TITLE: GCN CIRCULAR NUMBER: 19160 SUBJECT: GRB 160303A: GTC imaging of possible host galaxy DATE: 16/03/10 13:38:32 GMT FROM: Zach Cano at U of Iceland ​Z. Cano (U. Iceland), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), D. Malesani (DARK/NBI) and C. C. Thoene (IAA-CSIC) report on behalf of a larger collaboration: We observed the field of GRB 160303A (Beardmore et al., GCN Circ. 19126) with the 10.4-m GTC telescope (OSIRIS instrument) on 2016 March 8.15 UT (+4.7 days) in SDSS filters r and i. The optical transient associated with this event (Butler et al., GCN Circ. 19131) is no longer detected. Instead, we note the presence of a nearby, extended source with an r-band (AB) magnitude of 25.8 +- 0.3, as calibrated using nearby SDSS stars. After registering the astrometry of the GTC images against our NOT afterglow image (de Ugarte Postigo et al., GCN Circ. 19136), we find that the centroid of the extended object is offset by 1.7'' +- 0.3'' in the North-East direction. We note that the radial extent of this object nearly reaches the position of the previously detected optical transient. Using the equations from Bloom, Kulkarni & Djorgovski (2002, AJ, 123, 1111), we calculate that the chance probability of finding an object with r = 25.8 at a separation of 1.7'' is 14% -- small, but not negligible. If the extended object is associated with GRB 160303A, then for a standard cosmology this angular separation corresponds to 3.2, 5.6, 7.6, 10.5 kpc for redshifts of z=0.1, 0.2, 0.3, 0.5, respectively. This offset is consistent with that measured for short-duration GRBs at z=0.2 (median offset = 5 kpc for a range of 0.5-75 kpc; Fong, Berger & Fox 2010, ApJ, 708, 9). Instead, if GRB 160303A is a long-duration GRB, for this offset to be consistent with those measured for LGRBs (1 kpc, Bloom, Kulkarni & Djorgovski 2002, AJ, 123, 1111; de Ugarte Postigo et al. 2012, A&A, 548, 11), it would require a redshift of z=0.03. The luminosity distance at this redshift implies an absolute magnitude of M_r=-9.8, which is roughly eight to ten magnitudes fainter than those measured for LGRB host galaxies in sample studies (Savaglio, Glazebrook & Le Borgne, ApJ, 691, 182; Kruehler et al. 2015, A&A, 581, 125). Based on this line of argument, this observation suggests that GRB 160303A may be a short-duration GRB. We acknowledge the excellent support given by the GTC staff.