GCN Circular 22763

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
Not found
 
 
) associated to GW 170817 (LVC GCN Circ. 21509
Not found
 
 
, 21513
Not found
 
 
) 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
Not found
 
 
; 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
Not found
 
 
) 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
Not found
 
 
; Troja et al. GCN Circ. 
22693
Not found
 
 
; Dobie et al. arXiv:1803.06853
Not found
 
 
; Alexander at al. 2018; 
arXiv:1805.02870
Not found
 
 
).

We acknowledge the excellent support from the LBT staff in obtaining 
these observations.