TITLE: GCN CIRCULAR NUMBER: 43863 SUBJECT: GRB 260223A: 7DT detection and Medium-band SED of Afterglow DATE: 26/02/27 12:05:22 GMT FROM: YoungPyo Hong at Seoul National University <evan77333@gmail.com> YoungPyo Hong (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Donggeun Tak (SNU ARC/SNU), Hyeonho Choi (SNU ARC/SNU), Donghwan Hyun (SNU ARC/SNU), Gregory S.H. Paek (IfA, SNU ARC/SNU), Seo-Won Chang (SNU ARC/SNU), Ji Hoon Kim (SNU ARC/SNU), and Won-Hyeong Lee (SNU ARC/SNU), report on behalf of the 7 Dimensional Telescope (7DT) team: We report optical follow-up observations of GRB26023A (GCN 43808; GCN 43809), conducted with the 7-Dimensional Telescope (7DT), located in Chile. The 7DT follow-up began at 04:44:14 UTC on 2026-02-23, corresponding to T0 + 33 minutes, pointing to a region covering the target localization provided by Fermi (GCN 43808) at RA, DEC = 110.5 deg, -29.4 deg with an uncertainty of 1.7 degree. Initial observations were made with 15, units in 19 medium-band filters as well as r-band filters (20 filters in total). The following table summarizes the observations and the derived 5-sigma upper limits: Filter Mag Mag_err Exposure Date Time ---------------------------------------------------------------------------------------------------- r 19.05 0.08 300.0 2026-02-23T04:49:44.000 m400 >19.02 N/A 300.0 2026-02-23T04:44:14.000 m425 19.28 0.16 300.0 2026-02-23T04:44:31.667 m450 19.48 0.24 300.0 2026-02-23T04:44:16.667 m475 18.97 0.12 300.0 2026-02-23T04:44:15.000 m500 19.62 0.27 300.0 2026-02-23T04:44:20.667 m525 19.48 0.24 300.0 2026-02-23T04:44:23.000 m550 >19.29 N/A 300.0 2026-02-23T04:49:38.000 m575 18.95 0.14 300.0 2026-02-23T04:51:58.000 m600 18.92 0.20 300.0 2026-02-23T04:49:39.333 m625 18.79 0.22 300.0 2026-02-23T04:49:37.000 m650 18.66 0.22 300.0 2026-02-23T04:44:38.000 m675 >18.95 N/A 300.0 2026-02-23T04:44:23.000 m700 18.13 0.18 300.0 2026-02-23T04:44:18.000 m725 >18.46 N/A 300.0 2026-02-23T04:44:23.000 m750 18.23 0.22 300.0 2026-02-23T04:44:20.667 m775 17.99 0.31 300.0 2026-02-23T04:44:43.333 m800 >17.52 N/A 300.0 2026-02-23T04:44:21.667 m825 >17.41 N/A 300.0 2026-02-23T04:44:21.333 m850 >17.05 N/A 300.0 2026-02-23T04:44:21.000 ---------------------------------------------------------------------------------------------------- * Upper limits (marked with '>') are 5-sigma limits The optical counterpart was detected in most filters. 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. Note that 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 https://7ds.snu.ac.kr/ and http://gwuniverse.snu.ac.kr/. View this GCN Circular online at https://gcn.nasa.gov/circulars/43863. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...