AstroNote 2026-271

LSST Early Data Preview 2 (EDP2) coadds differenced against archival DECam images with the SLIDE package (Dong et el. 2025ApJ...994L...8D), have revealed transients with unusually long-lived emission, late-time flattening or rebrightening (Dong et al. 2026, Astronote 2026-238). Here we report Magellan/LDSS3 spectra of two objects from this sample, SN 2017hcc and SN 2023xbg, obtained on 2026 September 5 UT, which show that both are still powered by circumstellar interaction.

Spectra were obtained with the Low Dispersion Survey Spectrograph 3 (LDSS3) on the 6.5-m Magellan Clay telescope through program 2866 (PI: Das) in long-slit mode (VPH-All grism, 1.0" slit, ~3900–9800 Å, R ≈ 700–1000, seeing ~0.8"): 3 × 1200s for SN 2017hcc and 4 × 1200s for SN 2023xbg. We are grateful to Yuri Beletsky and the Magellan staff for carrying out the observations. The data were reduced with a version of PypeIt adapted for LDSS3.

1. SN 2017hcc: It is still interaction-dominated ~8.8 years after discovery. The spectrum shows a broad, asymmetric Hα profile whose emission extends to ≈ −4000 km/s on the blue side and ≈ +3000 km/s on the red. The [O I] λλ6300, 6364 and [Ca II] λλ7291, 7324 doublets are strongly blueshifted, with centroids at ≈ −2100 and ≈ −1300 km/s. The blueshifted [O I] and [Ca II] emission from the inner ejecta could be explained by dust attenuation. 

2. SN 2023xbg: Its spectrum at ~1000 days shows strong Hα at ~15,000 km/s, along with surprisingly strong He I lines at ~4000 km/s. Weaker emission from the Ca II near-infrared triplet and the [Ca II] λλ7291, 7324 doublet is present, while [O I] λλ6300, 6364 is not detected. The broad, luminous Hα together with the strong He I emission indicates that the spectrum at ~1000 days is likely powered by interaction with hydrogen- and helium-rich circumstellar material.