GCN Circular 18100
Subject
GRB 150424A: HST imaging
Date
2015-07-31T21:13:57Z (9 years ago)
From
Nial Tanvir at U.Leicester <nrt3@star.le.ac.uk>
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.