In late May 2015, scientists noticed something twice as bright as the brightest known supernova ever seen. Researchers initially thought the object, which they named “ASASSN-15lh,” was likely a supernova — and the brightest yet.
That startling discovery caught the interest of the astrophysics community, and an international team continued to observe the source for the next ten months. The team used an impressive array of ground-based and space-based telescopes, including the Very Large Telescope at ESO’s Paranal Observatory, the New Technology Telescope at ESO’s La Silla Observatory and the NASA/ESA Hubble Space Telescope. During that time, they’ve seen it behaving strangely for a supernova — so weird, in fact, that they’ve now concluded it’s probably not a supernova at all.
Instead, the pattern of 15lh’s changing temperature over time, its location in a distant galaxy, and a few other factors convinced the researchers that the bright flash was likely caused by a star being torn apart and becoming spaghetti-like. fied. Its staggering destruction is believed to be due to the incredible tidal forces of a nearby supermassive black hole, which has more than 100 million times the mass of the sun. (Its mass is implied by that of its host galaxy, since a galaxy’s mass is proportional to that of its supermassive black hole).
Under this model, published Monday in Nature Astronomy, the flash of light was generated by shocks in the star’s material as the disintegrating star material experienced collisions, as well as heat generated by friction as the material accelerated and fell into the black hole.
Spinning black holes
Black holes of that size are not uncommon, but objects they shred are. Above a certain mass, specifically 100 million solar masses, which is this black hole, the event horizon becomes large enough to extend beyond the point at which the star would be ripped apart. (The event horizon is the distance from the center of mass of the hole from which nothing can escape, which is also considered the “surface” of the black hole, even though it is not a true, solid surface). So if an incident star turned into ribbons, it would happen within the event horizon, and we could never see it, nor could our current understanding of physics successfully describe it.
However, Kerr black holes can destroy even a star with a mass of more than 100 million solar masses. This is due to the defining factor of such black holes: they spin quickly. This spin has a strong gravitational effect on the objects around it, meaning it can rip an infalling star nearly ten times farther from its center of mass than a non-spinning black hole. (Kerr black holes were first directly observed earlier this year in the LIGO experiment that detected gravitational waves).
“The tidal disturbance we propose cannot be explained with a non-spinning supermassive black hole,” said Nicholas Stone of Columbia University, one of the paper’s co-authors. “We argue that ASASSN-15lh was a tidal disturbance that originated from a very special kind of black hole.”
Artist’s impression, an animation showing what it might look like if the star is ripped apart. Courtesy: ESO, ESA/Hubble, M. Kornmesser
Although it is not yet certain that this is the correct explanation for the bright flash, many factors seem to point in that direction. “There are several independent aspects to the observations that suggest that this event was indeed a tidal disturbance and not a superluminous supernova.,said Morgan Fraser of the University of Cambridge, UK (now at University College Dublin, Ireland), one of the paper’s co-authors.
For starters, the researchers were able to perform spectroscopic analyzes of the light source, learning which elements were present and what wavelengths the light was emitting. From this they could collect clues about the object’s identity, such as the evolution of its temperature over the observation period.
The light output of 15lh (especially the ultraviolet) went through three different periods of dimming, relighting and dimming again. After about 100 days of observation, the spectra remained remarkably constant over time (albeit with a gradual decrease), which is inconsistent with post-supernova behavior, but consistent with a tidal-destroyed star, both in theory and in observation.
After a supernova explosion, the outer layers of the star are blown away and expand into a nebula (cloud of gas and dust in space). The researchers saw no evidence of nebulosity. However, a tidally disturbed star would not create a nebula. Its spaghetti-like remnants would continue to circle the disruptive black hole in an accretion disk as it is slowly consumed by the hole.
Computer simulation of the star being spaghetti into thin material trails. A few orbits later it has formed into an accretion disk. Credit:ESO, ESA/Hubble, N. Stone, K. Hayasaki
In addition, the researchers estimate the total energy output so far at 15lh, which according to current models is quite close to the maximum energy with which a supernova must operate. That doesn’t definitively rule out a supernova, but if the estimates are a little off, it could push the supernova over the edge.
A stronger argument that 15lh is a black hole ripping apart a star comes from its environment. The researchers were able to discover its location: about four billion light-years away near the core of a massive, red, silent galaxy whose stars are about a few billion years old.
However, the type of supernovae that could have produced 15lh are uncommon in that environment. They need massive stars, born in active, chaotic environments, with continuous star formation and stars that are typically only a few million years old. However, its location close to the center of the galaxy is suitable for a supermassive black hole, as every galaxy is believed to have one at its core.
That doesn’t definitively prove that 15lh isn’t a supernova; that is just a statistical argument, showing only probability. If you’re in an apple orchard and you see a fruit from afar that you can’t identify at first, it’s more likely to be an apple than a peach. It is always possible that a peach tree was accidentally planted there; it’s just less likely.
Implications
This discovery may provide an opportunity to learn more about the universe in general. Although we know that fast-spinning Kerr black holes exist, we don’t yet know how many of them exist among the population of supermassive black holes in the centers of galaxies. Observations of active galactic nuclei (galactic nuclei with extremely bright supermassive black holes) suggest that their spinning could be normal, but it is difficult to measure.
Other stars disrupted by the supermassive black holes they orbit could provide a perfect test for this. If a black hole has more than 100 million solar masses and makes a star spaghetti, we know it’s a Kerr black hole. And the more we find, the better statistics we will have to use in models of the universe as a whole.
Of course, that all depends on whether this is a tidal disturbance. While that now seems very likely, nothing is certain. “Even with all the data collected, we can’t say with 100 percent certainty that the ASASSN-15lh event was a tidal disturbance,” said Giorgos Leloudas of the Weizmann Institute of Science, Israel, the paper’s team leader and lead author. “But it’s by far the most likely explanation.”
Nature Astronomy2016. DOI:10.1038/s41550-016-0002 (About DOIs)