TITLE: GCN CIRCULAR NUMBER: 36352 SUBJECT: LIGO/Virgo/KAGRA S240422ed: GECKO/7DT ToO observations DATE: 24/04/30 03:56:19 GMT FROM: Hyeonho Choi at Seoul National University hhchoi1022@gmail.com
Hyeonho Choi, Myungshin Im , Gregory S.-H. Paek, Seo-Won Chang, Ji Hoon Kim and Hongjae Moon (SNU/SNU ARC), on behalf of the GECKO team
We report optical observations conducted with the 7-Dimensional Telescope (7DT) in the ~170 sq. degrees localization area of the LIGO/Virgo/KAGRA gravitational-wave event, S240422ed (The LVK Collaboration, GCN 36236).
The 7DT observation started at 01:19:40 UT on 2024-04-23, approximately 3.7 hours after the GW alert. We took a series of r-band images for 3x120 sec per field. The preliminary depth of the combined image is 19.5 magnitudes for a 5-sigma detection of point sources. The coverage of our observations is detailed in TreasureMap (Wyatt et al. 2020): https://treasuremap.space/alerts?graceids=S240422ed
Additionally, intensive observations were made in the most probable regions and reported transient candidates with 7DT units equipped with 18 medium-band filters with 25 nm widths from 400 nm to 825 nm at 25 nm intervals. We obtained 0.5 to 2 hours in each medium-band filter, reaching magnitudes of 19-20 for a 5-sigma limiting magnitude. 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 (Gregory S.H. Paek, in prep). Observations at low altitudes resulted in suboptimal conditions, potentially limiting our search sensitivity.
Gravitational-wave EM Counterpart Korean Observatory (GECKO) is a network of 0.5m to 1m class telescopes worldwide (Im et al. 2023, Proceedings of IAU Symp. Vol. 363, pp. 207, Paek et al. 2024, ApJ 960 113) including 7DT. 7-Dimensional Telescope (7DT) is a multiple telescope system in Chile to be made of 20 wide-field telescopes equipped with 40 medium-bandwidth (~25nm) filters as well as a suite of broad-band filters when completed. Currently, a partially constructed 7DT made of 10 telescopes with 20 medium-band filters and u, g, r, i, z filters is operational. Further information about the 7DT is available at http://gwuniverse.snu.ac.kr/%C2%A0. We thank the staff of the ObsTech at the El Sauce observatory for their support of the observation.
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