By PAIGE FRANK For The News-Letter
Science fiction has given the world plenty of ideas about black holes and how they might function. Recently, however, astronomers were able to make real-time observations of a star being consumed by a black hole, allowing them to collect data that will help to unravel some of the mystery behind the behavior of such phenomena.
The event astronomers observed was a “tidal disruption,” which occurs when a star gets trapped in the gravitational forces of a black hole and is ripped apart. The tidal destruction was captured by the optical light All-Sky Automated Survey for Supernovae (ASAS-SN) in November 2014 and was named ASASSN-14li.
After noticing the disruption, astronomers rushed to aim every tool they could toward the event, including NASA’s Chandra X-ray Observatory and Swift Gamma-ray Burst Explorer and the European Space Agency’s XMM-Newton satellite, in order to get a clearer picture of what the emissions from a tidal disruption look like.
Because ASASSN-14li happened only 290 million light-years from Earth (in what is known as the galaxy PGC 043234), it is the closest tidal disruption observed in almost a decade. Previously, most of the work done on tidal disruptions was theoretical. ASASSN-14li was the first opportunity to watch and collect data on the aftermath of a tidal disruption in real time. Scientists observed that after the star was ripped apart, the majority of its remains were drawn into the black hole. The speed of the particles as they fell toward the horizon of the hole resulted in great amounts of friction, heat and X-ray radiation.
During this stage, loads of emissions were recorded.
Then, immediately following the flux of X-ray activity, light emissions drastically decreased as the stellar material began to fall beyond the black hole’s horizon, where its light could no longer escape.
For the next stage of the tidal disruption, scientists watched carefully for the formation of an accretion disk, gas from the star that spirals inward toward the black hole, forming a visible disk shape. The specific process that results in these disks was largely untracked prior to ASASSN-14li. In the last stage of ASASSN-14li, scientists were not disappointed. They were able to track the X-ray wavelengths released and how they changed over time to gain a better understanding of accretion disk formation.
The data revealed that the gas farthest from the black hole releases the fewest number of x-rays. The least stable gas material, which is located near the horizon of the hole, produces the most radiation.
“The black hole tears the star apart and starts swallowing material really quickly, but that’s not the end of the story. The black hole can’t keep up that pace so it expels some of the material outwards,” Jelle Kaastra, co-author of the study and an astronomer at the Netherlands Institute for Space Research, said in a press release, describing the proposed force behind the disk formation.
What ASASSN-14li revealed was the presence of a stellar wind moving outward from the black hole.
This wind carries the gas from the star but does not move fast enough to escape the black hole’s gravitational field. The low speed keeps the gas circling, creating the observed accretion disk.
“We have seen evidence for a handful of tidal disruptions over the years and have developed a lot of ideas of what goes on,” lead author of the ASASSN-14li study, Jon Miller, said in a press release. “This one is the best chance we have had so far to really understand what happens when a black hole shreds a star.”
Scientists hope to apply the data gained from ASASSN-14li as well as any gained from future tidal disruptions, to further their knowledge of the black holes.
In particular, the goal is to gain a better understanding of how black holes interact with other stellar bodies.
