TITLE: GCN CIRCULAR NUMBER: 39252 SUBJECT: LIGO/Virgo/KAGRA S250206dm: Optical observations of candidate AT 2025bbo with Kinder DATE: 25/02/09 15:09:44 GMT FROM: Janet Chen at National Central University <janetstars@gmail.com> Y.-H. Lee, A. Aryan, T.-W. Chen, W.-J. Hou, H.-Y. Hsiao (all NCU), S. J. Smartt (Oxford/QUB), A. K. H. Kong (NTHU), J. Gillanders (Oxford), A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, Y. J. Yang, C.-H. Lai, C.-S. Lin, H.-C. Lin, 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: We observed the field of the optical transient AT 2025bbo (Smartt et al., GCN 39244) in the skymap of the NSBH merger event S250206dm (The LIGO-Virgo-Kagra Collaboration, GCN 39175) from the Pan-STARRS search described in Young et al. (GCN 39210). We used the 1m-LOT at Lulin observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024arXiv240609270C). The first epoch of 1m-LOT observations started at 11:06:23 UTC on 9th of February 2025. We employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the template subtraction using "sfft" (Hu, 2022, ApJ, 936, 157). We utilized the template image from PanSTARRS1 (Chambers et al. 2016 arXiv:1612.05560). In the difference image, we see a clear signature of the transient. We further utilized the AutoPhOT package to perform PSF photometry. The details of the observations and measured PSF magnitude from the differenced image (in the AB system) of AT 2025bbo are as follows: Telescope | Filter | MJD | Exposure | Magnitude | avg. Seeing | med. Airmass LOT | r | 60715.463 | 300 sec * 6 | 20.08 +/- 0.05 | 1".61 | 1.39 The presented magnitudes are calibrated using the field stars from the Pan-STARRS1 catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.24 mag in the direction of the burst (Schlafly & Finkbeiner 2011). Our photometry indicates that AT 2025bbo is brightening at a rate of approximately 0.3 mag per day, compared to the measurements reported by Smartt et al. (GCN 39244) and Liu et al. (GCN 39249). View this GCN Circular online at https://gcn.nasa.gov/circulars/39252. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...