GCN Circular 38725

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 a fast X-ray transient, EP240702a (Chen et al., GCN #36801
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 ) using the 7-Dimensional Telescopes (7DT). Approximately 26 hours following the initial detection (2024-07-02T00:50:05 UTC), we targeted the localization center provided by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at RA, Dec = 328.203 deg, -38.980 deg with an uncertainty of 3 arcmins. Observations were made with eleven 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.

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 16.3 to 19.6 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 20.2 AB magnitudes. Despite the increased depth, no significant transient detection was identified. 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 2024-07-03T03:01:13.000 300 19.181
m425 2024-07-03T03:06:51.000 300 19.510
m450 2024-07-03T03:01:15.000 300 19.643
m475 2024-07-03T03:06:45.000 300 18.943
m500 2024-07-03T03:01:13.000 300 19.643
m525 2024-07-03T03:06:42.000 300 19.497
m550 2024-07-03T03:01:13.000 300 18.947
m575 2024-07-03T03:06:41.000 300 19.092
m600 2024-07-03T03:01:19.000 300 19.009
m625 2024-07-03T03:06:53.000 300 18.688
m650 2024-07-03T03:01:14.000 300 18.777
m675 2024-07-03T03:06:42.000 300 18.853
m700 2024-07-03T03:01:08.000 300 18.502
m725 2024-07-03T03:06:34.000 300 18.062
m750 2024-07-03T03:01:15.000 300 17.998
m775 2024-07-03T03:06:43.000 300 17.450
m800 2024-07-03T03:01:19.000 300 17.131
m825 2024-07-03T03:06:44.000 300 16.989
m850 2024-07-03T03:01:15.000 300 16.692
m875 2024-07-03T03:06:42.000 300 16.306

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/.