TITLE: GCN CIRCULAR NUMBER: 42867 SUBJECT: LIGO/Virgo/KAGRA S251112cm: GECKO/KMTNet tiling observations and transient candidates DATE: 25/11/28 08:15:18 GMT FROM: Mankeun Jeong jmk5040@gmail.com

Mankeun Jeong, Myungshin Im, Seo-Won Chang, Hyeonho Choi (SNU/SNU ARC), Gregory S.-H. Paek (IfA), and Chung-Uk Lee (KASI), on behalf of the GECKO team

We observed the S251112cm sky localization region (Updated; GCN 42690) following the LIGO/Virgo/KAGRA trigger. Optical imaging was obtained with the three 1.6-m Korea Microlensing Telescope Network (KMTNet) telescopes at CTIO, SAAO, and SSO, with most of the data taken from SSO.

We surveyed the southern portion (decl. < -10 deg) of the S251112cm 90% localization region, covering ~1,012 deg^2 from 2025-11-13 to 2025-11-27. During the first three nights, we obtained R and I imaging with 480 sec total exposure per field (four dithered 120 sec exposures) over ~180 deg^2. For the remainder of the run, observations were conducted in R-band only with the same 480 sec per-field exposure.

A fraction of the surveyed area overlaps with regions having KMTNet Synoptic Survey of the Southern Sky (KS4) reference images. Difference image analysis has been completed for ~60 deg^2, with additional processing ongoing.

Below we list transient candidates re-identified from TNS and new GECKO sources detected in our imaging. Redshifts are adopted from the GLADE+ host-galaxy associations.

Name | RA (deg) | Dec (deg) | R (AB mag) | I (AB mag) | Date-Obs (UTC) | Redshift -----|----------|-----------|------------|------------|-------------------|--------- AT2025aebs | 354.16948 | -33.01666 | 20.42 ± 0.05 | 20.66 ± 0.07 | 2025-11-16T12:24:50 | 0.1642 AT2025adbw | 3.31215 | -36.40843 | 19.07 ± 0.02 | — | 2025-11-14T14:23:37 | — AT2025abnc | 7.50565 | -34.96515 | 19.95 ± 0.05 | 20.54 ± 0.08 | 2025-11-14T12:40:10 | 0.1067 GECKO25dryb | 1.69517 | -33.38614 | 18.95 ± 0.01 | 19.05 ± 0.05 | 2025-11-14T13:39:42 | 0.2094 GECKO25drxk | 1.53192 | -35.03043 | 18.89 ± 0.01 | 18.86 ± 0.03 | 2025-11-14T13:39:42 | 0.2408 GECKO25dulp | 358.59508 | -35.91373 | 19.58 ± 0.06 | 18.65 ± 0.03 | 2025-11-16T13:40:59 | 0.0859 GECKO25dukb | 356.10091 | -34.07094 | 18.37 ± 0.01 | 18.13 ± 0.01 | 2025-11-16T12:24:50 | 0.1269

All newly identified GECKO candidates lie close to the center of the host galaxy, whose redshifts exceed the estimated distance of S251112cm, indicating they are unrelated to the GW event's physical origin.

Further difference imaging and transient vetting across the remaining fields are underway.

The GECKO network (Im et al. 2023; Paek et al. 2024) operates 0.5–1 m class telescopes worldwide for EM follow-up of gravitational-wave events. KMTNet is operated by the Korea Astronomy and Space Science Institute. We thank the KMTNet staff for their support during these observations.

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