V. Sudilovsky (MPE Garching), A. Nicuesa Guelbenzu (Tautenburg Observatory) and J. Greiner (MPE Garching) report on behalf of the GROND team:
We observed the field of GRB 120327A (Swift trigger 518731; Sbarufatti et al., GCN #13123) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory (Chile).
Observations started at 03:34 UT, 39 minutes min after the GRB trigger, and are continuing. They were performed at an average seeing of 1.6" and at an average airmass of 2.3.
We found a single point source consistent with both the UVOT (Sbarufatti et al, GCN #13123) and Liverpool telescope (Smith et al., GCN #13125) afterglow position.
Based on the second of two 2.4 min of exposures in g'r'i'z' and 4 min in JHK, we estimate preliminary magnitudes (all in AB) at an observations midpoint 03:44 UT of
g' = 18.6 +- 0.1 mag,
r' = 17.5 +- 0.1 mag,
i' = 17.2 +- 0.1 mag,
z' = 17.0 +- 0.1 mag,
J = 16.3 +- 0.1 mag, and
H = 15.9 +- 0.1 mag
Given magnitudes are calibrated against GROND zeropoints as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.34 mag in the direction of the burst (Schlegel et al. 1998). If this value is correct for the line-of-sight to this GRB, then the substantially reduced g-band flux suggests a redshift around 3.