Fashionably Late: Why Some Supernovae Brighten Months After Exploding

Some supernovae unexpectedly increase in brightness in ways that our current models can’t explain. Can a new model that combines shock waves and sound waves make sense of this mystery?

Late to the Party

time series of supernova remnant SN 1987A from 1994 to 2016

This time series of Hubble images shows the expansion of supernova remnant SN 1987A. As shock waves interacted with the ambient gas, the supernova remnant brightened dramatically. [NASA, ESA and R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation) and P. Challis (Harvard-Smithsonian Center for Astrophysics)]

When a massive star explodes as a supernova, it briefly outshines its host galaxy before gradually fading from view. Sometimes, expanding supernova shock waves collide with nearby gas — such as circumstellar material ejected by the star before it went supernova — which causes a temporary increase in brightness. When this happens, the material being swept up is compressed, heated, and ionized, causing new emission lines to appear in the supernova’s spectrum. However, some supernovae brighten without showing new emission lines — what causes this behavior?

New work by Eric Coughlin (Syracuse University) and Jonathan Zrake (Clemson University) suggests that delayed brightening might not always be evidence for a new interaction with circumstellar material — but rather, an echo of a previous perturbation.

A Sound Solution

Coughlin and Zrake used linear perturbation theory — a way to mathematically describe the properties of a system in terms of a slowly varying background and a small perturbation in that background — to explore the scenario in which a supernova shock wave expanding into circumstellar material encounters an especially dense region of gas.

plot of the shock luminosity as a function of distance

The shock luminosity relative to the luminosity obtained when there is no density enhancement present (solid line) is plotted as a function of the shock’s position. The scaled density is shown by the dotted line. [Adapted from Coughlin & Zrake 2022]

In this scenario, Coughlin and Zrake predict that the supernova will brighten twice. As the shock wave expands into the circumstellar material, it encounters a denser region of gas and the supernova brightens for the first time and exhibits new emission lines. As the shock collects more and more material, it slows down and the brightness decreases.

So far, this is just a typical interaction between a shock wave and circumstellar material. Here’s where things change: the new model explored by Coughlin and Zrake incorporates a second wave — a slower-moving sound wave — which is launched by the initial collision between the shock wave and the denser circumstellar material. As the shock wave slows down, the sound wave catches up to it and hits it from behind. In the authors’ new framework, it’s the interaction between the initial shock wave and the secondary sound wave that causes the second brightening of the supernova — not a change in the density of the surrounding material. And since no additional material is being swept up and ionized, no new emission lines are produced.

Explaining Outliers

plot of shock luminosity as a function of time

The shock luminosity as a function of time for the density enhancement introduced in the previous figure. The increase in luminosity around 75 days agrees qualitatively with the observations of SN 2019tsf. [Adapted from Coughlin & Zrake 2022]

This model may explain the behavior of supernovae that have brightened long after their initial explosions, like SN 2019tsf, iPTF14hls, and SN2020faa. Should we expect delayed brightening to be a feature of all supernovae? Unlikely, say Coughlin and Zrake — extremely dense circumstellar material could obscure an increase in brightness, and if the density of the material decreases too quickly with distance, the sound wave won’t be able to catch up with the shock wave.

The authors note that there’s much more to explore, since real supernovae expanding into circumstellar gas are far more complex than the framework introduced in this article. Hopefully, future work will help us understand the wide variety of supernova behaviors seen so far!


“A Physical Model of Delayed Rebrightenings in Shock-interacting Supernovae without Narrow-line Emission,” Eric R. Coughlin and Jonathan Zrake 2022 ApJ 927 148. doi:10.3847/1538-4357/ac4033