Continued fading and photometric monitoring of AT2026cex

ATel #17669; M. S. Bisht, K. Belwal, A. Raj, D. Bisht, S. K. Chakrabarti, D. Bhowmick (Indian Centre for Space Physics)
on 11 Feb 2026; 08:32 UT
Credential Certification: Ashish Raj (ashishpink@gmail.com)

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AT2026cex was first reported as a candidate luminous fast blue optical transient (LFBOT) by Tsuboi, Nakaoka & Kawabata (ATel #17646) based on its rapid brightening and featureless blue spectrum. Subsequent optical and near-infrared follow-up observations revealed continued fading and blue colors (Gill et al. ATel #17647; Pereyra et al. ATel #17648; Schneider et al. ATel #17649; Ma et al. ATel #17652; Garton & Steele ATel #17655; Lin et al. ATel #17656; Perez-Fournon et al. ATel #17660), while spectroscopy showed a nearly featureless blue continuum similar to AT2018cow (Schneider et al. ATel #17649). Precise astrometry and archival survey data demonstrated a positional coincidence at the 0.1 arcsec level with the Gaia source 866940901044573184, suggesting that the transient is likely an outburst of a Gaia star and not an LFBOT associated with NGC 2398. No X-ray emission was detected by the Einstein Probe, with a 0.5-10 keV flux upper limit of ~ 3e-14 erg/cm^2/s (Sun et al. ATel #17651). More recently, quasi-periodic optical modulation with a characteristic timescale of P ~ 1.35 hr was reported from COLIBRI and SVOM/C-GFT observations, with statistically significant detections on four consecutive nights (false-alarm probability < 1e-3), further supporting an accreting binary interpretation (Pereyra et al. ATel #17662).

Additional photometric data for the present study were obtained with the 0.6-m VASISTHA reflecting telescope at the Ionospheric and Earthquake Research Centre and Optical Observatory (IERCOO), operated by the Indian Centre for Space Physics (ICSP), equipped with an Atik 460EX Mono CCD camera. We measured V = 16.98 +/- 0.06 (JD 2461081.26) and Rc = 17.14 +/- 0.05 (JD 2461081.15) on 2026 February 9, and V = 17.22 +/- 0.09 (JD 2461082.08) and Rc = 17.31 +/- 0.08 (JD 2461082.06) on 2026 February 10, confirming the continued fading of the source. Taken together, the positional coincidence with a Gaia counterpart and the statistically significant ~ 1.35 hr quasi-periodic modulation suggest that AT2026cex is an accreting binary system (Pereyra et al., ATel #17662). We strongly encourage continued spectroscopic follow-up observations to confirm the nature of AT2026cex.