The fraction of periodic SN Ib/c light curves

When a massive star runs out of fuel, it collapses and explodes, sending a brilliant flash of light across the universe. These explosions, known as supernovae, are among the most energetic events in nature. For a long time, astronomers believed that the light from these explosions simply faded away in a smooth, predictable curve, like a candle burning down. However, recent observations have revealed that some of these dying stars do not fade quietly. Instead, their brightness wiggles up and down in a regular pattern as they dim. This rhythmic pulsing is a rare and puzzling phenomenon. It suggests that the exploding star was not alone; it likely had a companion star orbiting it. As the explosion happened, the new, dense core left behind might have been interacting with this companion, creating a gravitational tug-of-war that caused the light to pulse. Understanding how often this happens helps scientists figure out how many stars live in pairs and how they evolve before they die.

A team of researchers at the Weizmann Institute of Science decided to take a systematic look at this behavior. They wanted to know if these pulsing explosions were just a few strange accidents or if they represented a significant group of supernovae. To find out, they turned to the Zwicky Transient Facility, a powerful telescope in California that scans the sky every night, capturing thousands of changing light sources. The team focused on a specific type of supernova called Type Ib or Ic. These are the explosions of stars that have already lost their outer layers of hydrogen and helium, often because a companion star stripped them away before the explosion. The researchers gathered light curves, which are records of how bright these stars appear over time, for thirty-four different candidates. They needed high-quality data with enough measurements to spot a pattern, so they selected only those events where the telescope had watched the star for a sufficient amount of time and taken enough pictures.

To find the hidden rhythms in the data, the team built a new computer method that was more careful than previous attempts. Instead of first smoothing out the general fading of the star and then looking for wiggles in the leftovers, their method treated the fading and the wiggles as a single, connected problem. They fit a mathematical curve to the overall decline of the light while simultaneously testing for a repeating wave pattern. This approach prevented the computer from accidentally hiding a real signal by smoothing it out too early. They applied this rigorous test to every light curve in their sample, searching for any period of repetition that stood out clearly from random noise. They set a very high standard for what counted as a discovery, requiring that the pattern appear clearly in two different colors of light observed by the telescope, ensuring the signal was real and not just a glitch in the data.

The search yielded three clear results. Two of them were stars that had already been identified by other astronomers as having this pulsing behavior: SN 2022jli and SN 2022esa. The team's method successfully recovered the known patterns in these stars, confirming that their new approach worked. SN 2022jli showed a pulse roughly every twelve days, while SN 2022esa pulsed about every thirty-four days. But the most exciting finding was a third star, SN 2020sgf, which had not been previously flagged as a periodic candidate. This star, which exploded in 2020, showed a clear, repeating pattern of about thirty days in one color of light and roughly thirty-six days in another. While the exact timing differed slightly between the two colors, the difference was small enough to be consistent with the same underlying rhythm. This discovery added a new member to the small club of pulsing supernovae.

To understand what these three discoveries meant for the wider population of exploding stars, the researchers had to account for the limitations of their search. They realized that their telescope could not see every possible rhythm. If a star pulsed very slowly, or if the pulse was very faint, the telescope might miss it entirely, especially if the star faded too quickly or was hidden by the sun's glare for part of the year. To measure this blind spot, they ran thousands of computer simulations. They took the actual data from their thirty-four stars and secretly added fake pulsing signals of different strengths and speeds. Then, they ran their detection method on this modified data to see how often it found the fake signals. This process created a map of their sensitivity, showing exactly which types of pulsing stars they could catch and which they would likely miss.

With this sensitivity map in hand, the team compared their findings to theoretical models of how stars behave in binary systems. These models predict that a certain percentage of stripped-envelope supernovae should show this pulsing behavior due to interactions with a companion. The researchers found that if only a tiny fraction of these stars were pulsing, they would have been very unlikely to find three of them in their sample. The fact that they found three suggests that the true number is higher. Their analysis indicates that models predicting that about twenty percent of these supernovae are periodic are consistent with their observations. This means that these rhythmic explosions are not exceptionally rare oddities. Instead, they likely represent a substantial sub-group of dying stars, perhaps one in five, that are interacting with a companion in the final moments of their lives.

The study also highlighted the importance of looking at the data with fresh eyes. The researchers noted that two of the periods they found were close to the twenty-nine-and-a-half-day cycle of the moon. While the moon's light can sometimes create false patterns in telescope data, the team found no evidence that their discoveries were caused by this effect. The signals were strong and consistent across different colors of light, pointing to a physical cause within the stars themselves. The work demonstrates that with the right tools and a large enough sample, astronomers can move beyond studying individual strange objects to understanding the broader population. The discovery of SN 2020sgf, along with the confirmation of the others, suggests that the universe is filled with these binary interactions, waiting to be found by the next generation of sky surveys.