AstroNote 2026-86

The BHTOM.space telescope network facilitated a dedicated observing campaign to track the event AT2026cex. AT2026cex was discovered by Tsuboi et al. 2026 (ATeL #17646) and flagged as a luminous fast blue optical transient (LFBOT) using the optical spectrum obtained with HOWPol on the 1.5-m Kanata Telescope at Higashi-Hiroshima Observatory on Feb 3, 2026, based on rapid brightening and featureless blue spectrum. Gill et al. 2026 (ATeL #17647) assumed that it is associated with a nearby galaxy NGC 2398, with a redshift of about 0.03, and their follow-up by the COLIBRI telescope suggested that it is consistent with the ones observed for AT2018cow, the prototype FBOT. The event was also observed by the ATLAS sky survey, ZTF, and 1 epoch observation by Swift UVOT.

Photometric follow-up by the BHTOM.space network commenced on Feb 6, 2026, just three days after initial discovery, and is still ongoing. During this period, approximately 736 measurements (PSF photometry) were obtained using 1-m LCO Teide Observatory (Tenerife, Spain), 1-m LCO McDonald Observatory (Davis Mountains of West Texas, USA), 1-m LCO Cerro Tololo Interamerican Observatory (Chile), 1-m LCO Siding Spring Observatory (Coonabarabran, Australia), 50-cm Corrected Dall-Kirkham telescope of the Astronomical Observatory of the Jagiellonian University (Kraków, Poland), 50-cm University of Zielona Góra Observatory (Chile), 30-cm Silesian University of Technology Observatories (Granada, Spain), 36-cm Celestron C14 Telescope UCL Observatory (London, UK), 68-cm Horten Telescope (Norway) and 60-cm RC telescope of Fan Mountains Observatory (Virginia, USA). Photometric observations were taken in g, r, i, z, B, V, I & R filters and subsequently transformed into Gaia Synthetic Photometry (GaiaSP) for consistency.

The BHTOM light curve exhibits a rapid decline in brightness for the entire period of observations. The inferred rise timescale is short, and the peak brightness suggests a highly energetic event. The light curve shows evidence for a secondary re-brightening (“bump”) around Feb 20, 2026. AT2026cexcoincides with Gaia DR3 866940901044573184, which is a star suggesting an outburst of that star and not an LFBOT associated with NGC 2398 (Perez-Fournon et al. 2026, ATeL #17660; Bisht et al. 2026, ATeL #17669). Further photometric monitoring will help to better understand its behavior.

Detailed light curves and associated statistics are presented on the webpage: https://bhtom.space/public/targets/AT2026cex. 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).