Interacting supernovae and where to find them

Context . The early interaction of supernova blast waves with circumstellar material has the potential to accelerate particles to petaelectronvolt energies, although this has not yet been detected. Current models for this interaction assume that the blast wave expands into a smooth freely expanding stellar wind, although multiwavelength observations of many supernovae do not support this assumption. Aims . We extend previous work by considering blast waves expanding into complex density profiles consisting of smooth winds with dense circumstellar shells at various distances from the progenitor star. We aim to predict the gamma-ray and multiwavelength signatures of circumstellar interaction. Methods . We used the code PION to model the circumstellar medium around luminous blue variables including a brief episode of enhanced mass-loss and to simulate the formation of photoionization-confined shells around red supergiants. Consequently, we used the time-dependent acceleration code RATPaC to study the acceleration of cosmic rays in supernovae expanding into these media and to evaluate the emitted radiation (both thermal and nonthermal) across the whole electromagnetic spectrum. Results . We find that the interaction with the circumstellar shells can significantly boost the gamma-ray emission of a remnant, with the emission peaking weeks to years after the explosion when γγ absorption has reduced to negligible levels. The peak luminosity for Type IIP and Type IIn remnants can exceed the luminosity expected for smooth winds by several orders of magnitude. For Type IIP explosions, the light-curve peak is only reached years after the explosion, when the blast wave reaches the circumstellar shell. We evaluated the multiwavelength signatures expected from the interaction of the blast wave with a dense circumstellar shell from radio to optical and thermal X-rays. Conclusions . High-cadence optical surveys and continuous monitoring of nearby supernovae in radio and millimeter wavelengths are the best-suited strategies for identifying targets. They should be followed-up by gamma-ray observatories. We predict that gamma-rays from interaction with dense circumstellar shells may be detectable out to a few megaparsec for late interaction and out to tens of megaparsec for an early interaction.