TITLE: GCN CIRCULAR NUMBER: 39129 SUBJECT: GRB 250129A: 7DT Detection and Medium-band SED of Afterglow DATE: 25/02/03 06:14:53 GMT FROM: Gregory Paek at Seoul National University <gregorypaek94@gmail.com> Gregory S.H. Paek (IfA, SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Hyeonho Choi (SNU ARC/SNU), Donggeun Tak (SNU ARC/SNU), Seo-Won Chang (SNU ARC/SNU), and Ji Hoon Kim (SNU ARC/SNU) report on behalf of the 7-Dimensional Telescope collaboration We detected the optical counterpart of GRB 250129A using the 7-Dimensional Telescopes (7DT). Approximately 1 hour and 14 minutes following the initial detection by Swift (Swift team, Beardmore et al., GCN #39066), we targeted the localization center provided by Swift/UVOT at RA, Dec = 198.67673 deg, +5.03063 deg with an uncertainty of 1.10 arcseconds. Observations were made with eleven 7DT units in r-band and nineteen medium-band filters, denoted as m400, m425, then through m875, in which the numeric values indicate their central wavelengths in nanometers. Each medium-band filter has a bandwidth of 25nm. 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. The optical counterpart was detected in most filters. However, only marginal detection was observed in m800, m825, m850, and m875. This is based on preliminary photometry, and no extinction correction has been applied. The 5-sigma upper limits (AB) and detections in relevant filters are summarized below. ------ Filter Mag Mag_err Date-obs[UT] Exp.time[s] Depth(5sigma) Note m425 17.3 0.0 2025-01-29T05:59:21 300 19.9 m450 17.2 0.0 2025-01-29T05:53:51 300 19.2 m475 17.2 0.0 2025-01-29T05:59:21 300 19.5 m500 17.2 0.0 2025-01-29T05:53:51 300 20.0 m525 17.2 0.0 2025-01-29T05:59:20 300 20.1 m550 17.0 0.1 2025-01-29T05:53:52 300 18.6 m575 17.0 0.0 2025-01-29T05:59:19 300 18.8 m600 16.9 0.0 2025-01-29T05:53:58 300 19.7 m625 16.9 0.0 2025-01-29T05:59:36 300 19.4 m650 16.9 0.0 2025-01-29T05:53:56 300 19.4 m675 17.1 0.1 2025-01-29T05:59:22 300 19.4 m700 16.9 0.0 2025-01-29T05:53:49 300 19.2 m725 17.0 0.0 2025-01-29T05:59:17 300 18.9 m750 17.0 0.1 2025-01-29T05:53:54 300 18.8 m775 17.0 0.1 2025-01-29T05:59:22 300 18.3 m800 2025-01-29T05:53:59 300 17.8 n/d m825 2025-01-29T05:59:32 300 17.6 n/d m850 2025-01-29T05:53:54 300 17.2 n/d m875 2025-01-29T05:59:15 300 17.1 n/d r 17.0 0.0 2025-01-29T05:59:07 600 20.9 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://gwuniverse.snu.ac.kr/. View this GCN Circular online at https://gcn.nasa.gov/circulars/39129. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...