GCN Circular 9773
Subject
GRB 090812: GROND Observations of the Optical/NIR Afterglow
Event
Date
2009-08-12T11:54:31Z (16 years ago)
From
Thomas Kruehler at MPE/MPI <kruehler@mpe.mpg.de>
Adria Updike (Clemson University), Arne Rau, Thomas Kruehler, Felipe
Olivares, and Jochen Greiner (all MPE Garching) report on behalf of the
GROND team:
We observed the field of GRB 090812 (Swift trigger 359711; Stamatikos et
al., GCN #9768) 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).
Due to cloud coverage, observations started only at 08:13 UT on August 12,
2 hours and 11 minutes after the GRB trigger. They were performed at an
average seeing of 1.1" and at an average airmass of 1.2 with light cirrus,
and continued until twilight.
We clearly detect the afterglow in stacked images (Stamatikos et al. GCN
#9768, Cenko GCN #9769). Based on the first 4 min of total exposures, we
estimate preliminary magnitudes and upper limits (all in AB system) of
g' = 22.15 +- 0.15 mag,
r' = 21.50 +- 0.08 mag,
i' = 21.30 +- 0.13 mag,
z' = 21.02 +- 0.12 mag,
J = 20.57 +- 0.15 mag,
H > 19.8 mag,
K > 18.9 mag
The given magnitudes are calibrated against SDSS as well as 2MASS field
stars and are not corrected for the expected Galactic foreground extinction
corresponding to a reddening of E_(B-V)=0.02 mag in the direction of the
burst (Schlegel et al. 1998).
The foreground extinction corrected spectral energy distribution is well
fit with a pure power law and a spectral index of beta = 1.1+/-0.2 and no
strong evidence of excess extinction.
The flux difference between the g' band magnitude and the extrapolation of
the power law as defined by r'i'z'J is consistent with Lyman-alpha
absorption at a redshift of 2.45 as reported by de Ugarte Postigo et al.
(GCN #9771).