TITLE: GCN CIRCULAR NUMBER: 22724 SUBJECT: GRB 180512A: VLT/HAWK-I NIR Observations DATE: 18/05/14 15:28:16 GMT FROM: Andrea Rossi at INAF 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.