TITLE: GCN CIRCULAR NUMBER: 43270 SUBJECT: GRB251230A: 7DT optical upper limits DATE: 25/12/30 13:38:44 GMT FROM: Donggeun Tak <donggeun.tak@gmail.com> Donggeun Tak (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), YoungPyo Hong (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), and Ji Hoon Kim (SNU ARC/SNU) report on behalf of the 7 Dimensional Telescope (7DT) team: We report optical follow-up observations of GRB251230A (GCN 43252; GCN 43263), conducted with the 7-Dimensional Telescope (7DT), located in Chile. The 7DT follow-up began at 06:52:54 UTC on 2025-12-30, corresponding to T0 + 5.5 hours, pointing to a region covering the target localization provided by Swift (GCN 43252) at RA, DEC = 179.89760 deg, 11.43870 deg. Initial observations were made with 12 units in 20 medium-band filters as well as g-, r-, and i-band filters (23 filters in total). The following table summarizes the derived 5-sigma upper limits: Filter Mag Mag_err Exposure Date Time ---------------------------------------------------------------------------------------------------- g >20.04 N/A 300s 2025-12-30T06:49:06.000 r >19.42 N/A 300s 2025-12-30T06:54:28.333 i >18.34 N/A 300s 2025-12-30T06:49:04.000 m400 >18.32 N/A 300s 2025-12-30T06:54:34.667 m425 >18.89 N/A 300s 2025-12-30T06:54:20.000 m450 >18.95 N/A 300s 2025-12-30T06:49:02.667 m475 >19.00 N/A 300s 2025-12-30T06:54:26.000 m500 >19.04 N/A 300s 2025-12-30T06:49:10.667 m525 >18.93 N/A 300s 2025-12-30T06:54:29.000 m550 >18.60 N/A 300s 2025-12-30T06:54:34.000 m575 >18.79 N/A 300s 2025-12-30T06:49:09.667 m600 >18.60 N/A 300s 2025-12-30T06:49:08.667 m625 >18.21 N/A 300s 2025-12-30T06:54:42.667 m650 >18.25 N/A 300s 2025-12-30T06:49:03.000 m675 >18.28 N/A 300s 2025-12-30T06:54:38.333 m700 >18.16 N/A 300s 2025-12-30T06:49:07.000 m725 >17.72 N/A 300s 2025-12-30T06:54:36.333 m750 >17.56 N/A 600s 2025-12-30T06:51:49.833 m775 >17.04 N/A 300s 2025-12-30T06:54:24.667 m800 >16.81 N/A 300s 2025-12-30T06:49:05.667 m825 >16.65 N/A 300s 2025-12-30T06:54:29.667 m850 >16.44 N/A 300s 2025-12-30T06:48:59.000 m875 >15.91 N/A 300s 2025-12-30T06:49:08.000 ---------------------------------------------------------------------------------------------------- No significant transient event was identified in the preliminary result. Preliminary analysis yields 5σ limiting magnitudes in the range of 15.91–20.04 mag depending on sky brightness and airmass. 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/43270. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...