TITLE: GCN CIRCULAR NUMBER: 38724 SUBJECT: GRB 240606B: 7DT Optical upper limits DATE: 24/12/30 10:45:36 GMT FROM: Gregory Paek at Seoul National University <gregorypaek94@gmail.com> Gregory S.H. Paek (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 Telescope collaboration We searched for the optical counterpart of the GRB, GRB 240606B (DeLaunay et al., GCN #36628) using the 7-Dimensional Telescopes (7DT). Approximately 20.2 hours following the initial detection (2024-06-06T13:33:06.5 UTC), we targeted the localization center provided by the Swift XRT at RA, Dec = 40.29507 deg, -60.61201 deg with an uncertainty of 2.4 arcsecs (Osborne et al., GCN #36631). Observations were made with eleven 7DT units in eight medium-band filters, denoted as m450, m500, m550, m650, m700, m750, m800, and m850, in which the numeric values indicate their central wavelengths in nanometers. Each medium-band filter has a bandwidth of 25nm. No significant transient event was identified in the preliminary result. Photometric flux calibration was performed using synthetic photometries derived from the Gaia DR3 XP catalog (Gaia Collaboration et al. 2022) within the AB magnitude system. The 5-sigma upper limits (AB) range from 17.8 to 21.0 mag in the medium-band filters. To improve the depth for detection, we combined all images taken with the medium-band filters from m400 to m875. The combined image was treated as an r-band equivalent, and photometric measurements were performed. This approach yielded a 5-sigma upper limit of 20.9 AB magnitudes. Despite the increased depth, no significant transient detection was identified. Observations were conducted under suboptimal conditions, potentially limiting our search sensitivity. Filter Date-obs[UT] Exp.time[s] Depth(5sigma) m450 2024-06-07T09:47:55 1200 20.7 m500 2024-06-07T09:47:23 1200 21.0 m550 2024-06-07T09:47:22 1200 20.1 m650 2024-06-07T09:46:09 1200 20.1 m700 2024-06-07T09:47:20 1200 19.6 m750 2024-06-07T09:47:16 1200 19.2 m800 2024-06-07T09:47:21 1200 18.0 m850 2024-06-07T09:47:18 1200 17.8 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/38724. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...