AstroNote 2026-268

AT 2026aabo was discovered by Anirudh Salgundi on behalf of the Zwicky Transient Facility (ZTF; TNS Astronomical Transient Report No. 321735). The first ZTF detection was obtained on 2026-08-29 at 09:28:57 UT, at a magnitude of g-ZTF = 18.578 ± 0.100 mag. Subsequent ATLAS forced photometry showed a strong and fast brightening of the transient (ATel #18027). Independent GROWTH-India Telescope (GIT) observations reported in ATel #18029 showed that the transient reached g = 15.3 ± 0.01 mag on 2026-08-31T23:21:19, corresponding to an increase of more than 3.3 mag in less than three days.

Spectroscopic observations reported in ATel #18029 classified AT 2026aabo as a very bright nova in M31. Two spectra obtained with the Himalayan Chandra Telescope on 2026-09-01 and 2026-09-02 showed prominent Fe II multiplets and P-Cygni Balmer features, leading to its classification as a nova in the Fe II phase.

Another spectrum of AT 2026aabo (TNS spectrum ID 24617) taken on 2026-09-02T00:37:33 UT with the low-resolution CAFOS instrument mounted on the 2.2 m telescope at Calar Alto Observatory showed narrow Fe II, Ti II, and H absorption lines with blueshifted profiles. The absorption velocity was approximately −800 km s⁻¹ after correction for the instrumental resolution of CAFOS (~600 km s⁻¹). These narrow absorption features were interpreted as being consistent with the spectroscopic properties expected for an early luminous red nova (LRN) candidate, and the transient was then classified as an early LRN candidate.

The new spectrum reported here was obtained on 2026-09-05 at 01:26:38 UT using the CAFOS spectrograph mounted on the 2.2 m telescope at Calar Alto Observatory. Two 1200 s exposures were taken using the G-200 grism and a 1.5 arcsec slit. The data were reduced using a custom-developed CAFOS pipeline, and flux calibration was performed using the spectrophotometric standard star BD+33_2642.

The resulting spectrum shows prominent Fe II and O I lines, as well as broad hydrogen emission lines. The Hα absorption component reaches a maximum velocity of approximately 2000 km s⁻¹. These spectral characteristics are consistent with those observed in Fe II novae (e.g. Shafter et al. 2011). Our observations provide additional spectroscopic confirmation that AT 2026aabo is an Fe II nova in M31.