TITLE: GCN CIRCULAR NUMBER: 43206 SUBJECT: GRB251221A: 7DT optical upper limits DATE: 25/12/23 06:57:34 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 GRB251221A (Fermi/GBM GCN 43164; Swift GCN 43166) conducted with the 7-Dimensional Telescope (7DT), located in Chile. The 7DT follow-up observations began at 06:25:41 UTC on 2025-12-21, corresponding to approximately 1 hour after the trigger. The observations covered the target localization provided by Swift-XRT at RA, Dec = 101.42801 deg, 0.73241 deg, with an uncertainty radius of 2.1 arcseconds. Initial observations were carried out using 12 telescope units with 20 medium-band filters as well as g-, r-, i-band filters (23 filters in total). The following table summarizes (the observations and) the derived 5-sigma upper limits: Filter Mag Mag_err Exposure Date Time ---------------------------------------------------------------------------------------------------- g >20.32 N/A 600s 2025-12-21T06:27:52.833 r >19.54 N/A 600s 2025-12-21T06:34:05.833 i >18.54 N/A 600s 2025-12-21T06:27:56.167 m400 >18.61 N/A 600s 2025-12-21T06:34:09.167 m425 >19.09 N/A 600s 2025-12-21T06:34:04.667 m450 >19.23 N/A 600s 2025-12-21T06:27:50.667 m475 >19.21 N/A 600s 2025-12-21T06:34:05.167 m500 >19.36 N/A 600s 2025-12-21T06:27:57.333 m525 >19.05 N/A 600s 2025-12-21T06:34:08.167 m550 >18.66 N/A 600s 2025-12-21T06:34:12.833 m575 >18.94 N/A 600s 2025-12-21T06:28:00.500 m600 >18.88 N/A 600s 2025-12-21T06:27:58.500 m625 >18.48 N/A 600s 2025-12-21T06:34:30.000 m650 >18.42 N/A 600s 2025-12-21T06:27:52.667 m675 >18.35 N/A 600s 2025-12-21T06:34:26.333 m700 >18.40 N/A 600s 2025-12-21T06:27:55.000 m725 >17.92 N/A 600s 2025-12-21T06:34:13.500 m750 >17.75 N/A 1200s 2025-12-21T06:31:10.667 m775 >17.28 N/A 600s 2025-12-21T06:34:04.000 m800 >17.14 N/A 600s 2025-12-21T06:27:53.333 m825 >16.85 N/A 600s 2025-12-21T06:34:06.000 m850 >16.69 N/A 600s 2025-12-21T06:27:51.667 m875 >16.14 N/A 600s 2025-12-21T06:27:54.167 ---------------------------------------------------------------------------------------------------- No significant transient event was identified in the preliminary result. Preliminary analysis yields 5 sigma limiting magnitudes in the range of 16.14 – 20.32 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 http://7ds.snu.ac.kr/ and http://gwuniverse.snu.ac.kr/. View this GCN Circular online at https://gcn.nasa.gov/circulars/43206. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...