���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
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.