Interacting supernovae and where to find them

Early interaction of supernova blast waves with CSM has the potential to accelerate particles to PeV energies, although this has not yet been detected. Current models for this interaction assume the shock expands into a smooth stellar wind, although observations of many SNe do not support this assumption. We extend previous work by considering shocks expanding into complex density profiles consisting of smooth winds with dense CSM shells at various distances from the progenitor star. We aim to predict the gamma-ray and multiwavelength signatures of CSM interaction. We used the PION code to model the CSM around LBV including a brief episode of enhanced mass-loss and to simulate the formation of photoionization-confined shells around RSGs. Consequently, we used the time-dependent acceleration-code RATPaC to study the acceleration of cosmic rays in SNe expanding into these media and to evaluate the emitted radiation across the whole electromagnetic spectrum. We find that the interaction with the CSM shells can significantly boost the gamma-ray emission, with the emission peaking weeks to years after the explosion. The peak luminosity for Type-IIP and Type-IIn remnants can exceed the luminosity expected for smooth winds by orders of magnitude. For Type-IIP explosions, the light-curve peak is only reached years after the explosion. We evaluate the multiwavelength signatures expected from the interaction of the blast wave with a dense CSM shell from radio, over optical, to thermal X-rays. We identify high-cadence optical surveys and continuous monitoring of nearby SN in radio and mm wavelengths as the best-suited strategies for identifying targets that should be followed-up by gamma-ray observatories. We predict that gamma-rays from interaction with dense CSM shells may be detectable out to a few Mpc for late interaction, and tens of Mpc for early interaction.

💡 Research Summary

This paper investigates how the interaction of young supernova (SN) blast waves with structured circumstellar material (CSM) can dramatically enhance particle acceleration and high‑energy emission, challenging the prevailing assumption of a smooth 1/r² stellar wind. The authors focus on two progenitor classes—Luminous Blue Variables (LBVs) and Red Supergiants (RSGs)—that are expected to produce dense CSM shells through episodic mass‑loss or external photo‑ionization.

Using the PION hydrodynamics code, they first generate realistic CSM density profiles. For LBVs, a brief outburst with a mass‑loss rate of 1 M⊙ yr⁻¹ lasting two years creates a dense shell that expands to ∼1 pc. For RSGs, two cases are modeled: a high‑mass shell (Ṁ = 10⁻⁴ M⊙ yr⁻¹, v = 15 km s⁻¹) and a low‑mass shell (Ṁ = 2×10⁻⁵ M⊙ yr⁻¹), both exposed to external ionizing fluxes that generate photo‑ionization‑confined shells at ≈0.03 pc. The resulting density structures are fitted with Gaussian profiles and fed into the time‑dependent cosmic‑ray (CR) acceleration code RATPaC.

RATPaC solves the kinetic diffusion‑advection equation for CRs, couples it with a thermal leakage injection model, and evolves magnetic turbulence self‑consistently within a 1‑D spherical PLUTO‑based hydrodynamic framework. This setup yields the evolution of the CR spectrum, maximum particle energy, and the non‑thermal (synchrotron, inverse‑Compton, pion‑decay) as well as thermal emission across the electromagnetic spectrum.

Key findings:

1. CSM Shell Interaction Boosts Acceleration: When the blast wave hits a dense shell, the shock decelerates sharply, magnetic fields are amplified, and the CR acceleration efficiency rises to ∼1 % (10⁻²). Maximum energies can exceed the PeV scale, especially for LBV‑type IIn SNe where the shell is encountered within weeks.

2. Gamma‑Ray Light Curves Peak After γγ‑Absorption Declines: The γ‑ray luminosity, initially suppressed by pair‑production on the SN photospheric photons, rises sharply once the optical depth drops (weeks to years). For Type‑IIP SNe, the peak occurs years after explosion, coincident with the shock reaching the shell.

3. Multi‑wavelength Signatures Appear Simultaneously: Radio and mm bands show a synchrotron flare (10–100× increase) preceding the γ‑ray peak by 1–2 yr. Optical spectra develop strong Hα and He I emission from CSM interaction, while thermal X‑rays brighten due to shock‑heated dense gas (kT ≈ 0.1–1 keV).

4. Detectability Horizons Depend on Shell Location: Early interaction (shell at ≲0.01 pc) yields γ‑ray fluxes ≳10⁻¹² erg cm⁻² s⁻¹, detectable out to several × 10 Mpc with current instruments (Fermi‑LAT, H.E.S.S.) and well within the reach of CTA. Late interaction (shell at 0.1–1 pc) reduces the flux to ≈10⁻¹³ erg cm⁻² s⁻¹, limiting detection to a few Mpc.

5. Observational Strategy Recommendations: High‑cadence optical surveys (e.g., ZTF, LSST) combined with continuous radio/mm monitoring (VLA, ALMA) can identify CSM interaction early. A radio flare can serve as a trigger for targeted γ‑ray observations, ensuring that both early and late interaction phases are covered.

In summary, incorporating realistic, clumpy CSM structures into SN models reveals that dense shells can dramatically amplify both particle acceleration and high‑energy emission, producing observable signatures from radio to γ‑rays. The work underscores the need for coordinated, multi‑wavelength monitoring of nearby core‑collapse supernovae to capture these transient high‑energy events.