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"Brightest Supernova” Not a Supernova, but a Star Ripped Apart by a Spinning Black Hole

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This artist’s impression depicts a star close to a rapidly spinning supermassive black hole in the center of a distant galaxy. Its large mass bends the light from stars and gas behind it. The gravitational pull of the supermassive black hole rips the the star apart in a Tidal Disruption Event, which generates a burst of light. Credit: ESO, ESA/Hubble, M. Kornmesser

Scientists at the Las Cumbres Observatory are part of a team of astronomers that has discovered a star being torn apart by a supermassive, spinning, black hole. The event, named ASASSN-15lh, was originally classified as the brightest supernova ever observed. The study published this week in Nature Astronomy describes ASASSN-15lh as a Tidal Disruption Event (TDE) — the destruction of a star by a rapidly rotating supermassive black hole.  This significant discovery is the first time that a TDE has been used to probe the spin of a black hole, a property which is very difficult to measure.  The study made heavy use of astronomical observation data from the Las Cumbres Observatory (LCO), a global robotic telescope network headquartered in Goleta, California. The team was led by Giorgos Leloudas, of the Weizmann Institute of Science in Israel and the Dark Cosmology Centre in Denmark.

The very luminous event was first discovered by the All-Sky Automated Search for Supernovae (ASAS-SN) in 2015. In a paper published in Science earlier this year by Subo Dong of the Kavli Institute for Astronomy & Astrophysics in Beijing, the authors declared ASASSN-15lh to be the brightest supernova ever discovered. Initially, these authors rejected the possibility that the event was caused by a star being ripped apart by the supermassive black hole at the center of the galaxy. They concluded that it was a supernova because the data indicated that this black hole was so large that it would have swallowed the star whole.

Some astronomers were hesitant from the beginning about classifying ASASSN-15lh as a supernova, in part because the event happened in a large elliptical galaxy.  In this type of galaxy, stars are no longer forming and do not die in supernova explosions. Also, the event released more energy than was believed to be possible in a supernova.

There were other indications that the event was not a supernova — in a normal supernova, the light rises to a peak and then declines. ASASSN-15lh was observed to brighten in ultraviolet light, decline, and then brighten again. This type of light curve is more consistent with a Tidal Disruption Event than a supernova.  Observations of the chemical elements ejected during the event were also more consistent with a TDE.

In the study described in Nature Astronomy, astronomers present new data and point to several lines of evidence indicating that ASASSN-15lh is a Tidal Disruption Event.  Using more precise images from the Hubble Space Telescope, they have found that the event occured at the center of the galaxy, where the supermassive black hole resides. With new theoretical models of spinning black holes, they also show that they have the ability to violently rip apart stars, rather than eating them whole. These rapidly rotating black holes are known as Kerr black holes.

The observed event ASASSN-15lh occurred when the star strayed too close to the supermassive black hole and was torn apart by extreme tides generated by the strong gravity of the black hole.  The stellar material orbited around the black hole, collided with itself at high velocity, and started falling onto the black hole. This released prodigious amounts of energy and generated the bright flare observed as ASASSN-15lh.

This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by a disc of rotating material that consists of the leftovers of a star which was ripped apart by the tidal forces of the black hole. Shocks in the colliding debris as well as heat generated in accretion led to a burst of light, resembling a supernova explosion. Credit: ESO, ESA/Hubble, M. Kornmesser

Las Cumbres Observatory provided essential data that was key to the analysis of this event. The robotic network of telescopes, located around the world, is critical for the rapid-response monitoring of events like ASASSN-15lh. The network provides regular data every few days and these data are vital for determining the nature of the event.

“We've only been studying the optical flares of tidal disruptions for the last few years,” said Iair Arcavi, principal investigator of the program used to observe ASASSN-15lh on Las Cumbres Observatory, and also an author on the current paper. "ASASSN-15lh is similar in some ways to the other events we've been seeing, but is different in ways we didn't expect. It turns out that these events, and the black holes that make them, are more diverse than we had previously imagined."

“Years ago we just wouldn’t have been able to follow an event like this,” said Andy Howell, the leader of the supernova group at LCO, and an author on the study. “This study shows that large-area surveys, a global robotic telescope network, and a NASA satellite can come together to reveal dramatic new discoveries that wouldn’t be possible without each piece of that puzzle.”

“This is like discovering a new kind of dinosaur,” says Howell.  “Now that we have the right tools and know what to look for, we’re going to find more and get a better sense of the population.  It is so exciting to have new ways of learning about black holes and stellar death!”

Las Cumbres astronomers involved in the current study include Einstein Fellow Iair Arcavi, postdoctoral fellow Curtis McCully, graduate student Griffin Hosseinzadeh, and staff scientist Andrew Howell.  All are also affiliated with the University of California, Santa Barbara, where Howell is also adjunct faculty.

This animation shows how ASASSN-15lh most likely happened. A star gets close to a rapidly spinning supermassive black hole in the center of a distant galaxy. While its orbit moves constantly closer to the black hole, the star gets ripped apart and creates a bright flash, similar to a superluminous supernova. Credit: ESO, ESA/Hubble, M. Kornmesse

The paper is available on Nature Astronomy at this link: http://nature.com/articles/doi:10.1038/s41550-016-0002
The paper is available also through the arXiv pre-print server at this link: http://arxiv.org/abs/1609.02927

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