AstroNote 2026-27

The BHTOM.space network coordinated an observing campaign to monitor the SN2025ahsa (=ZTF25acjpvvr) event discovered on December 20, 2025, by ZTF and reported here: https://www.wis-tns.org/object/2025ahsa and identified autonomously by BTSbot (Rehemtulla et al. 2024; arXiv:2401.15167).

Follow-up photometric observations commenced on December 24, 2025, less than a week after initial discovery and are still ongoing. During this period, approximately 200 measurements (PSF photometry) were obtained using a 14-inch reflector of the ROAD Observatory (Atacama, Chile). Photometric data were taken in B, V & I filters and subsequently transformed into Gaia Synthetic Photometry (GaiaSP) for consistency.

The multi-band light curve in BHTOM shows a steady decline in brightness in the B and V bands for the entire period of observations. Still, there is a significant rise in the I band beginning sometime in mid-January and lasting for about three weeks. The light curve is consistent with a Type-Ia Supernova explosion as classified previously by C. Balcon here: https://www.wis-tns.org/object/2025ahsa/discovery-cert .

The event was also observed by the ATLAS & ZTF sky surveys. There are some archival measurements of the host galaxy from PTF, GALEX & WISE surveys. Detailed light curves and associated statistics are presented on the webpage: https://bhtom.space/public/targets/SN2025ahsa. Further details and the full photometric dataset are available for download by registered users of the BHTOM.space platform.

Observations of the object with BHTOM will continue until it disappears in the background. Follow-up observations with the use of bigger telescopes are strongly encouraged. 

Acknowledgements: BHTOM.space is based on the open-source TOM Toolkit by LCO and has been supported by the European Union's research and innovation programmes under grant agreements No 101004719 (OPTICON-RadioNet Pilot, ORP) and 101131928 (ACME).