GCN Circular 9901
Subject
GRB 090904B: GROND detection of the optical afterglow
Event
Date
2009-09-07T12:54:08Z (16 years ago)
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.