TITLE: GCN CIRCULAR NUMBER: 21529 SUBJECT: LIGO/Virgo G298048: Potential optical counterpart discovered by Swope telescope DATE: 17/08/18 01:05:23 GMT FROM: Edo Berger at Harvard U D. A. Coulter, C. D. Kilpatrick, M. R. Siebert, R. J. Foley (UCSC), B. J. Shappee, M. R. Drout, J. S. Simon, A. L. Piro (Carnegie), and A. Rest (STScI) report on behalf of the One-Meter Two-Hemisphere (1M2H) collaboration: On 2017 Aug 18 UT in the process of observing several galaxies coincident with the highest-likelihood localization region for the LIGO/Virgo G298048 trigger (LVC GCNs 21509, 21513) with the 1-m Swope telescope at Las Campanas Observatory, we detect a source 5.3¿ E and 8.8¿ N of NGC 4993, an S0 galaxy in the NGC 4993 / ESO 508-G018 group at a distance of ~40 Mpc (Tully-Fisher distance to the group; Freedman et al., ApJ, 553, 47, 2001). The object is: SSS17a 13:09:48.089 -23:22:53.35 and had a brightness of i = 16.0 mag. We have checked the minor planet center and previous SN discoveries, and have found no cross-matches. This source could possibly be optical emission associated with the LIGO/Virgo G298048 trigger. Spectroscopic observations are under way. Additional photometry has been obtained and is being analyzed. Additional observations are encouraged. The contact person for this circular is Ryan Foley (foley@ucsc.edu). [GCN OPS NOTE(28sep17): Per a combination of requests and editorial policy: (1) AR was added to the author list. (2) The contact-name sentence was added. The editor notes that RF was having sign-up problems with the LV-EM Forum and E.Berger offered to submit in his place since publication latency was important. (3) And the EB signature block was removed.]