AT 2024ahzi: A Type IIP Supernova Discovered by the LSST Commissioning Camera

Abstract

As part of its commissioning, the Vera C. Rubin Observatory observed several fields repeatedly for a month with ComCam, an instrument that uses the same hardware as the LSST camera but covers a smaller field of view. We photometrically classify AT 2024ahzi, a transient discovered by ComCam, as a Type IIP supernova (SN IIP) using both ComCam and DECam photometry. We find that the duration, luminosity, and color of AT 2024ahzi's photometric plateau are all consistent with those from a large sample of SNe II. By comparing its multi-band light curves to SN II models and analytic relations, we place constraints on the SN progenitor, explosion dynamics, and circumstellar environment. We argue that the progenitor has an extended density profile indistinguishable from a slowly accelerating CSM. We discuss how a similar workflow can identify and characterize future Rubin SNe II.

Figures (8)

• Figure 1: Left: ComCam exposure map showing the location of AT 2024ahzi and a single DECam pointing overlaid. Grey rectangles represent single DECam CCDs. Right: A ComCam $gri$ color-composite image of the field surrounding AT 2024ahzi. The SN is circled and the crosshairs show the location of the host, which has a spectroscopic redshift $z=0.211\pm0.002$.
• Figure 2: Subset of nightly co-added observations of AT 2024ahzi, uncorrected for Milky Way or host extinction. We include the first $5\sigma$ detection preceded by the last two non-detections. All uncertainties are $1\sigma$. The full single-exposure and nightly co-added photometry are available as machine readable tables online.
• Figure 3: Left: Comparison of ComCam $griz$-band difference-image photometry of AT 2024ahzi as queried directly from RSP (i.e., using the ComCam templates; circles), versus corrected using robust DES templates (squares). The DECam difference-image photometry at early times is also included (triangles) to highlight the improved continuity between instruments when using the DES-subtracted ComCam photometry. Right: The combined AT 2024ahzi light curves in magnitude space. Translucent points represent $3$-sigma upper limits of non-detections (SNR $\leq 3$). For clarity, only the DES-subtracted ComCam photometry is shown.
• Figure 4: Observed photometry (colored circles) and spectrum (gold line) obtained of AT 2024ahzi's host galaxy, compared to the Prospector model spectrum (purple line) and corresponding photometry (black squares). The model suggests that the host is actively star-forming, consistent with the blue observed color of the host in the composite image.
• Figure 5: Left: Superphot+ fit to AT 2024ahzi used for initial classification. Both the redshift-inclusive and redshift-agnostic classifiers assign a SN II label to the best-fit parameter set, with confidences of 98% and 92%, respectively. Right: SN IIP-specific Superphot+ fit to AT 2024ahzi. The flat plateau from hydrogen recombination is differentiated from the steeper dimming and cooling region following shock-breakout, possibly lengthened by an extended CSM. We define "plateau duration" as the time from explosion to where the flux drops halfway after hydrogen recombination, as bounded by the vertical gray dashed lines. We measure the plateau magnitude and color from midway along the hydrogen recombination region (dotted yellow line)