A. Rossi (INAF-OAS), M. Cantiello (INAF-OA Abruzzo) V. Testa, D. Paris
(INAF-OAR), A. Melandri, S. Covino, O. S. Salafia, P. D'Avanzo, S.
Campana (INAF-OAB), L. Nicastro, E. Palazzi, F. Cusano (INAF-OAS), G.
Stratta (Urbino University/INFN Firenze), R. Carini, S. Piranomonte, E.
Brocato (INAF-OAR), V. D'Elia (ASDC), and M. Branchesi (GSSI) report on
behalf of the GRAWITA collaboration and its partners:
We observed the optical counterpart of GRB 170817A (Kienlin et al., GCN
21520) associated to GW 170817 (LVC GCN Circ. 21509, 21513) with the LBC
imager mounted on the Large Binocular Telescope (Mt Graham, AZ, USA).
Observations were performed in the r-sloan filter on 2018-01-23, i.e.,
~160 days after the GW/GRB trigger.
At the location of the optical transient (e.g., Coulter et al., GCN
21529; Adams et al., 21816) we detect the optical afterglow of GRB
170817A with magnitude r-sloan=26.2+-0.4, calibrated against Pan-STARRS
field stars. Image analysis was performed after preliminary removal of
an elliptical model of the underlying host galaxy from each single
frame. However, some residual emission is left which contributes for
~0.2 mags to the uncertainty of the photometry.
Our detection is the first one from a ground-based optical telescope. It
is in agreement with a turnover/flattening in the optical light curve of
GW 170817/GRB 170817A as inferred by Alexander at al. 2018
(arXiv:1805.02870) and with the overall flattening/declining temporal
evolution observed in the X-ray and radio bands (D'Avanzo et al. 2018,
A&A, 613 L1; Hajela et al. GCN Circ. 22692; Troja et al. GCN Circ.
22693; Dobie et al. arXiv:1803.06853; Alexander at al. 2018;
We acknowledge the excellent support from the LBT staff in obtaining