GCN Circular 33833

Gregory S.-H. Paek, Myungshin Im, Mankeun Jeong, Seo-Won Chang, Hyeonho Choi, Sophia Kim, Elahe Khalouei (SNU/SNU ARC), and Chung-Uk Lee (KASI), on behalf of the GECKO Team We report the optical observations conducted with the KMTNet telescopes of the 90 % localization area of of the LIGO/Virgo/KAGRA gravitational-wave event, S230518h (The LVK Collaboration, GCN 33816). The KMTNet is a system made of three 1.6-meter class telescopes each at SAAO in South Africa, CTIO in Chile, and SSO in Australia. The KMTNet field of view of a single pointing covers a ~2 deg by 2 deg area. The KMTNet observation started at 2023-05-18 17:31 (UT) or about 4.53 hrs after the GW alert, and we took a series of R-band images for 2*120sec per field. As a result, we covered 59 tiles (~230 deg^2) within the localization area. The preliminary depths of these images are 22 magnitudes for a 5-sigma detection of point sources. ------------------------------------------------------------------------ t-t0[day] DATE-OBS[UTC] OBSERVAT RA DEC FILTER ------------------------------------------------------------------------ 0.189 2023-05-18T17:31:36 SAAO 07:07:21.280 -31:59:59.00 R 0.194 2023-05-18T17:37:50 SAAO 07:10:00.030 -37:59:59.90 R 0.198 2023-05-18T17:44:29 SAAO 07:08:06.490 -35:59:59.80 R 0.203 2023-05-18T17:51:32 SAAO 08:11:42.420 -77:59:59.90 R 0.207 2023-05-18T17:57:52 SAAO 08:13:42.860 -79:59:59.80 R 0.212 2023-05-18T18:03:48 SAAO 07:43:43.690 -71:59:59.80 R 0.216 2023-05-18T18:09:58 SAAO 07:38:10.850 -83:59:59.80 R 0.220 2023-05-18T18:16:06 SAAO 07:45:13.770 -69:59:59.80 R 0.225 2023-05-18T18:22:25 SAAO 08:16:33.060 -81:59:59.70 R 0.229 2023-05-18T18:28:44 SAAO 08:10:12.780 -75:59:59.80 R 0.233 2023-05-18T18:35:04 SAAO 08:06:45.600 -67:59:59.90 R 0.238 2023-05-18T18:41:23 SAAO 07:39:34.490 -75:59:59.90 R 0.242 2023-05-18T18:47:36 SAAO 08:43:38.220 -83:59:59.90 R 0.428 2023-05-18T23:15:22 CTIO 07:09:08.320 -33:59:59.90 R 0.449 2023-05-18T23:46:11 CTIO 07:34:44.200 -52:00:00.00 R 0.454 2023-05-18T23:52:30 CTIO 07:42:23.090 -53:59:59.90 R 0.459 2023-05-18T23:59:28 CTIO 07:45:27.280 -58:00:00.00 R 0.463 2023-05-19T00:05:48 CTIO 07:36:34.750 -78:00:00.40 R 0.467 2023-05-19T00:12:00 CTIO 07:36:55.300 -56:00:00.00 R 0.476 2023-05-19T00:24:14 CTIO 08:08:07.970 -72:00:00.20 R 0.482 2023-05-19T00:33:33 CTIO 07:44:30.960 -60:00:00.00 R 0.487 2023-05-19T00:40:41 CTIO 07:24:42.400 -41:59:59.90 R 0.496 2023-05-19T00:53:27 CTIO 08:09:03.220 -74:00:00.20 R 0.498 2023-05-19T00:56:48 CTIO 07:36:35.100 -47:59:59.90 R 0.503 2023-05-19T01:03:04 CTIO 08:07:23.000 -70:00:00.10 R 0.508 2023-05-19T01:10:00 CTIO 07:47:31.840 -66:00:00.00 R 0.512 2023-05-19T01:16:57 CTIO 08:46:49.440 -78:00:00.40 R 0.517 2023-05-19T01:23:20 CTIO 08:40:50.860 -76:00:00.30 R 0.521 2023-05-19T01:29:31 CTIO 08:36:13.420 -74:00:00.20 R 0.525 2023-05-19T01:35:43 CTIO 08:32:32.370 -72:00:00.10 R 0.793 2023-05-19T08:01:18 SSO 07:46:28.480 -67:59:59.80 R 0.837 2023-05-19T09:04:10 SSO 07:17:50.270 -35:59:59.60 R 0.841 2023-05-19T09:10:46 SSO 07:29:01.780 -59:59:59.60 R 0.846 2023-05-19T09:17:26 SSO 07:41:52.780 -73:59:59.70 R 0.853 2023-05-19T09:26:53 SSO 07:55:06.650 -61:52:59.60 R 0.855 2023-05-19T09:29:57 SSO 08:06:13.800 -65:59:59.60 R 0.859 2023-05-19T09:36:25 SSO 08:29:32.030 -69:59:59.70 R 1.174 2023-05-19T17:09:20 SAAO 06:44:29.820 -19:08:55.90 R 1.178 2023-05-19T17:15:29 SAAO 06:21:24.190 -11:29:19.70 R 1.185 2023-05-19T17:25:58 SAAO 06:50:13.920 -24:53:37.90 R 1.188 2023-05-19T17:29:38 SAAO 06:45:31.020 -22:58:43.90 R 1.192 2023-05-19T17:35:49 SAAO 06:48:59.840 -26:48:31.90 R 1.197 2023-05-19T17:42:12 SAAO 06:57:31.010 -26:48:32.00 R 1.201 2023-05-19T17:48:32 SAAO 06:40:54.490 -21:03:49.90 R 1.206 2023-05-19T17:55:17 SAAO 06:56:23.010 -28:43:22.90 R 1.210 2023-05-19T18:01:27 SAAO 06:53:47.500 -22:58:43.80 R 1.220 2023-05-19T18:15:24 SAAO 06:59:32.900 -29:52:59.80 R 1.222 2023-05-19T18:19:12 SAAO 06:58:36.200 -24:53:37.80 R 1.226 2023-05-19T18:24:25 SAAO 07:05:03.610 -28:43:23.10 R 1.230 2023-05-19T18:29:49 SAAO 07:26:53.800 -81:59:59.90 R 1.234 2023-05-19T18:36:37 SAAO 07:27:16.320 -43:59:59.70 R 1.238 2023-05-19T18:42:34 SAAO 07:27:43.890 -49:59:59.90 R 1.245 2023-05-19T18:51:37 SAAO 07:39:49.890 -49:59:59.90 R 1.249 2023-05-19T18:57:44 SAAO 07:38:10.800 -61:59:59.80 R 1.252 2023-05-19T19:02:27 SAAO 07:50:46.190 -55:59:59.90 R 1.263 2023-05-19T19:17:49 SAAO 08:00:00.020 -59:59:59.90 R 1.270 2023-05-19T19:28:05 SAAO 08:56:23.500 -79:52:59.80 R 1.272 2023-05-19T19:31:16 SAAO 09:06:12.490 -81:59:59.90 R 1.277 2023-05-19T19:37:38 SAAO 09:49:05.440 -83:59:59.80 R We have reference images for most of the observed fields from the KMTNet Synoptic Survey of Southern SKy (KS4) project. Detailed photometry and transient search analyses are underway. Gravitational-wave EM Counterpart Korean Observatory (GECKO) is a network of 10+ 0.5m to 1m class telescopes worldwide. KMTNet is operated by the Korea Astronomy and Space Science Institute. We thank the KMTNet operators for their support for the KMTNet observations.