TITLE:   GCN CIRCULAR
NUMBER:  9901
SUBJECT: GRB 090904B: GROND detection of the optical afterglow
DATE:    09/09/07 12:54:08 GMT
FROM:    Thomas Kruehler at MPE/MPI  <kruehler@mpe.mpg.de>

F. Olivares, P. Afonso, T. Kruehler, J. Greiner  (all MPE Garching), report 
on behalf of the GROND team:

The field of GRB 090904B (Swift trigger 361831, D'Elia et al., GCN #9881, 
also GBM trigger 273720255/090904058, Goldstein, GCN #9895) was observed 
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 
405) mounted at the 2.2 m ESO/MPI telescope at La Silla Observatory 
(Chile). Observations started at 01:28 UT, on September 4, 2009, 4 minutes 
after the GRB trigger. The field was observed during a second epoch, 
starting at 01:28 UT, on September 5, 2009, 24.07 hours after the GRB 
trigger. Image subtraction between the two epochs reveals a fading source 
on the edge of the enhanced Swift/XRT error circle (Osborne et al., GCN 
#9887). The position of the afterglow is

RA (J2000.0) = 17h 36m 44.59s
DEC (J2000.0) = -25d 12' 46.5"

with an uncertainty of 0.5" in each coordinate.

The afterglow is located roughly 1.1" South/East of the 2MASS/USNO star 
reported by REM (Covino et al., GCN #9882) and MOA-II (Nishimoto et al., 
GCN #9892), and both sources are strongly blended in the GROND images of 
1.0" seeing.

At a midtime of 6.5 minutes after the burst, we measure rough preliminary 
AB magnitudes of the optical afterglow of r' ~ 22.2, i ~ 20.6 and z' ~ 19.7.

Calibrations were done against GROND zero points, and given magnitudes are 
not corrected for the significant Galactic extinction corresponding to a 
reddening of E(B-V) = 1.76 mag (Schlegel et al. 1998). The object is 
detected in all bands except g', which, given the large Galactic foreground 
extinction, does not constrain the redshift.