AstroNote 2025-270

The transient AT 2025seuwas discovered by GOTO (GOTO25fkt, Kumar et al., TNS Astronomical Transient Report No. 263793) on 2025-07-26 00:58:58 UT at L-GOTO = 20.2 +/- 0.19. Detections were reported to TNS also by ATLAS (ATLAS25jgm, Tonry et al.) and ZTF (ZTF25abdegyy, Sollerman et al.). The first detection is the ZTF one on 2025-07-19 06:55:42 UT at g-ZTF = 20.5955 +/- 0.1158. 

AT 2025seu is located 0.83" S, 1.07" W from the nucleus of the red galaxy DESI J268.3778+61.7777 with a DESI DR1 public spectrum (TARGETID = 39633408455870113) that provides a spectroscopic redshift of z = 0.2544. Assuming AT 2025seu is at the redshift of this galaxy, the peak g-band absolute magnitude is of about Mg = -21 for a light curve peak of g-ZTF = 19.34 +/- 0.13 on 2025-08-18 08:59:34 UT. The ZTF g-r color is becoming red after maximum light.

The ALeRCE light classifier (Sánchez-Saéz et al. 2021, AJ, 161, 141) gives a classification of superluminous supernova (SLSN) with a large probability of 0.546. The large peak luminosity (if the transient is at the redshift of the likely host galaxy) and the ALeRCE light curve classification support that AT 2025seu is a superluminous supernova. Alternatively, AT 2025seu could be a gravitationally lensed supernova.

We encourage the spectroscopic classification.

Links to LSST/ZTF brokers:

Lasair, ALeRCE, ANTARES, and Fink.

Position on the Legacy Surveys Viewer.

Host galaxy DESI DR1 spectrum.