C. Delvaux, T. Schweyer (both MPE Garching), D. A. Kann (TLS Tautenburg),
and J. Greiner (MPE Garching) report on behalf of the GROND team:
We observed the field of GRB 160228A (Swift trigger 676595; Malesani et
al., GCN #19107) simultaneously in g'r'i'z'JHK with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2m MPG telescope at the ESO La Silla
Observations started at 00:02 UT on 2016-02-29, 6.4h after the GRB
trigger. They were performed at an average seeing of 1.2" and at an
average airmass of 1.9.
We detect a single source at the edge of the Swift-XRT error circle
reported by Beardmore et al. (GCN #19110) at
RA (J2000.0) = 07h09m15.82s
Dec. (J2000.0) = +26d55'53.8"
with an uncertainty of 0.2" in each coordinate.
Based on 132 min of exposure in g'r'i'z' and 110 min in JHK, at a mid-time
of 01:46 UT, we derive the following preliminary magnitudes and upper
limits (in AB system):
g' = 24.4 +/- 0.1 mag,
r' = 24.4 +/- 0.1 mag,
i' = 23.8 +/- 0.2 mag,
z' = 24.1 +/- 0.3 mag,
J > 22.3 mag,
H > 21.6 mag, and
K > 19.4 mag.
We note the presence of a very close-by (separation < 2") extended bluish
object, lying outside the Swift-XRT error circle, which could be the host
galaxy of the candidate. Another host could be a larger, brighter galaxy
lying at the north of the Swift-XRT error circle (separation ~ 6.6").
We found no evidence of fading of the candidate. Therefore, we cannot
affirm the detected source is the optical afterglow.
Given detections and upper limits are calibrated against GROND zeropoints
as well as 2MASS field stars. No correction for the expected Galactic
foreground extinction corresponding to a reddening of E_(B-V)=0.056 mag in
the direction of the burst (Schlafly & Finkbeiner 2011) has been applied.