AstroNote 2026-115

AT2026brq was first reported by the Pan-STARRS survey on UT 2026-01-29 22:42:12 at r=22.04 mag. This source was independently detected by the Vera Rubin Observatory’s Legacy Survey of Space and Time (LSST, diaObjectid=313888627043074080), with several detections from December 2025 to April 2026 that reveal a reddening of the source from g - r ~ 0 mag  to  g - r ~ 1.5 mag over ~100 days. The source is spatially coincident with the galaxy 2MASXJ10044169B0246137, which has a confirmed spectroscopic redshift from the Dark Energy Spectroscopy Instrument (DESI) of z=0.046. This corresponds to a peak absolute magnitude in the r-band M_r ~ -14.5, indicating a low-luminosity transient akin to subluminous Type II supernovae and LRN/ILRT supernova impostors.

We flagged this transient as part of a dedicated effort to track subluminous, red transients in LSST using a filter set up with the Babamul alert brokering system (du Laz et al. 2026). We obtained a spectrum of this source with the Low Resolution Imaging Spectrograph (LRIS, Oke et al. 1995) on the Keck I telescope beginning UT 2026-04-24 06:58:00, as part of our program to follow-up transients with late-time IR excess. Our observations comprised a total exposure time of 40 min, with the D560 dichroic mirror, 400/8500 grating, and 400/3400 grism -- covering 3500-10000 A with the spectral resolution of 1000. Data reduction was performed with PypeIt.

The spectrum shows clear features of Hα and the Ca II NIR triplet with P-Cygni profiles, with low velocities of ~ 3000 km/s. Cross-matching to supernova templates using SNID gives good matches to Type IIp supernovae ~130 days post-peak. We therefore classify AT2026brq as a low luminosity Type IIp supernova.