Spectroscopic Follow-up Observation of WNTR23bzdiq = TCP J00384865+4046079 = AT 2025abao: a Luminous Red Nova in M31?

ATel #17468; Kenta Taguchi, Keiichi Maeda (Kyoto University)
on 2 Nov 2025; 14:42 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)

Tweet

We report the spectroscopic follow-up observation of WNTR23bzdiq = TCP J00384865+4046079 = AT 2025abao. We obtained its spectra by the fiber-fed integral field spectrograph (KOOLS-IFU; Matsubayashi et al. 2025, 2019) mounted on the 3.8 m Seimei telescope (Kurita et al. 2020) at Okayama Observatory of Kyoto University on 2025-10-29.61 and 2025-10-31.72 UT using the VPH-blue grism (covering 4100 - 8500 Å, resolution ∼ 500).

As presented by Karambelkar et al. (2025), this transient was discovered by WINTER in the J band on 2023-12-17. As reported to CBAT, this object has been brightening in the optical, discovered at 17.8 mag (unfiltered) on 2025-10-17.4798 UT by Koichi Itagaki (also independently discovered by Robert Fidrich on 2025-10-18.8112 and MASTER on 2025-10-19.8196 UT). Our spectra taken by Seimei/KOOLS-IFU are dominated by a blue continuum. The emission line of Hα is prominently seen in both epochs (with Lorentzian FWHM ∼ 630 and 700 km/s on 2025-10-29.61 and 2025-10-31.72, respectively), while Hβ is only prominent on 2025-10-29.61 but has significantly weakened on 2025-10-31.72. The absorption line of Na_I D is clearly seen in both epochs, suggesting that this object suffers from extinction. The evolution is similar to that of the luminous red nova AT 2019zhd, reported by Pastorello et al. (2021). From this similarity and the luminosity of the optical event, we propose that this event is likely a luminous red nova in M31, consistent with the common-envelope transient scenario of WNTR23bzdiq proposed by Karambelkar et al. (2025). Further follow-up observation of this event is strongly encouraged.

The spectra