AstroNote 2026-53

We report the detection of the Ca-rich SN Ia-pec SN 2024kce in the early LSST alerts (Babamul link: https://babamul.caltech.edu/objects/LSST/170028529317052506; LSST id: LSST-AP-D0-170028529317052506). The source was identified through a dedicated filter operating within the Babamul event broker and BOOM software infrastructure (du Laz et al., 2025), designed to discover late-time and rebrightening supernovae in LSST alerts by crossmatching with SNe from ZTF, PTF, iPTF, and older events cataloged on TNS. 

The science goal is to obtain late-time light curves of archival SNe and assess their consistency with radioactive decay, while identifying cases that deviate from this expectation through rebrightening or flattening driven by circumstellar interaction, compact-remnant power, or fallback accretion, and flagging them for spectroscopic follow-up.

The last public ZTF alert detection in the r band occurred approximately 475 days earlier (MJD 60622) at ZTF r ≈ 19.7 mag. For the LSST detection associated with SN 2024kce, the alert photometry reports g ≈ 22.0 mag, i ≈ 22.1 mag, and r ≈ 23.3 mag at MJD 61098 (∼640 days since explosion); these magnitudes correspond to the absolute values of negative difference-flux measurements. Given the non-detection in the recent science images, the measured brightness reflects the reference-image epoch. The LSST reference-image epoch is not explicitly available in the alert header; assuming the template was constructed sometime between July and November 2025, this apparent brightness is significantly higher than a simple extrapolation of the SN 2024kce late-time tail using a fitted decline slope of 0.021 mag day^-1. This underscores the need for reference-image epoch metadata and for timely access to LSST prompt imaging data products to enable robust early science on rebrightening supernovae.

SN 2024kce (ZTF internal name ZTF24aaqaroi) was discovered by ZTF on 2024-06-02 04:41:20.99 UTC (2024TNSTR1768....1S). It was later classified as a peculiar SN Ia based on its spectral properties (2024TNSCR1809....1T). We subsequently obtained a spectrum with the Next Generation Palomar Spectrograph (NGPS; 2024TNSAN.340....1K) during commissioning on 2024-12-02, which shows strong forbidden [Ca II] doublet λλ7291, 7324 Å emission, supporting the Ca-rich interpretation and motivating continued investigation of the late-time power mechanism.

The Babamul alerts broker and BOOM software infrastructure (du Laz et al. 2026) is co-developed by the California Institute of Technology and the University of Minnesota. This work acknowledges support from the National Science Foundation through AST Award No. 2432476 (PI Kasliwal; co-PI Coughlin) and leverages experience from the Zwicky Transient Facility (co-PIs Graham and Kasliwal).