TITLE: GCN CIRCULAR NUMBER: 17829 SUBJECT: GRB 150518A: Xinglong2.16m optical afterglow detection DATE: 15/05/19 23:26:00 GMT FROM: Dong Xu at NAOC/CAS Dong Xu (NAOC/CAS), Xue-Bing Wu, Qian Yang (PKU) report We observed the field of the XRT transient (Sbarufatti et al., GCN 17827), which is very likely the X-ray counterpart of GRB 150518A (Kawamuro et al., GCN 17825) , using the 2.16m telescope located at Xinglong, Hebei, China. Observations started at 14:23:06 UT (i.e., 16.68 hr after the burst) in a rather poor seeing of 3", and 4x300s R-band frames were obtained. At the XRT-UVOT enhanced position (R.A. = 15:36:48.25, Dec. = +16:19:47.3 , Err. Rad: = 2.2′′), an optical source is well detected at m(R) = 20.3 +/- 0.2 mag, calibrated with nearby SDSS stars. The source largely looks round in our stacked image. We note that there exists a source at the XRT-UVOT position that is already detected and classified as a galaxy by SDSS (u=22.88, g=22.14, r=21.43, i=21.24, z=20.74), with a photometric redshift of z_ph=0.35 +/- 0.11. The Xinglong detection is brighter than the SDSS detection by about 1 magnitude in the R/r filters, and we thus think that it is the optical afterglow of the GRB. If the afterglow is indeed associated with the galaxy at such a redshift, it would be doable to search for a GRB-associated supernova. We thank Junjun Jia for carrying out these observations.