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

1. Problem Statement

The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) is poised to detect hundreds of thousands of core-collapse supernovae (CCSNe) annually, offering a statistical leap in understanding stellar evolution. However, two primary challenges exist in processing this data:

1. Classification at High Redshift: Traditional classification relies on spectroscopy, which is resource-intensive. Photometric classification is necessary but difficult at higher redshifts ($z \sim 0.2$) where filter shifts (K-corrections) and limited bandpasses (e.g., lacking near-UV) can lead to biased temperature and radius estimates.
2. The "Missing Red Supergiant" Problem: There is a discrepancy between the observed upper mass limit of Red Supergiant (RSG) progenitors of Type II supernovae ($\sim 17 M_\odot$) and theoretical stellar evolution models predicting explosions up to $25-30 M_\odot$. Large, high-redshift samples are needed to resolve whether this is due to failed supernovae, observational biases, or small-number statistics.
3. Circumstellar Matter (CSM) Diversity: The prevalence and nature of CSM around Type II supernovae (whether they form a continuum between Type IIP and IIL or distinct classes) remain open questions, particularly regarding how CSM interaction affects early-phase light curves.

2. Methodology

The authors present a comprehensive workflow for identifying, classifying, and characterizing a transient discovered during the LSST commissioning phase using the ComCam (Commissioning Camera) and DECam (Dark Energy Camera).

• Data Acquisition & Cross-Matching:

  • Discovery: AT 2024ahzi was detected by ComCam (using LSST camera hardware but a smaller field of view) during its commissioning run (Nov–Dec 2024).
  • Follow-up: The Young Supernova Experiment (YSE) used DECam to observe the same fields, providing a longer baseline and deeper coverage of the supernova's plateau phase.
  • Template Subtraction: To improve ComCam photometry, the authors replaced the standard ComCam difference-imaging templates (which contained the SN signal) with deep, pre-explosion DES DR2 (Dark Energy Survey) templates. This was performed using the SLIDE package.
  • Host Association: The transient was associated with a host galaxy (WISEA J035320.41-484500.0) using the Bayesian code Pröbst. Spectroscopic follow-up with Magellan/LDSS-3 confirmed the host redshift ($z = 0.211 \pm 0.002$).

• Photometric Classification:

  • The authors used Superphot+, a parametric light curve fitting tool, to classify the transient.
  • They fitted the multi-band light curves to both a general SN model and a specific Type IIP model (incorporating shock cooling, recombination plateau, and radioactive decay tails).
  • Classification was performed using classifiers trained on ZTF data, with robustness checks applied for K-corrections and host extinction.

• Physical Characterization:

  • Blackbody Evolution: The code extrabol was used to fit the multi-band photometry to an evolving blackbody to derive effective temperature ($T_{\text{eff}}$) and photospheric radius ($R_{\text{phot}}$).
  • Progenitor Constraints: Analytic scaling relations (Goldberg et al. 2019) linking plateau luminosity and duration to progenitor mass, radius, and explosion energy were applied.
  • CSM Modeling: The early-phase light curve was fitted against a grid of synthetic SN II models (Moriya et al. 2023) generated with the STELLA hydrodynamics code. These models included variable CSM density profiles parameterized by wind acceleration ($\beta$) and mass-loss rates ($\dot{M}$).
  • Host Analysis: Prospector was used to fit the host galaxy's spectral energy distribution (SED) and spectrum to determine stellar mass, star formation rate (SFR), and dust extinction ($A_V$).

3. Key Contributions

• First Rubin Photometric Classification: AT 2024ahzi is the first supernova photometrically classified as a Type IIP following detection by a Rubin Observatory instrument (ComCam).
• Improved Template Subtraction: The paper demonstrates that using deep DES templates for ComCam difference imaging significantly reduces noise and corrects flux jumps between instruments, enabling robust early-phase characterization.
• Workflow for Future LSST: The authors outline a complete pipeline for future Rubin SNe II: cross-matching with external surveys (like DECam/ZTF) for full light curves, using deep templates for subtraction, and applying probabilistic host association to estimate redshift without immediate spectroscopy.
• CSM vs. Extended Envelope Analysis: The study provides a nuanced discussion on distinguishing between a true CSM interaction and an extended, static progenitor envelope, noting that slow wind acceleration can mimic an extended density profile.

4. Results

• Classification: AT 2024ahzi is classified as a Type IIP supernova with $>99\%$ calibrated confidence. The classification remains robust even when accounting for redshift effects and host extinction.
• Light Curve Properties:
  • Plateau Duration: $103.4 \pm 7.3$ days.
  • Mid-Plateau Absolute Magnitude: $M_r \approx -17.39$ mag.
  • Temperature: The photosphere cools to $\sim 6000$ K during the plateau, consistent with hydrogen recombination, ruling out a Type IIn classification.
• Progenitor & Explosion Parameters:
  • Based on analytic scalings, the progenitor likely has a Zero-Age Main Sequence (ZAMS) mass of 12–14 $M_\odot$.
  • Explosion energy is estimated at $\sim 10^{51}$ erg.
  • The H-rich ejecta mass is constrained to $7.8 - 8.1 M_\odot$.
• CSM Characterization:
  • The best-fit models require a CSM density profile with slow wind acceleration ($\beta \approx 4.5$) and a moderate mass-loss rate ($\dot{M} \approx 10^{-2} M_\odot/\text{yr}$).
  • The derived CSM mass is high ($0.7 - 2.1 M_\odot$), typical of Type IIn SNe. However, the authors argue this may be an overestimation if the "CSM" is actually an extended, hydrostatic progenitor envelope.
  • When calculating mass only above the photosphere ($R_{\text{phot}} \approx 1300 R_\odot$), the mass drops to $0.4 M_\odot$, which is more consistent with standard Type IIP progenitors.
• Host Galaxy: The host is a star-forming spiral galaxy with a stellar mass of $\log(M_*/M_\odot) \approx 9.86$ and a specific SFR consistent with the "main sequence" of star-forming galaxies.

5. Significance

• Validation of Rubin's Capabilities: This work proves that the Rubin Observatory, even during commissioning with a smaller camera, can detect and characterize high-redshift supernovae when combined with complementary surveys.
• Solving the "Missing RSG" Problem: By establishing a pipeline to classify SNe II photometrically at $z \sim 0.2$, Rubin will generate a sample large enough to statistically probe the high-mass tail of the progenitor distribution, potentially resolving the missing red supergiant problem.
• CSM Physics: The results highlight the degeneracy between extended stellar envelopes and CSM. The finding that slow wind acceleration mimics an extended profile suggests that "CSM" estimates in Type IIP SNe may often be tracing the outer layers of the progenitor star itself rather than detached shells.
• Future Strategy: The paper advocates for a multi-instrument approach (Rubin + DECam/ZTF/LS4) to capture full light curves, which is essential for accurate temperature/radius derivation and distinguishing between different SN II subclasses.

In summary, AT 2024ahzi serves as a "proof of concept" for the Rubin Observatory's future supernova science, demonstrating that photometric classification and progenitor characterization are feasible for high-redshift events, paving the way for a million-transient era in time-domain astronomy.