[vsnet-grb-info 6926] GRB 081109: GROND redshift limit and host extinction

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8515
SUBJECT: GRB 081109: GROND redshift limit and host extinction
DATE:    08/11/10 19:25:43 GMT
FROM:    Jochen Greiner at MPI  <jcg at mpe.mpg.de>

C. Clemens, T. Kruehler, J. Greiner (all MPE Garching) report on
behalf of the GROND team:

We report on further analysis of the GROND data of GRB 081109 detected
by Swift (trigger 334112; Immler et al., GCN #8500) and Fermi/GBM
(von Kienlin 2008, GCN #8505) taken simultaneously in g'r'i'z'JHK.

In a combined 2~h exposure at a midtime of 18.5~hours after the burst, 
we clearly detect the afterglow reported by D'Avanzo et al. (GCN 
#8501) and Clemens et al. (GCN #8510) in all filters.

Using 2MASS field stars in JHK and the GROND zeropoints in g'r'i'z' we 
estimate preliminary AB magnitudes of

g' = 23.14 +- 0.18 mag,
r' = 22.55 +- 0.15 mag,
i' = 22.00 +- 0.16 mag,
z' = 21.54 +- 0.15 mag,
J  = 20.49 +- 0.07 mag,
H  = 19.56 +- 0.06 mag and
K  = 18.84 +- 0.10 mag

which have been corrected for the expected Galactic reddening of 
E_(B-V)=0.02 (Schlegel et al. 1998).

Assuming Ly-alpha absorption, the g' band detection implies a redshift 
of z < 3.5.

The Galactic foreground corrected broad band SED is best fit with a 
power law template and significant extinction in the host. Depending 
on the used model, the  best fit intrinsic extinction A_V^{host} 
varies between 0.6 and 1.2. We caution however, that this is strongly 
dependent on the unknown redshift and subject to changes due to an
future improved calibration.