{
  "format": "text/plain",
  "bibcode": "2024GCN.38725....1P",
  "body": "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\n\nWe searched for the optical counterpart of a fast X-ray transient, EP240702a (Chen et al., GCN #36801) 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.\n\nNo 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.\n------\nFilter Date-obs[UT] Exp.time[s] Depth(5sigma)\nm400 2024-07-03T03:01:13.000 300 19.181\nm425 2024-07-03T03:06:51.000 300 19.510\nm450 2024-07-03T03:01:15.000 300 19.643\nm475 2024-07-03T03:06:45.000 300 18.943\nm500 2024-07-03T03:01:13.000 300 19.643\nm525 2024-07-03T03:06:42.000 300 19.497\nm550 2024-07-03T03:01:13.000 300 18.947\nm575 2024-07-03T03:06:41.000 300 19.092\nm600 2024-07-03T03:01:19.000 300 19.009\nm625 2024-07-03T03:06:53.000 300 18.688\nm650 2024-07-03T03:01:14.000 300 18.777\nm675 2024-07-03T03:06:42.000 300 18.853\nm700 2024-07-03T03:01:08.000 300 18.502\nm725 2024-07-03T03:06:34.000 300 18.062\nm750 2024-07-03T03:01:15.000 300 17.998\nm775 2024-07-03T03:06:43.000 300 17.450\nm800 2024-07-03T03:01:19.000 300 17.131\nm825 2024-07-03T03:06:44.000 300 16.989\nm850 2024-07-03T03:01:15.000 300 16.692\nm875 2024-07-03T03:06:42.000 300 16.306\n\nThe 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/.",
  "createdOn": 1735555648806,
  "submitter": "Gregory Paek at Seoul National University <gregorypaek94@gmail.com>",
  "subject": "EP240702a: 7DT Optical upper limits",
  "circularId": 38725,
  "eventId": "EP240702a",
  "submittedHow": "web"
}