TITLE:   GCN CIRCULAR
NUMBER:  13810
SUBJECT: GRB 120922A: GROND photometric redshift
DATE:    12/09/23 21:23:29 GMT
FROM:    Fabian Knust at MPE/GROND  <fabian.knust@hotmail.de>

F. Knust (MPE Garching), T. Kruehler (DARK/NBI), S. Klose (TLS Tautenburg) and J. Greiner (MPE Garching) report on behalf of the GROND team:

We report on further analysis of the GROND (Greiner et al. 2008, PASP 120, 405) optical/NIR imaging of the afterglow (Knust et al. GCN 13795, Klotz et al., GCN 13799, Yershov et al., GCN 13803, Kuin et al., GCN 13808) of GRB 120922A (Yershov et al., GCN 13793). Tying our photometric solution to field stars from the SDSS in the optical filters and 2MASS in the NIR, we derive the following AB magnitudes centered at a midtime of 00:01 UTC on September 23 (approx 1.5 hrs after the trigger):

g' = 20.53 +/- 0.05
r' = 19.69 +/- 0.04
i' = 19.44 +/- 0.04
z' = 19.28 +/- 0.04 
J = 18.92 +/- 0.06
H = 18.79 +/- 0.07
K = 18.63 +/- 0.10

After correcting these magnitudes for the expected Galactic foreground extinction corresponding to a reddening of E_B-V = 0.15 mag (Schlegel et al. 1998), the SED is well fit with a simple power law of spectral index \beta = 0.5 +/- 0.2 and no strong evidence for host-intrinsic reddening. The significant drop in flux in the g'-band suggests a photometric redshift of 3.1 +/- 0.2, consistent with the lack of detection in the bluer UVOT filters (Kuin et al., GCN 13808).