TITLE: GCN CIRCULAR NUMBER: 19186 SUBJECT: GRB 160228A: GROND afterglow confirmation and X-shooter host candidate redshift DATE: 16/03/14 17:46:43 GMT FROM: Thomas Kruehler at MPE Garching T. Kruehler (MPE Garching), D. Malesani (DARK/NBI), D. Xu (NAOC/CAS), J. Bolmer (MPE Garching), J. P. U. Fynbo, D. Perley (both DARK/NBI), D. A. Kann (TLS Tautenburg), J. Greiner and J. F. Graham (both MPE Garching) report on behalf of a larger collaboration: We re-observed the field of GRB 160228A (Swift-trigger 676595, Malesani et al. GCN #19107) with GROND mounted at the 2.2m MPG telescope at ESO La Silla observatory. Simultaneous observations in seven filters (g'r'i'z'JHK) started at 01:13 UT on 2016-03-03 (80 h after the GRB) and consisted of images with a total integration time of 72 minutes in g'r'i'z' and 60 minutes in JHK. The source reported as candidate afterglow in our earlier imaging (Delvaux et al. GCN #19114) has faded beyond the detection limit of the new data (r' > 25.1 mag in the AB system), confirming it as the optical afterglow of GRB 160228A. The nearby source (angular separation of 1.7") mentioned in Delvaux et al. (GCN #19114) is still clearly present and located at RA (J2000.0) = 07:09:15.79 Dec. (J2000.0) = +26:55:52.4 with a preliminary brightness of r' = 24.2 +/- 0.3 mag. The chance coincidence probability of finding an unrelated object of this magnitude within a distance of 1.7" is 0.04 (following Bloom et al. 2002, AJ, 123, 1111), and we hence consider this object as a host galaxy candidate of GRB 160228A. While the chance coincidence probability is quite low, we note that the angular separation would be unusually large for long GRBs. A spectrum of the possible host was taken on 2016-03-12 with the VLT equipped with the X-shooter spectrograph. The spectrum covers the wavelength range 3000-20000 AA and has an exposure time of 4x1200 s. In the red part of the spectrum, we detect two emission lines which we interpret as [O III] (5007) and Halpha at a common redshift z = 1.64. We acknowledge excellent support from the observing staff at La Silla and Paranal, in particular Giovanni Carraro, Boris Haeussler, Jose Velasquez, and Willem-Jan de Wit.