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
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.