TITLE: GCN CIRCULAR NUMBER: 42569 SUBJECT: GRB 251013C: 7DT detection and Medium-band SED of Afterglow DATE: 25/11/04 06:14:11 GMT FROM: YoungPyo Hong at Seoul National University <evan77333@gmail.com> YoungPyo Hong (SNU ARC/SNU), Donggeun Tak (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Hyeonho Choi (SNU ARC/SNU), Seo-Won Chang (SNU ARC/SNU), and Ji Hoon Kim (SNU ARC/SNU) report on behalf of the 7-Dimensional Sky Survey team: At 2025-10-14T00:02:30 UTC (about 6 hours after the GRB), we observed the optical counterpart of GRB 251013C (Fermi GBM team GCN 42221, Rakotondrainibe et al. GCN 42222), first reported by SVOM/VT (Palmerio et al. GCN 42223) using 7-Dimensional Telescope (7DT) at the El Sauce Observatory in Chile. Observations were made with 10 7DT units in g, r, i-band and 14 medium-band filters, in which the numeric values indicate their central wavelengths in nanometers. Each medium-band filter has a bandwidth of 25nm. The counterpart was detected at the following position: RA(J2000) = 345.8356 (deg) / 23h03m20.54s RA(J2000) = -0.2103 (deg) / -00°12'37.08" We report our photometry result with 14 medium-band and g, r, i band filters: Filter Mag Mag_err Exposure Date Time T-T0 --------------------------------------------------------------------------------------------------- g 18.11 0.02 3x100s 2025-10-14T00:06:00 6:15:48 r 17.77 0.02 3x100s 2025-10-14T00:12:58 6:22:46 i 17.55 0.05 3x100s 2025-10-14T00:06:10 6:15:59 m400 18.34 0.08 3x100s 2025-10-14T00:12:53 6:22:42 m425 18.29 0.05 3x100s 2025-10-14T00:13:00 6:22:49 m450 18.00 0.04 3x100s 2025-10-14T00:05:55 6:15:43 m475 18.04 0.03 3x100s 2025-10-14T00:12:53 6:22:42 m500 18.07 0.04 3x100s 2025-10-14T00:13:40 6:23:28 m525 17.98 0.04 3x100s 2025-10-14T00:12:54 6:22:43 m550 17.88 0.04 3x100s 2025-10-14T00:13:00 6:22:49 m575 17.85 0.05 3x100s 2025-10-14T00:06:19 6:16:07 m600 17.86 0.07 3x100s 2025-10-14T00:06:16 6:16:05 m625 17.64 0.08 3x100s 2025-10-14T00:06:03 6:15:52 m650 17.57 0.07 3x100s 2025-10-14T00:05:54 6:15:42 m675 17.73 0.05 3x100s 2025-10-14T00:13:29 6:23:17 m700 17.58 0.06 3x100s 2025-10-14T00:05:57 6:15:46 m750 17.41 0.09 3x100s 2025-10-14T00:05:51 6:15:40 m750 17.36 0.08 3x100s 2025-10-14T00:13:41 6:23:30 Photometric flux calibration was performed using synthetic photometry based on the Gaia DR3 XP catalog (Gaia Collaboration et al. 2022) within the AB magnitude system. This is based on preliminary photometry, and no extinction correction has been applied. The 7-Dimensional Telescope (7DT), located in Chile and comprising 20 wide-field telescopes equipped with 40 medium-bandwidth (~25nm) filters, aims to detect optical counterparts of GW sources and conduct the 7-Dimensional Sky Survey (7DS) of the Southern Hemisphere. Further information about the 7DT is available at [http://7ds.snu.ac.kr/](http://7ds.snu.ac.kr/). View this GCN Circular online at https://gcn.nasa.gov/circulars/42569. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...