Red supergiant forensics: LST-1's Search for High-Energy Emission from SN 2024bch

In this episode, we dive into the latest findings on **Supernova (SN) 2024bch**, a spectacular stellar death event observed in the nearby galaxy NGC 3206 ($\sim 20$ Mpc). Scientists conducted a multiwavelength analysis, combining **Very High-Energy (VHE) gamma-ray observations** with optical photometry and spectroscopy.

**Key Findings:**

* **Classification:** SN 2024bch is classified as a potential **Type IIn-L supernova**. This type of core-collapse supernova (CCSNe) exhibits a fast linear decay in its light curve, similar to Type II-L SNe, but shows early-time spectral features indicating interaction with a dense circumstellar medium (CSM) (Type IIn-like).

* **The Progenitor:** The data strongly suggest that the progenitor star was consistent with a **Red Supergiant (RSG)**. The progenitor parameters derived from optical modeling and pre-explosion images fall within the typical range for RSGs: mass $M_{pr} = 11 – 20 M_{\odot}$, radius $R_{pr} = 531 \pm 125 R_{\odot}$, luminosity $L_{pr} \le 10^{4.82} L_{\odot}$, and temperature $T_{pr} \le 4000 \text{ K}$.

* **The Gamma-Ray Search:** VHE observations were carried out using the **LST-1** (Large-Sized Telescope 1) prototype of the Cherenkov Telescope Array Observatory (CTAO). No significant VHE gamma-ray emission was detected above $100 \text{ GeV}$.

* **Setting Limits:** Researchers calculated an integral upper limit on the photon flux of **$F\gamma(> 100 \text{ GeV}) \le 3.61 \times 10^{-12} \text{ cm}^{-2} \text{ s}^{-1}$**. This measurement is significant because it represents the **first ever determined gamma-flux upper limit for a SN of the IIn-L class**, and the first CTAO LST-1 observation of a CCSN with such a low energy threshold.

* **Mass-Loss Constraints:** The non-detection allowed researchers to place an upper limit on the mass-loss-rate to wind-velocity ratio ($\dot{M}/u_w \le 10^{-4} M_{\odot} \frac{\text{ yr } \text{ s}}{\text{ km}}$). However, the constraints are subject to uncertainty due to **gamma-gamma absorption**, a process where VHE gamma rays are attenuated by optical photons from the supernova photosphere, especially at early times.

**Further Reading:**

The results discussed here are based on the article:

**"Constraining the TeV gamma-ray emission of SN 2024bch, a possible type IIn-L from a red supergiant progenitor"** published in *Astronomy & Astrophysics manuscript no. aa54721-25*.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: CTAO gGmbH