[vsnet-grb-info 11828] GRB 120401A: GROND Detection of an optical/NIR afterglow candidate

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13219
SUBJECT: GRB 120401A: GROND Detection of an optical/NIR afterglow candidate
DATE:    12/04/04 23:27:18 GMT
FROM:    Vladimir Sudilovsky at MPE  <vsudilov at mpe.mpg.de>

V. Sudilovsky, J. Elliott, J. Greiner (all MPE Garching), T. Kruehler (DARK) and A. Rau (also MPE Garching), report on behalf of the GROND team:

We observed the field of GRB120401A (Swift trigger 519043; Beardmore et al., GCN #13182) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla Observatory (Chile).

First observations started at 23:32 UT on 2012-04-01, 18.1 hours after the GRB trigger. For an exposure time of 18.76 min in g'r'i'z' and 15 min in JHK performed at an average seeing of 1".8 and at an average airmass of 2.1, we find a single point source within the 1".6 enhanced Swift-XRT error circle reported by Evans et al. (GCN #13184) at

RA (J2000.0)   = 03:52:19.82 = 58.08258
Dec. (J2000.0) = -17:38:08.5 = -17.63569

with an uncertainty of 0.3" in each coordinate.

We estimate preliminary magnitudes (all in AB) of

g' > 23,
r' = 22.7 +/- 0.2
i' = 21.5 +/- 0.1
z' = 21.0 +/- 0.1
J  = 20.7 +/- 0.2,
H  > 20.7, and
K  > 18.5

The SED, constructed from the Galactic foreground extinction (E_(B-V)=0.053 mag, Schlegel et al. 1998) corrected magnitudes of the object gives a photometric redshift of z=4.5 ± 0.5, while intrinsic or host galaxy extinction alone cannot account for the r'-i' colour. 

A second observation began at 23:29 UT on 2012-04-02, 1.75 days after the GRB trigger. Due to telescope pointing problems, JHK were the only filters to cover the GRB location. For an exposure time of 38.3 min in JHK at an average seeing of 1".4 and at an average airmass of 2.0, we do not detect the source. With an upper limit of J (AB) > 21.4, it suggests a decay of at least 0.7 mag.

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS field stars.