Tidal Disruption Flares Lead Researchers to Supermassive Black Holes

A supermassive black hole “choking” on an unexpected influx of stellar debris has been discovered by researchers at the center of a distant galaxy, located nearly 300 million light years from our planet.

Scientists from MIT and NASA’s Goddard Space Flight Center have discovered a “tidal disruption flare,” that is, a striking burst of electromagnetic activity that takes place when a star is obliterated by a nearby black hole.

The researchers have reported their discovery in a paper published today in the journal Astrophysical Journal Letters. The researchers first discovered the flare on 11 November 2014, and from that time they have observed the event through telescopes in order to understand more about the way in which black holes grow and evolve.

The MIT-led researchers analyzed the data collected from two varied telescopes and recognized a peculiar pattern in the energy emitted by the flare: When the dust from the obliterated star fell into the black hole, minute fluctuations in the ultraviolet (UV) and optical bands of the electromagnetic spectrum were observed by the researchers. The same pattern repeated itself after a time period of 32 days, but now in the X-ray band.

The scientists used simulations of the event carried out by other researchers to come to a conclusion that energy “echoes” of that kind were generated as a result of the following condition: When a star gets very close to the black hole, it is swiftly ripped apart by the gravitational energy of the black hole.

The emerging stellar debris swirls even closer to the black hole and collides with itself, thus generating bursts of UV and optical light at the collision sites. When the colliding debris is further pulled in, it gets heated up, generating X-ray flares, in a pattern very similar to that of the optical bursts, shortly before the stellar debris fell into the black hole.

In essence, this black hole has not had much to feed on for a while, and suddenly along comes an unlucky star full of matter. What we’re seeing is, this stellar material is not just continuously being fed onto the black hole, but it’s interacting with itself - stopping and going, stopping and going. This is telling us that the black hole is ‘choking’ on this sudden supply of stellar debris.

Dheeraj Pasham, Postdoc, MIT

Co-authors who worked with Pasham are MIT Kavli postdoc Aleksander Sadowski and researchers from NASA’s Goddard Space Flight Center, the University of Maryland, the Harvard-Smithsonian Center for Astrophysics, Columbia University, and Johns Hopkins University.

A “Lucky” Sighting

Pasham expressed that tidal disruption flares can prove to be a prospective way to observe many “hidden” black holes in the universe that are neither actively accreting nor feeding on material.

“Almost every massive galaxy contains a supermassive black hole,” stated Pasham. “But we won’t know about them if they’re sitting around doing nothing, unless there’s an event like a tidal disruption flare.”

Flares like these appear if a star moving very near to a black hole is ripped apart by the immense gravitational energy of the black hole. Such a stellar obliteration might result in spectacular bursts of energy along the entire electromagnetic spectrum, that is, from the radio band to the optical and UV bands, and also to the X-ray and high-energy gamma ray wavelengths. Since such tidal disruption flares are extreme and occur less frequently, they are highly difficult to observe.

You’d have to stare at one galaxy for roughly 10,000 to 100,000 years to see a star getting disrupted by the black hole at the center.

Dheeraj Pasham, Postdoc, MIT

However, on 11 November 2014, a global network of robotic telescopes called All Sky Automated Survey for SuperNovae (ASASSN) found out signals of a probable tidal disruption flare from a galaxy located 300 million light years from Earth. The researchers swiftly focused other telescopes, such as the X-ray telescope aboard NASA’s Swift satellite, on the occurrence. The Swift satellite is a spacecraft that orbits around and scans the sky to find out extreme high-energy bursts.

“Only recently have telescopes started ‘talking’ to each other, and for this particular event we were lucky because a lot of people were ready for it,” stated Pasham. “It just resulted in a lot of data.”

A Light-on Collision

Having gained access to this data, Pasham and his collaborators headed to find the answer for a long-pending puzzle: From where do the bursts of light in a flare first appear?

The researchers employed models of black hole dynamics and evaluated that when a star is pulled apart by a black hole, the emerging tidal disruption flare has the ability to generate X-ray emissions nearer to the black hole.

However, precisely identifying the origin of UV and optical emissions has been challenging. Solving this will lead us one step closer to understanding events that occur during disruption of a star.

“Supermassive black holes and their host galaxies grow in-situ,” stated Pasham. “Knowing exactly what happens in tidal disruption flares could help us understand this black hole and galaxy coevolution process.”

The scientists analyzed the first 270 days after the tidal disruption flare called ASASSN-14li was detected. Specifically, they studied the UV/optical and X-ray data acquired by the Swift satellite and the Las Cumbres Observatory Global Telescope.

They were able to recognize fluctuations in the X-ray band: there were two broad peaks, one closer to day 50 and the other closer to day 110, and a subsequent short dip closer to day 80. They could recognize a similar pattern in the UV/optical data before nearly 32 days.

In order to provide an explanation for such emission “echoes,” the researchers used simulations of a tidal disruption flare generated when a star obliterated a black hole. They modeled the ensuing accretion disc, that is, an elliptical disc of stellar debris that swirls around a black hole, including its prospective radius, speed, and rate of infall (i.e. the speed at which the material is drawn into the black hole).

Using the simulations run by other researchers, the MIT-led team derived that the UV and optical bursts probably originated due to collision of the stellar debris on the black hole’s outer perimeter. When this colliding material circles very close to the black hole, it gets heated up, ultimately producing X-ray emissions, which lag behind the optical emissions, identical to the pattern observed by the researchers in the data.

For supermassive black holes steadily accreting, you wouldn’t expect this choking to happen. The material around the black hole would be slowly rotating and losing some energy with each circular orbit. But that’s not what’s happening here. Because you have a lot of material falling onto the black hole, it’s interacting with itself, falling in again, and interacting again. If there are more events in the future, maybe we can see if this is what happens for other tidal disruption flares.

Dheeraj Pasham, Postdoc, MIT

NASA partially supported this research.