Interacting supernovae and where to find them

TL;DR

This study demonstrates that supernova blast waves interacting with complex CSM, including LBV-generated shells and photoionization-confined RSG shells, can dramatically boost gamma-ray emission well after the initial explosion. By coupling PION-based CSM modeling with time-dependent cosmic-ray acceleration (RATPaC) and comprehensive radiation/absorption treatments, the authors predict strong, multiwavelength signatures and quantify detectability horizons for current and upcoming gamma-ray facilities. The results show that shell interactions can boost gamma-ray luminosities by orders of magnitude and shift peak emission to weeks-to-years post-explosion, with clear observational implications for triggering strategies in Fermi-LAT, H.E.S.S., and CTA-type instruments. The work provides practical guidance for multiwavelength monitoring campaigns and offers a framework to interpret potential late-time gamma-ray detections (e.g., SN 2004dj) and to plan targeted follow-ups of nearby events.

Abstract

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.

Figures (14)

• Figure 1: Density profile of the CSM for the two RSG scenarios listed in table \ref{['tab:ProgenitorModels']}. The low mass loss case creates a more dilute (yellow) shell compared to the high mass-loss case (purple). The red dashed lines are the desnity-profiles adopted in our simulations.
• Figure 2: The total energy in CRs in units of $10^{51}\,$erg for the LBV-cases (solid) and RSG-cases (dashed) as a function of time.
• Figure 3: Gamma-ray luminosities in the Fermi-LAT energy range (dashed) and H.E.S.S. energy range (solid) accounting for $\gamma\gamma$-absorption. Top: SNR of LBV progenitors interacting with the dense shell at different times (models as described in table \ref{['tab:ProgenitorModels']}). The $\gamma\gamma$-absorption has ceased completely after about 200 days. Bottom: Interaction of SNRs with RSG progenitors interacting with dense shells created by external photoionization. The purple case represents an upper limit in terms of shell-mass, cause by a high mass-loss wind and an strong ambient photon field. The dilute case represents typical parameters for the population of RSGs. (See table \ref{['tab:ProgenitorModels']} for details.)
• Figure 4: Fermi detectability for LBV 0.1, LBV 2.0, RSG high mass and RSG low mass scenarios from top to bottom. White lines indicate detected SNe explosions at their inferred distance and their overlap with operation-window of Fermi-LAT. Shown are Type-IIn events in the upper two panels and Type-IIP events in the lower panels.
• Figure 5: Top panel: Radio luminosity including the effects of free-free absorbtion for Type-IIn explosions. The green area indicates the $1\sigma$ uncertainty region for the rise time and peak radio-luminosity for Type-IIn SN taken from 2021ApJ...908...75B. Bottom panel: Radio spectral index $\alpha$ of the absorbed radio flux at 4 GHz.