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 <gregorypaek94@gmail.com>

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:
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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:
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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:
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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.