TITLE: GCN CIRCULAR NUMBER: 38314 SUBJECT: LIGO/Virgo/KAGRA S241125n: Kinder optical upper limit in the BAT error region DATE: 24/11/25 22:44:46 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> T.-W. Chen (NCU), S. J. Brennan (OKC), S. Ronchini (PSU), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. L. Thakur (INAF-IAPS), S. J. Smartt (Oxford/QUB), C.-S. Lin, C.-C. Ngeow, A. Aryan, C.-H. Lai, H.-Y. Miao (all NCU), I. Mandel (Monash), A. K. H. Kong (NTHU), S. Yang (HNAS), Morgan Fraser (UCD), S. Srivastav (Oxford), M. Nicholl, M. Fulton, T. Moore, D. R. Young, and K. W. Smith (QUB) report: We observed the field of the Swift/BAT-GUANO candidate counterpart (DeLaunay et al., GCN 38308) for the LIGO/Virgo/KAGRA S241125n (LIGO/Virgo/KAGRA, GCN 38305; GCN 38309; GCN 38312) using the 1m LOT and 40cm SLT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024 arXiv:2406.09270). The first SLT epoch of observations started at 18:55 UTC on 25th November 2024 (MJD 60639.788), 17.90 hr after the S241125n trigger. We utilized the astroalign (Beroiz et al., 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al., 2022, ApJ, 935, 167) packages to align and stack the individual frames. We do not find any new and uncataloged optical source in the stacked frames within the 5 arcmin 50% error circle of the Swift/BAT-GUANO localization, in comparison to the Pan-STARRS1 3Pi archive images (Chambers et al., 2016 arXiv:1612.05560). The LOT has a field of view of 13 x 13 arcmin and SLT's is 12.8 x 12.8 arcmin. Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on our stacked frames, after subtracting Pan-STARRS1 references images using SFFT (Lei Hu et al., 2022, ApJ, 936, 157). The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows: ---------------------------------------------------------------------------------------------------- Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 60639.786 | 17.85 | 300 * 6 | > 22.6 |0".929 | 1.75 SLT | i | 60639.788 | 17.90 | 300 * 10 | > 21.05 | 1".42 | 1.75 The presented magnitudes are calibrated using Pan-STARRS1 field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 1.24 mag and A_i = 0.93 mag in the direction of the transient (Schlafly & Finkbeiner 2011) taken by NED. View this GCN Circular online at https://gcn.nasa.gov/circulars/38314. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...