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Evaporating Stars, Magnetars, and Bumpy Supernova Light Curves

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JWST image of Wolf–Rayet star WR 140
The Wolf–Rayet star WR 140 as seen by JWST. This image shows the shells of dust created by the interaction of WR 140 with its binary companion. New research shows that the explosion of a Wolf–Rayet star with a binary companion can account for the bumpy light curves of certain supernovae. [NASA, ESA, CSA, STScI, NASA-JPL, Caltech]

Instead of fading smoothly, some supernova light curves take a bumpy road from brilliance to obscurity. Can unusual binary systems containing a rapidly spinning, wind-emitting magnetar and a stellar companion explain these light curves?

Light Curve Wiggles

illustration of a magnetar

Artist’s impression of a highly magnetized stellar remnant called a magnetar. [ESO/L.Calçada; CC BY 4.0]

When a star explodes, researchers record the light curve from its final moments and attempt to understand its life and death. Certain supernovae show bumps and wiggles in their light curves, the cause of which is not yet agreed upon. Researchers suspect that some of these light-curve bumps crop up when the expanding shock wave of the supernova slams into gas and dust surrounding the star. Other brightness increases might occur when the explosion leaves behind a magnetar — an extremely dense, city-size stellar remnant that spins rapidly and has a strong magnetic field — that injects energy into its surroundings.

In a recent research article, Jin-Ping Zhu (Monash University) and collaborators expanded on the latter possibility, pairing a powerful magnetar with an unlucky companion star to explain bumpy features in the light curves of certain supernovae.

diagram illustrating the stages of the magnetar–star binary engine model

A diagram illustrating the stages of the magnetar–star binary engine model. Click to enlarge. [Zhu et al. 2024]

What Goes Bump in a Supernova Light Curve

The proposed theory starts with an ordinary star and a massive star in a close binary system. As the massive star evolves, it sheds its outer layers through rapid rotation and fierce winds, exposing its super-hot core and becoming a rare Wolf–Rayet star. As the Wolf–Rayet star continues to evolve, tidal interactions between the stars in the binary system spin the Wolf–Rayet star up to high speeds. It eventually explodes in a core-collapse supernova, leaving behind a rapidly spinning magnetar.

Other models have invoked magnetars to explain bumpy supernova light curves, but this theory goes a step further, giving the companion an important role to play. As the newborn magnetar and the companion star swing around each other on their tight orbits, the magnetar’s powerful particle wind collides with the other star, evaporating some of the unlucky companion. The evaporated stellar material is then heated and accelerated by the magnetar wind, producing a bump in the light curve.

A Fitting Theory

plot of supernova light curves and model results

Example of multi-band light curves for a supernova that is well fit by the authors’ model. [Adapted from Zhu et al. 2024]

That’s the theory — how does it compare to observations? Zhu and collaborators applied their magnetar–star binary engine model to the light curves of supernovae with a single bump after maximum brightness. They found that the model generally fits the observations well, with the best-fitting results implying that a significant chunk — about 25–60% — of the companion star gets evaporated.

Zhu and collaborators suspect that their model may apply to light curves with multiple bumps, as well. If the companion star remains bound to the magnetar after the supernova explosion but is kicked into a new, highly eccentric orbit, a bump could be created each time the stars draw close to one another on their orbits.

The team notes that there isn’t yet firm observational or theoretical evidence that rapidly rotating massive stars leave behind magnetars, and it’s not clear whether a magnetar embedded within a supernova remnant can sustain a magnetar wind, as is required here. Future work may shore up the needed evidence, and in the meantime, this work provides a new way to interpret bumpy light curves.

Citation

“Bumpy Superluminous Supernovae Powered by a Magnetar–Star Binary Engine,” Jin-Ping Zhu et al 2024 ApJL 970 L42. doi:10.3847/2041-8213/ad63a8