TITLE: GCN CIRCULAR NUMBER: 39297 SUBJECT: LIGO/Virgo/KAGRA S250206dm: GECKO Continued Observaiton with 7DT DATE: 25/02/12 02:15:38 GMT FROM: Gregory Paek at Seoul National University gregorypaek94@gmail.com
Gregory S.H. Paek (IfA), Myungshin Im, Mankeun Jeong, Hyeonho Choi, Seo-Won Chang, Ji Hoon Kim, Donggeun Tak, Hongjae Moon (SNU/SNU ARC), Dong-Jin Kim and Chung-Uk Lee (KASI), on behalf of the GECKO team
We report ongoing follow-up observations of the LIGO/Virgo/Kagra (LVK) gravitational-wave (GW) event, S250206dm (LVK Collaboration, GCN 39175) with the 7-Dimensional Telescope (7DT) following previous report (Paek et al., GCN 39241).
Observations with the 7DT array began on 2025-02-07 at 01:24 UT, roughly four hours after the GW event. Pre-defined tiles were chosen to overlap with the 90% credible region of the GW skymap, and a wide-field search strategy was implemented by assigning each telescope a distinct sky area to maximize coverage.
Thirteen 7DT units were employed to sequentially image the region—initially in the g-band and subsequently in the r-band. Per-tile exposure times ranged from 3*100 seconds to 6*100 seconds. Over the two observing epochs from 2025-02-07 (UT) to 2025-02-08 (UT), our observations covered a total of 590 tiles (~590 deg2) in the g-band and 320 tiles (~320 deg2) in the r-band.
The achieved 5-sigma depth varied by band, reaching 19.0–21.7 mag in the g-band and 18.4–21.2 mag in the r-band. Notably, following the initial g-band and r-band imaging, two additional epochs of g-band exposures are planned for the updated GW skymap region (LVK Collaboration, GCN 39231)—covering 75 tiles—in coordination with KMTNet’s R- and I-band imaging to ensure comprehensive multi-color coverage. Additionally, a dedicated transient search is scheduled for forthcoming analysis. The coverage of observations is detailed in TreasureMap (Wyatt et al. 2020; https://treasuremap.space/alerts?graceids=S250206dm).
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 https://7ds.snu.ac.kr/ and http://gwuniverse.snu.ac.kr/.
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