TITLE: GCN CIRCULAR NUMBER: 18100 SUBJECT: GRB 150424A: HST imaging DATE: 15/07/31 21:13:57 GMT FROM: Nial Tanvir at U.Leicester N. R. Tanvir (U. Leicester), A. J. Levan (U. Warwick), A. S. Fruchter (STScI), J. Hjorth, D. Watson (DARK), D. Perley (Caltech), J. Greiner (MPE), A. de Ugarte Postigo, C. Thoene (IAA-CSIC), R. A. Hounsell (Illinois), S. Rosswog (U. Stockholm) report: We observed the optical/nIR counterpart of short-GRB 150424A (Perley et al. GCN 17745) with HST at three epochs, roughly 6.7, 9.3 and 13.9 days post-trigger. On each occasion we obtained imaging with the WFC3/UVIS F606W and the WFC3/IR F125W and F160W filters. The optical transient is visible at all epochs, fading from J(AB)=25.3 to J(AB)=26.9 over the course of the observations. There is a suggestion of a colour change, from r-H(AB)~1.1 to r-H(AB)~2 between the first and last epoch, although the errors are large (and in particular suffer a rather uncertain background correction - see below). Significantly, the OT is superimposed on a faint source that is extended over about 1 arcsec. We propose that this extended source is most likely the host galaxy of the burst, rather than the bright nearby spiral whose redshift (z=0.3) was reported by Castro-Tirado et al. (GCN 17758). Our current best estimates of the magnitude of this source are r(AB)=26.8, J(AB)=26.1 and H(AB)=25.7, although we note that much of the flux in the r-band is due to a single blue knot of emission, whereas the nIR is more diffuse. Given the luminosity, size and colour of the proposed host, it is likely to be at z>0.7, where the 4000A break would help explain the red r-J colour of most of this galaxy. If the burst was at z=0.3 (e.g. if the faint extended source is a satellite of the spiral, or just a background galaxy aligned by chance), then we place limits on any kilonova/macronova emission of M_J(AB)>~-14.7 at 7 days post-burst in the rest frame. A more likely redshift of z~1, for example, would mean that our reddest filter only corresponds to the rest-frame I-band, here providing a limit of M_I(AB)>~-17.0 at 7 days in the rest-frame (i.e. ~1 mag fainter than typical for a collapsar-produced supernova at the same time). Further analysis is in progress.