In 2015, a group of astronomers spotted one of the brightest and most powerful supernovae ever discovered — a star explosion so luminous that it was poised to reshape our understanding of stellar eruptions. But now, astronomers have taken a closer look at this “supernova” with the Hubble Space Telescope, and they say that it wasn’t an exploding star after all. Instead, they pose a much bleaker scenario: the bright event was the result of a supermassive black hole tearing apart a star that passed way too close.
Called ASASSN-15lh, the event was so bright that at one point it was 20 times more luminous than all of the stars in the Milky Way combined. That led astronomers to think that a very powerful type of star was fueling the event — a super dense, collapsed star known as a magnetar. These types of stars spin incredibly fast, creating intense magnetic fields. And that field may have provided enough energy to sustain the super bright explosion, the astronomers argued.
But even at the time, they weren’t completely sure. If ASASSN-15h was caused by a magnetar, the star would have to be spinning really fast and converting energy very efficiently to make an event that bright. "Can we even actually explain it with the magnetar?" Kris Stanek, an astronomer at Ohio State University who found ASASSN-15lh, told The Verge in January. "If the magnetar model doesn’t work, then we’d have to find some new way to explain this."
Now, a new group of astronomers say they have that other explanation. Thanks to observations from Hubble and telescopes from the European Southern Observatory, the international team found that ASASSN-15lh went through “three distinct phases,” which is more indicative of a black hole tearing something apart than a supernova, the astronomers claim. They also noticed temperature increases and a re-brightening of ultraviolet light — changes that don’t really happen when stars explode. Plus, ASASSN-15lh is located in a type of galaxy that doesn’t typically have super luminous supernovae.
Given this new data, the team argues that a supermassive black hole caused ASASSN-15lh by pulverizing a passing Sun-like star. Known as a tidal disruption event, the black hole’s intense gravity would have pulled the star apart, stretching it out like spaghetti. This process would have sent debris colliding into each other and generated intense heat as the star fell toward the black hole. Such an event would have been able to produce the intense burst of light seen by the original astronomy team.
The astronomers say the new theory is not a done deal, however. Tidal disruption events are pretty rare to see, with only 10 having been observed so far. Plus, the supermassive black hole would also have to be spinning super fast in order to disrupt the passing star. So there are some constraints on this theory, but the astronomers say this concept is the better argument.
“Even with all the collected data we cannot say with 100 percent certainty that the ASASSN-15lh event was a tidal disruption event,” team leader Giorgos Leloudas at the Weizmann Institute of Science, Israel, said in a statement. “But it is by far the most likely explanation.”