Want to find more supernovae? Follow the light

Is there anything more dramatic than an exploding star? More than just extraordinarily bright, energetic events that can light up the sky for months, these explosions play important roles in the cosmos. Supernovas create heavy elements and spread them out into their surroundings, where they can be taken up in the next round of planet and star formation.

Their blast waves also compress the surrounding gas, triggering more starbirth. Each time a new star forms, the cosmos rolls the dice, and if the right numbers come up, a planet with the potential to host life could come into being around one of these stars. This is why astrophysicists are so interested in SN explosions.

Supernova explosions are anything but neat and tidy. Stars are essentially balancing acts between their outward force of radiation and their inward gravitational pull. Those forces can be in balance for billions of years, like in our sun. But as stars fuse hydrogen into helium, they slowly and inexorably lose mass.

This mass loss leads to instability. Aging stars go through pulsations, releasing waves of material that create a circumstellar medium (CSM) around the star. As convulsions wrack the star, it emits more material into shells. Eventually, the star can no longer support itself, and for stars several times more massive than the sun, there’s an explosion as the star can’t support its own mass and collapses in on itself.

The final explosion releases a powerful blast wave that slams into the CSM. Modeling the stellar winds in that blast has proven to be challenging. But it’s important to get it right, since much of the light signal from a supernova is generated by the interaction between the CSM and the wind.

Astrophysicists have worked hard to model these winds, and previous models show them as a steady, smooth force. But observations haven’t always supported this. New research modeled these winds with more complexity as they slam into the CSM. The results from the modeling could provide a new way to observe and study supernova explosions.

The research is titled “Interacting supernovae and where to find them.” It’ll be published in the journal Astronomy and Astrophysics and is currently available online on the arXiv preprint server. The lead author is Robert Brose, from the Institute of Physics and Astronomy at the University of Potsdam, Germany.

“Early interaction of supernova blast waves with circumstellar material has the potential to accelerate particles to PeV energies, although this has not yet been detected,” the researchers write. PeV stands for Peta-electron Volts, and is the realm of gamma-rays. “Current models for this interaction assume the blast wave expands into a smooth, freely-expanding stellar wind, although multiwavelength observations of many supernovae do not support this assumption.”

Brose and his colleagues extended the work of previous researchers by modeling blast waves slamming into CSM with more complex density profiles, rather than into more smooth, featureless CSM. They included dense, multilayered shells of CSM at different distances from the supernova’s progenitor star.

“We aim to predict the gamma-ray and multiwavelength signatures of circumstellar interaction,” they explain.

They found that the interaction between the wind and the CSM can create a significant boost in gamma ray production from a supernova remnant. This elevated gamma ray production can persist for a long time, and its peak can appear years after the explosion. For some types of SN remnants, those from Type-IIP and Type-IIn supernova, the luminosity can exceed the luminosity from smooth stellar winds by several orders of magnitude.

Type IIn supernova are core collapse supernova with narrow hydrogen lines. Much of their light comes from their blast waves reaching and interacting with a shell. In Type IIP supernova, the P stands for plateau, meaning their light emissions reach a plateau and indicates that their emissions reach a steady state during their decline.

“For Type-IIP explosions, the light-curve peak is only reached years after the explosion, when the blast wave reaches the circumstellar shell,” the authors write.

The researchers examined the complex, multiwavelength light signatures that can be expected from SN as their blast waves slammed into the CSM. They considered light from radio, to optical, and to X-rays.

But the critical result concerns gamma-rays. They come from the most energetic processes in a SN. Different isotopes created in the blast wave create gamma rays with different fingerprints, and these can explain what reactions are taking place. In core-collapse supernovae, gamma-rays can also reveal what’s happening in the central part of the SN, the engine that drives the explosion. This work shows that gamma-rays can peak much later than thought.

The research shows that the gamma rays should be detectable for tens of Megaparsecs from the explosion, at least for some types of progenitors. Brose and his co-researchers propose observational strategies that can find these SN with high-cadence optical surveys and continuous radio and millimeter-wavelength monitoring. In this way, they can find promising targets for follow-up observations with gamma-ray observatories.

Very few SN have been initially detected by gamma-ray observatories. SN are usually spotted by other telescopes in other wavelengths, and then gamma ray observatories are aimed at them for further study. This research suggests a different approach.

“The late peak of the gamma-ray emission is at odds with current observation strategies of IACTs,” the authors write. IACTs are Imaging Atmospheric Cherenkov Telescopes. They detect gamma rays as they strike Earth’s atmosphere and allow physicists to detect them in energy regimes inaccessible to orbiting gamma ray telescopes. They expand the observation of gamma rays.

“After evaluating the thermal emission in the optical and X-rays, which both peak after the gamma-ray emission, we identify high-cadence optical surveys as a potentially suitable tool to capture the most extreme TypeIIPs interacting with dense shells,” the researchers explain.

“Due to the low sample sizes in the nearby universe and the short required observation times, a systematic radio and mm monitoring of close-by SNe for a few years after their explosions might prove valuable for identifying late shell-interactions as well,” they conclude.