GCN Circular 10246

GRB 091109A: GROND confirmation of redshift z = 3.5

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

GROND (Greiner et al. 2008, PASP 120, 405), mounted at the 2.2m ESO/MPI
telescope at La Silla Observatory (Chile), reobserved the field of GRB
091109A (Oates et al., GCN #10138) simultaneously in the g'r'i'z'JHK
bands.

Observations started 2009-11-20 00:42, 10.8 days after the trigger and
lasted for an effective exposure of 75 min in g'r'i'z' and 60 min in JHK.

At the position of the optical afterglow (Guidorzi et al. GCN#10142,
Oates GCN#10143, Afonso et al., GCN #10158) we do not detect any source
down to a limiting magnitude of r' > 25.1.

Comparing with the brightness of r' = 23.6 +- 0.1 mag (AB) reported for
the 1st epoch (Afonso et al., GCN #10158), the last imaging clearly
shows a fading of more than 1.5 magnitudes. The 1st epoch was hence
dominated by afterglow light, favoring a redshift of around z = 3.5 +/-
0.4.

A thorough analysis of the VLT data (GCN # 10158) was limited by the
very low S/N of the optical spectrum and does not yield more precise
redshift constraints.