TITLE:   GCN CIRCULAR
NUMBER:  22060
SUBJECT: GRB 171027A: VLT optical upper limits
DATE:    17/10/27 12:01:58 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani@dark-cosmology.dk>

D. Malesani (DARK/NBI), N. R. Tanvir (Univ. Leicester), P. Schady (MPE), 
A. de Ugarte Postigo (IAA/CSIC and DARK/NBI), K. E. Heintz (Univ. 
Iceland and DARK/NBI), G. Pugliese (API, Univ. Amsterdam), report on 
behalf of the Stargate collaboration:

We observed the field of GRB 171027A (D'Avanzo et al., GCN 22053) with 
the ESO Very Large Telescope (VLT) UT2 equipped with X-shooter. 
Observations started on 2017 Oct 27.379 UT (42.8 min after the trigger) 
using the acquisition camera, entirely during twilight. For this reason, 
only short exposures could be secured (45 s in r, 60 s in z).

Within the prompt-enhanced XRT position (Evans, GCN 22054) we find no 
new sources down to r > 23, z > 22.7 mag (all AB, calibrated against the 
Pan-STARRS catalog). Our results are consistent with the lack of optical 
detection by GROND (Schady, GCNs 22056, 22058), RATIR (Watson et al., 
GCN 22057), and LCO (Guidorzi, GCN 22059). In particular, we confirm 
that no object is visible at the position of the NIR candidate reported 
by Watson et al. (GCN 22057).

We acknowledge excellent support from the ESO staff, in particular 
Michael Hilker and Francisco Labrana.