AstroNote 2026-238

LSST released Early Data Preview 2 (EDP2) on 2026 July 27. EDP2 includes deep stacks from images obtained with the LSST Camera between April 2025 and January 2026. To demonstrate LSST's potential to systematically investigate rare transient phenomena, including supernova precursors and late-time light-curve flattening or rebrightening, we performed image subtraction between the EDP2 coadds and archival DECam images using the SLIDE package (Dong et al. 2025ApJ...994L...8D). 

We identified 8 candidate precursors associated with SNe 2024lwa, 2025uue, 2026muw, 2025aawk, 2026gzf, 2026ejf, 2026fgl, and TDE 2026pag. Precursors associated with SNe 2024lwa (#2026-206) and 2026gzf (Chen et al., 2026) were also detected in Pan-STARRS data and were previously reported publicly. In addition, we identified 13 transients exhibiting unusually long-lived emission, late-time light-curve flattening, or rebrightening: SNe 2017hcc, 2019iid, 2019mav, 2020izc, 2020jfv, 2020usa, 2020zpf, 2021zon, 2022pul, 2022rvy, and 2023swf, as well as TDEs 2020zso and 2021qth. These objects span several transient classes, including Types Ia, II, IIn, and IIb supernovae, SLSNe, and TDEs. Some events, including SNe 2017hcc and 2020jfv, may still be sufficiently bright for spectroscopic observations, and we encourage further follow-up. The results are summarized in the table below. 

As part of this search, we also recovered 154 additional transients whose expected active phases overlap the time span covered by the EDP2 deep stacks. As reported in #2026-53, SN 2024kce was detected by LSST at late times and is present in the LSST template images. We clearly detected SN 2024kce in the EDP2 deep stacks and measured g=22.01±0.01, r=22.07±0.01, and i=23.37±0.04 mag at around MJD 60798, consistent with the estimates reported in #2026-53.

Although we cannot fully exclude contributions from nuclear activity in some cases, these findings demonstrate LSST's potential to study transients both before explosion and at very late phases. Such observations will provide new constraints on the mass-loss histories and final-stage activity of massive stars.

Transient precursor candidates:

* The reference MJD is either the discovery date reported to TNS or the approximate explosion epoch.

Late-time detections of supernovae and TDEs:

*The reference MJD is either the discovery date reported to TNS or the approximate explosion epoch.