GCN Circular 22724
Subject
GRB 180512A: VLT/HAWK-I NIR Observations
Event
Date
2018-05-14T15:28:16Z (8 years ago)
From
Andrea Rossi at INAF <andrea.rossi@inaf.it>
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.