GCN Circular 22724

A. Rossi (INAF-OAS), J. Palmerio (IAP, Paris), J. Japelj (API, U. 
Amsterdam), D. Malesani (DAWN/NBI and DARK/NBI), N. R. Tanvir (Univ. 
Leicester), D. A. Kann (HETH/IAA-CSIC), J. P. U. Fynbo (DAWN/NBI), G. 
Pugliese (API, U. Amsterdam),  A. de Ugarte Postigo (HETH/IAA-CSIC and 
DARK/NBI), K. E. Heintz (Univ. Iceland and DAWN/NBI), and S. D. Vergani 
(GEPI/Obs. Paris) report on behalf of the Stargate Consortium:

We observed the Swift/XRT afterglow localization of GRB 180512A (Swift 
trigger 832119; Deich et al., GCN #22710) with the ESO VLT UT4 equipped 
with the HAWK-I near-infrared imager. Observations started on 2018 May 
13 at 04:27 UT and ended at 05:28 UT, for a total of 36 min on source 
and corresponding to a midtime of ~7 hours after the GRB trigger. We do 
not detect the optical source observed by Rossi et al. (GCN #22718), nor 
any other object within the XRT error circle, down to H > 22.8 (Vega), 
calibrated against 2MASS field stars.

Moreover, we note that after comparing the early GROND upper limit (r' > 
24.9; Bolmer, GCN #22714) with the Swift/XRT observations, we obtain an 
optical to X-ray spectral slope beta_OX < ~0.1 (using the convention 
F_nu ~ nu^-beta). This value is lower than the minimum expected 
following standard afterglow modelling (beta_OX >= 0.5) and, together 
with the r-band detection with LBT as well as the high X-ray column 
density (Burrows et al., GCN #22721), suggests that a combination of 
moderate redshift, intrinsic faintness and dust extinction is 
responsible for the faint optical afterglow.

We acknowledge the excellent support from the ESO staff, particularly 
Cyrielle Opitom, Fuyan Bian, and Steffen Mieske in obtaining these 
observations.