TITLE: GCN CIRCULAR NUMBER: 35790 SUBJECT: Transient EPW20240219aa: 7DT Optical Upper Limits DATE: 24/02/25 16:06:42 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 bright X-ray flare, EPW20240219aa (Zhang et al., ATel #16463; Zhang et al., GCN #35773) using the 7-Dimensional Telescopes (7DT). Approximately 3.8 days following the initial detection (2024-02-19T06:23:27 UTC), we targeted the localization center provided by the Wide-field X-ray Telescope (WXT) modules aboard the Einstein Probe (EP) mission at RA, Dec = 80.016 deg, 25.541 deg with an uncertainty of 3 arcmin. Observations were made with nine 7DT units in medium-band filters, denoted as m400, m425, then through m825, in which the numeric values indicate their central wavelengths in nanometers. Each filter has a bandwidth of 25nm. In the preliminary result, no significant transient event was identified. 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. Observations at low altitudes resulted in suboptimal conditions, potentially limiting our search sensitivity. ------ Filter Date-obs[UT] Exp.time[s] Depth(5sigma) m400 2024-02-23T01:42:11.629 900 18.166 m425 2024-02-23T02:00:22.606 900 18.338 m450 2024-02-23T01:42:53.577 900 18.330 m475 2024-02-23T02:00:47.055 900 18.447 m500 2024-02-23T01:42:28.304 900 18.597 m525 2024-02-23T01:59:53.492 900 18.678 m550 2024-02-23T01:48:38.922 900 18.379 m575 2024-02-23T02:05:57.204 900 18.486 m600 2024-02-23T01:44:36.663 900 17.712 m625 2024-02-23T02:01:47.678 900 18.365 m650 2024-02-23T01:43:27.232 900 18.110 m675 2024-02-23T02:00:54.580 900 18.425 m700 2024-02-23T01:43:15.587 900 17.894 m725 2024-02-23T02:00:44.848 900 18.029 m750 2024-02-23T01:43:56.489 900 17.154 m775 2024-02-23T02:01:45.824 900 17.641 m800 2024-02-23T01:47:43.573 900 17.177 m825 2024-02-23T02:05:38.685 900 17.014 The 7-Dimensional Telescope (7DT), comprising 20 wide-field telescopes equipped with 40 medium-bandwidth (~25nm) filters located in Chile, 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/35790. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...