GCN Circular 38610

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, EP241119a (Zhang et al., GCN #38281
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 ) using the 7-Dimensional Telescopes (7DT). Approximately 13.6 hours following the initial detection (2024-11-19T17:53:20 UTC), we targeted the localization center provided by the Wide-field X-ray Telescope (WXT) and confirmed by the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission at RA, Dec = 84.1062 deg, 3.8404 deg with an uncertainty of 10 arcsecs. Observations were made with thirteen 7DT units in twenty 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. Observations for two broad-band filters were also carried out. 

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. The 5-sigma upper limits (AB) range from 16.8 to 19.3 mag in the medium-band filters, reaching 20.1 mag in the g-band and 19.9 mag in the r-band. Observations were conducted under suboptimal conditions, potentially limiting our search sensitivity.
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Filter Date-obs[UT] Exp.time[s] Depth(5sigma)
m400 2024-11-20T07:34:48 300 18.767
m425 2024-11-20T07:34:47 300 18.977
m450 2024-11-20T07:29:19 300 19.008
m475 2024-11-20T07:34:49 300 18.947
m500 2024-11-20T07:29:17 300 19.280
m525 2024-11-20T07:34:46 300 19.228
m550 2024-11-20T07:29:19 300 18.866
m575 2024-11-20T07:34:49 300 18.892
m600 2024-11-20T07:29:23 300 19.166
m625 2024-11-20T07:35:00 300 18.964
m650 2024-11-20T07:29:20 300 18.466
m675 2024-11-20T07:34:48 300 18.537
m700 2024-11-20T07:29:18 300 18.382
m725 2024-11-20T07:34:47 300 18.159
m750 2024-11-20T07:29:18 300 18.215
m775 2024-11-20T07:34:48 300 17.981
m800 2024-11-20T07:29:19 300 17.748
m825 2024-11-20T07:34:47 300 17.491
m850 2024-11-20T07:29:18 300 17.214
m875 2024-11-20T07:34:54 300 16.784
g    2024-11-20T07:29:19 300 20.120
r    2024-11-20T07:34:47 300 19.922

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/.