AstroNote 2026-21

The BHTOM.space network coordinated an observing campaign to monitor AT2025abao, reported on October 20, 2025, by the MASTER network (https://www.wis-tns.org/object/2025abao/discovery-cert). The event was spotted in the direction of the Andromeda constellation within the Messier 31 spiral galaxy.

Observations commenced on November 3, 2025, approximately two weeks after the discovery by MASTER on October 20, 2025, and are still ongoing. During this period, almost 800 measurements were obtained using a variety of telescopes within the BHTOM network (including LCOGT 1-m and 40-cm telescopes). Photometric data were taken in both Sloan- and Johnson-like filters and subsequently transformed into Gaia Synthetic Photometry (GaiaSP) for consistency.

The AT2025abao event was also observed by a number of sky surveys: ZTF (and PTF), ATLAS, Gaia, GALEX, 2MASS, WISE (All-WISE), Pan-STARRS. Also ultravioltet observations from Swift UVOT are present in the lightcurve available through BHTOM.space.

The multi-band light curve in BHTOM shows a steady rise in near-infrared and red bands (r/R/i/I), no change in g/V bands and a decline in blue bands (B/u/U). This trend abruptly changed around 2026/01/17, and the lightcurve is declining steadily in all filters. 

The event is now confirmed to be a red nova by several sources, both professional: https://www.wis-tns.org/object/2025abao and popular: https://www.stsci.de/en/observations/at2025abao-a-red-nova-in-our-neighboring-galaxy/,https://skyandtelescope.org/online-gallery/luminous-red-nova-at2025abao.

Observations of the object with BHTOM will continue until it disappears in the background. Further photometric observations with the use of larger telescopes are advised, as well as spectra taken during different phases of dimming. Detailed light curves and associated statistics are presented on the webpage: https://bhtom.space/public/targets/AT2025abao. Further details and the full photometric dataset are available for download by registered users of the BHTOM.space platform. 

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).