TITLE:   GCN CIRCULAR
NUMBER:  37454
SUBJECT: GRB 240825A: 7DT Optical Upper Limit
DATE:    24/09/11 10:38:02 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 bright optical counterpart of the gamma-ray burst, GRB240825A (Fermi GBM team, GCN 37273; Gupta et al., GCN 37274) using the 7-Dimensional Telescopes (7DT). Approximately 10 hours following the initial detection (2024-08-25T15:53:00 UTC), we targeted the UVOT coordinate (Gupta et al., GCN 37274). 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 signal was found. 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 first unit of 7DT (m400 and m425 filters) temporarily experienced an electronic issue with the camera, resulting in relatively low sensitivity.

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Filter Date-obs[UT] Exp.time[s] Depth(5sigma)
m400 2024-08-26T01:54:47 300 15.415
m425 2024-08-26T02:00:23 300 17.851
m450 2024-08-26T01:54:43 300 19.651
m475 2024-08-26T02:00:09 300 19.790
m500 2024-08-26T01:54:42 300 19.952
m525 2024-08-26T02:00:07 300 19.883
m550 2024-08-26T01:54:43 300 19.304
m575 2024-08-26T02:00:10 300 19.456
m600 2024-08-26T01:54:48 300 19.313
m625 2024-08-26T02:00:21 300 18.892
m650 2024-08-26T01:54:44 300 19.068
m675 2024-08-26T02:00:09 300 19.156
m700 2024-08-26T01:54:39 300 19.041
m725 2024-08-26T02:00:02 300 18.658
m750 2024-08-26T01:54:48 300 18.258
m775 2024-08-26T02:00:14 300 17.792
m850 2024-08-26T01:54:46 300 17.067
m875 2024-08-26T02:00:11 300 16.834

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/.