TITLE: GCN CIRCULAR NUMBER: 39249 SUBJECT: LIGO/Virgo/KAGRA S250206dm: WFST follow-up observations DATE: 25/02/09 14:02:04 GMT FROM: Zhengyan Liu at USTC ustclzy@mail.ustc.edu.cn
Z. Y. Liu, L. He, W. Zhao, J.-A. Jiang, Z. L. Xu, D. Z. Meng, Z. Y. Cai, M. X. Cai, T. G. Wang, X. Kong, Z. G. Dai, L. L. Fan (USTC), Z. P. Jin, X. F. Wu (PMO) report on behalf of the WFST Collaboration: Following the detection of the gravitational wave event S250206dm (the LIGO-Virgo-Kagra Collaboration, GCN 39175), we conducted follow-up observations at the high-probability region using the newly deployed Wide Field Survey Telescope (WFST Collaboration; Wang et al., 2023) at Lenghu Astronomical Observation Base (Qinghai province, China) from 2025-02-07T12:10:00 to 2025-02-07T17:00:04 UTC and 2025-02-08T12:14:53 to 2025-02-08T17:14:15 UTC, corresponding to 14.74 and 38.82 hours after the merger event (UTC 2025-02-06T21:25:30.44), respectively. We observed in WFST r, i, and z bands with exposure time of 60-90 seconds. A total area of about 333 square degrees of the 90% area in the Bilby.fits skymap (GCN 39231) was observed, covering 64% of the event localization. We checked WFST data for the previously reported transient candidates AT2025azm, AT2025azn, and AT2025bbo (GCN 39191; GCN 39244). Our targeted observations include 90s exposures in r and i bands for AT2025azm, 60s exposures in r, i, and z bands for AT2025azn, and 60s exposures in r and i bands for AT2025bbo. Observations of AT2025azm began at 2025-02-07T12:30:01 UTC, 15.08 hours post-merger; AT2025azn was observed starting at 2025-02-07T14:38:44 UTC, 17.21 hours post-merger; and AT2025bbo was observed starting at 2025-02-07T13:50:35 UTC, 16.42 hours post-merger. All images were processed with the WFST pipeline. After astrometric and photometric calibration, image subtraction was performed using templates of the PS1 stacked images (Waters et al., 2020). For AT2025azm and AT2025azn, we did not detect any transient in observed bands. The observations were carried out under seeing conditions of 0.9-1.3 arcseconds during the first night (starting at 2025-02-07T12:10:00 UTC). The 5-sigma non-detection limits for the two objects are as follows: AT2025azm: r > 22.2, i > 21.8; AT2025azn: r > 21.7, i > 21.4, z > 20.5. For AT2025bbo, we detected the transient in both r and i bands on the first night. However, the target was not found on the second night due to cloudy weather. The 1st-night photometric results for AT2025bbo are: r = 20.5 +/- 0.1, i = 20.3 +/- 0.1. In addition, the r-band magnitude reported by Pan-STARRS (GCN 39244) is r = 20.45 +/- 0.2 and the time interval between our observations is ~0.7 day, suggesting a slow evolution for AT2025bbo. Subsequent WFST observations and a systematic search of potential transients are ongoing.
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