redshifts. They could be produced by some kind of massive star core-collapse, like ordinary long-GRBs but perhaps with a more extended progenitor star, or alternatively via a relativistic jet produced in a tidal disruption event, similar to the well-studied source Swift J1644+57. Recently a new ultra-long duration burst was identified, GRB 130925A, at a redshift of only z=0.35, which is close enough to allow a deep search for any accompanying supernova event with HST, and also detailed constraints on the location of the transient with respect to the nucleus of its host galaxy. This promises to provide the definitive test of core-collapse vs TDE models for these events. We propose three pairs visits, the first pair as soon as possible to monitor around the peak of any supernova light, the second pair a few weeks later when the transient light should largely have disappeared, and a final visits a few months later which will be assuredly only host light. Each pair of visits is intended to be split between IR (F110 and F160) and optical (F814) exposures. 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