TITLE:   GCN CIRCULAR
NUMBER:  23114
SUBJECT: GRB 180809B: VLT optical upper limits
DATE:    18/08/10 10:18:56 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani@dark-cosmology.dk>

J. Japelj (U. Amsterdam), D. B. Malesani (DAWN/NBI and DARK/NBI), J. 
Selsing (DAWN/NBI), N. R. Tanvir (U. Leicester), D. A. Kann 
(HETH/IAA-CSIC), V. D'Elia (ASI-SSDC), G. Pugliese (U. Amsterdam), K. E. 
Heintz (Univ. Iceland and DAWN/NBI), J. P. U. Fynbo (DAWN/NBI), A. J. 
Levan (Univ. Warwick), P. Schady (University of Bath), report on behalf 
of the Stargate collaboration:

We observed the field of GRB 180809B with the ESO VLT UT2 (Kueyen), 
using the X-shooter acquisition camera. In a single 120-s r-band 
exposure, with a mean epoch Aug 10.055 UT (4.84 hr after the GRB), we 
detect no object at the position of the optical afterglow (e.g., Moss et 
al., GCN 23105; Lipunov et al., GCN 23109; Schweyer et al. GCN 23111; 
Guidorzi et al., GCN 23112, Tiurina et al., GCN 23113), down to a 
limiting magnitude r > 24.7 AB (calibrated against nearby Pan-STARRS 
sources).

Compared to the r-band measurement by Guidorzi et al. (GCN 23112), and 
assuming an unbroken power-law decay F(t) propto t^-alpha, the 
corresponding decay index is alpha > 2.2 between 0.55 and 4.84 hr after 
the GRB.

We acknowledge excellent support from the observing team in Paranal, 
including Steffen Mieske and Boris Haeussler.