AstroNote 2025-24

We report the classification of the optical transient ATLAS24pgn / AT2024xuo (RA=10:08:49.736, Dec=-67:02:50.70, z=0.0047), first detected by ATLAS on October 11th 2024 with an apparent magnitude in o-band of 17.46+/-0.16 [2024-291]. Just a few days before, on October 6th 2024, there was a non-detection reported by GOTO (see Dyer et al. 2020, SPIE, 11445 2020SPIE11445E..7GD) up to 19.5 mag in the L-band, indicating that the ATLAS detection was recent. The source was promptly observed by the ePESSTO+ (Public ESO Spectroscopic Survey for Transient Objects) collaboration (Smartt et al. 2015, 2015A&A...579A..40S) on 2024 October 13, using the EFOSC2 instrument mounted on the ESO-NTT telescope) 2024-291 [2024-297]. The source showed a reddened continuum and narrow emission lines for H and for the Ca II triplet, as well as the [CaII] doublet in emission. The classification pointed to a possible Luminous Red Transient (LRT) event. At a distance of 23.5 Mpc (from NED, DM=31.8), the outburst reached -14.7 absolute magnitude (we use Ao=0.4 to correct for extinction) in the ATLAS o-band before fading beyond its detection limit a month later. 

After a two-month period of non-detections, the source has quickly rebrightened in the ATLAS data: from a 19.3 mag non-detection 5 sigma limit on January 9th it rose to a magnitude of 17.2 on January 11th 2025, and 16.1 mag on January 13th, implying a brightening of approximately 0.5 mag per day. This is consistent with a GOTO L band (400-700 nm) measurement of 16.0 mag one day later on January 14th. At the distance of the transient, this corresponds to an absolute magnitude of  ~ -16.1 mag in the o-band.

We obtained a classification spectrum with the low-resolution (R~175) Mookodi spectrograph and imager mounted on the 1 m Lesedi telescope located at the SAAO Sutherland site. The observation started at 20:54:32 UT on 2025 January 15 and consisted of a 900 s exposure. The spectrum shows strong Balmer emission lines, with an emission peak that is redshifted by ~400 km/s to the rest frame velocity for the reported host galaxy redshift of z=0.0047 obtained from NED. The H-alpha emission profile shows a broader base, with a narrower central emission. We also identify strong Na I doublet absorption and possible Fe II, Fe I, and He I narrow absorption lines in the spectrum, which could be attributed to absorption and recombination of radiatively excited circumstellar, slowly expanding gas.

Although some of the transient’s properties are shared with LRTs, based on our latest follow-up optical spectrum and the increasing absolute magnitude of this event (beyond the transitional LRT), we conclude that the most recent outburst is likely a IIn interacting supernova, where the fast ejecta is interacting with the slower circumstellar material.

We encourage further spectroscopic follow-up observations of this peculiar SN candidate.