TITLE: GCN CIRCULAR NUMBER: 34146 SUBJECT: LIGO/Virgo/KAGRA S230627c: GECKO optical follow-up observations of host galaxy candidates and ZTF23aaptsuy/AT2023lxu DATE: 23/07/06 00:01:29 GMT FROM: Gregory Paek at Seoul National University Gregory S.-H. Paek, Myungshin Im, Jinguk Seo, Hyeonho Choi, Hongjae Moon (SNUARC/SNU), Joh-Na Yoon (CBNU),  Sumin Lee, Changgon Kim, Soojong Pak (KHU), and Hyun-Il Sung (KASI), on behalf of the GECKO Team We report optical follow-up observations associated with the LIGO/Virgo/KAGRA gravitational-wave event, S230627c (GCN 34086), conducted with the LOAO, KHAO, CBNUO, and SAO telescopes. The LOAO is a 1.0-meter telescope located in the USA. The KHAO and SAO are 1.0-meter and 0.8-meter telescopes, respectively, located in Korea. The CBNUO is a 0.6-meter telescope with a wide field of view, also located in Korea. Detailed information is in Im+21. The LOAO and KHAO observed the seven highest-scoring host galaxy candidates. The SAO observed one of the optical counterpart candidates, ZTF23aaptsuy/AT2023lxu, reported by ZTF (GCN 34089), and the CBNUO observed the highest-ranked tile with a wide field-of-view (~1.4 deg2). The host galaxy candidates from the GLADE+ catalog (Dálya+21) were ranked based on a calculated score that combines the probability in the GW localization area with the K-band luminosity, the latter of which is used as a proxy for stellar mass. Each tile for wide field-of-view instruments is ranked by summing the scores of galaxies within the tile. Observations began at 2023-06-27 04:25 (UT), approximately 0.11 days after the GW alert. For the LOAO images, we subtracted each observed image from the reference image in the r-band from the Pan-STARRS1 surveys (Chambers+16) and filtered transient candidates, which we classified either through parameters from SourceEXtractor (Bertin+96) or visual inspection. Despite our efforts, we could not find any significant signals. We used the Pan-STARRS1 catalog for flux calibration and employed an AB magnitude system. The preliminary 5 sigma depths of LOAO and KHAO images are as follows: ------ OBS OBJECT RANK RA DEC DATE-OBS[UTC] t-t0[days] FILTER EXPTIME[s] DEPTH LOAO G135600 0 10:44:53.7 +51:59:22.2 2023-06-27T04:25:33 0.11 R 4860 20.9 LOAO G1053369 1 10:48:32.0 +54:05:12.0 2023-06-28T04:12:01 1.10 R 1800 20.8 LOAO G1047395 2 10:41:23.0 +46:56:05.0 2023-06-28T04:47:23 1.12 R 1800 20.5 LOAO G1079423 3 10:49:12.0 +57:50:13.2 2023-06-29T04:00:59 2.09 R 1800 20.8 LOAO G1078790 4 10:51:27.1 +57:42:36.0 2023-06-29T04:46:02 2.12 R 1800 20.7 LOAO G1047181 5 10:36:31.9 +45:16:44.4 2023-06-30T03:57:02 3.09 R 1800 20.7 LOAO G1079513 6 10:51:47.2 +55:23:09.6 2023-06-30T04:34:44 3.11 R 1800 20.7 KHAO G135600 0 10:44:53.1 +51:59:09.8 2023-06-27T12:10:16 0.43 R 900 16.7 The preliminary 5-sigma depth of the CBNUO image is as follows: ------ OBS OBJECT RANK RA DEC DATE-OBS[UTC] t-t0[days] FILTER EXPTIME[s] DEPTH CBNUO S230627C 0 11:10:14.110 +75:41:28.640 2023-06-27T12:21:49 0.44 R 300 17.1 The preliminary 5 sigma depths of SAO images are as follows: ------ OBS OBJECT DATE-OBS[UTC] t-t0[days] FILTER EXPTIME[s] DEPTH SAO AT2023lxu 2023-06-27T12:32:39 0.44 R 120 16.6 SAO AT2023lxu 2023-07-01T11:45:01 4.41 R 2520 19.7 Gravitational-wave EM Counterpart Korean Observatory (GECKO) is a network of 10+ 0.5m to 1m class telescopes worldwide. The Korea Astronomy and Space Science Institute operates LOAO. We thank the LOAO, KHAO, CBNUO, and SAO operators for their support for the observations.