TITLE: GCN CIRCULAR NUMBER: 42825 SUBJECT: LIGO/Virgo/KAGRA S251112cm: AT 2025adht Kinder follow-up observations DATE: 25/11/24 17:22:31 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> J. H. Gillanders (Oxford), A. Aryan, T.-W. Chen, Y.-H. Lee, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt (Oxford), Y. J. Yang (NYUAD), A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, C.-H. Lai, W.-J. Hou, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report: Here we report follow-up observations of the candidate optical counterpart AT 2025adht (first reported by Franz et al., GCN 42675; see also Anand et al., GCN 42677; Burkhonov et al., GCN 42798) to the LIGO-Virgo-KAGRA (LVK) gravitational-wave event S251112cm (LVK Collaboration, GCN 42650). We observed AT 2025adht utilizing the 1m LOT at Lulin Observatory in Taiwan, as part of the Kinder collaboration (Chen & Yang et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations began at 20:31 UTC on November 21, 2025 (MJD 61000.855), 9.225 days after the GW trigger. The second LOT epoch began at 20:16 UTC, or MJD 61001.845, 10.207 days after the GW 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 utilized the Python-based package AutoPhOT (Brennan & Fraser 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 images using the 'hotpants' (Becker 2015, ascl.soft. ascl:1504.004) algorithm. Moreover, we further utilized AutoPhOT to perform PSF photometry. The details of the observations and the measured magnitudes (in the AB system) are as follows: Telescope | Filter | MJD (start) | t - t0 (d) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass LOT | r | 61000.855 | 9.225 | 300 * 6 | 21.83 +/- 0.06 | 1".2 | 1.41 LOT | r | 61001.845 | 10.207 | 300 * 6 | 21.86 +/- 0.08 | 1".2 | 1.46 Burkhonov et al. (GCN 42698) report a (PS1-equivalent) r-band magnitude of 21.64 +/- 0.1 on MJD 60994.009 (2.37 days post-GW trigger). Our observations indicate that AT 2025adht has faded only slightly in the ~week since detection. The slow evolution indicates that AT 2025adht is not some rapidly declining kilonova-like transient; we instead suspect that AT 2025adht is a background supernova near peak, hence the slow evolution. This interpretation is supported by the Legacy Survey's host galaxy phot-z estimate (0.139 +/- 0.033; see also Anand et al., GCN 42677). The presented magnitudes are calibrated using field stars from the ATLAS-RefCat2 catalog from MAST (Tonry et al. 2018, ApJ, 867, 105) and are not corrected for the expected Galactic foreground extinction of A_r = 0.06 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology and details on the capabilities of Lulin observatory telescopes can be found in Aryan et al. 2025, ApJS, 281, 20. doi:10.3847/1538-4365/adfc69. View this GCN Circular online at https://gcn.nasa.gov/circulars/42825. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...