Whenever a star moves to close to a supermassive black hole, a phenomenon called the tidal disruption event takes place. The name itself is scary and can give you an idea of what happens. The black hole doesn’t like its personal space being invaded and it shows its anger by generating strong tidal forces to shred the star as it’s pulled into the void. Scientists have reopened a similar cosmic incident that took place some 290 million years ago to better understand this phenomenon.
A team led by MIT recently found evidence that suggested that the tidal disruption is even more disastrous than we initially thought it was. In 2014, astronomers observed signs of a tidal disruption event in a galaxy far away called the PGC 043234. Intense flaring was observed as a super massive black hole consumed a star much like our own Sun. The event was referred to as the ASASSN–14li and it was first registered in 2014 even though the event took place some 290 million years ago. It took this long for the flares to reach us for the exact same reason. 290 light years is a massive distance to cross you know.
NASA’s Swift satellite caught something unusual in the form of X-rays, visible and UV light and that the flaring observed has a month long lag between some of the readings. According to MIT astrophysicist Dheeraj Pasham, "We discovered brightness changes in X-rays that occurred about a month after similar changes were observed in visible and UV light.” The team believes that the lag was due to emission of light from two different sources.
The team’s hypothesis suggests that when the star went too close to the black hole, it got converted into a stream of tidal debris. This debris was then sucked into the black hole’s accretion disk. Before entering this disk, the debris overshot the hole in an elliptical arc. Pretty interesting and theatrical I must add. While in the arc, the debris collided with each other violently and produced emissions. "We think this means the optical and UV emission arose far from the black hole, where elliptical streams of orbiting matter crashed into each other," explains Pasham. Bradley Cenko another team member added, “Returning clumps of debris strike the incoming stream, which results in shock waves that emit visible and ultraviolet light.”
A bad case of indigestion
A month after the collisions, the star matter flared again as it got heated and compressed as it settled into the hole’s accretion disk. According to Cenko, “As these clumps fall down to the black hole, they also modulate the X-ray emission there.” The team referred to this entire phenomenon as a cosmic indigestion situation. Pasham said, “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."
We don’t know much about this consuming and choking and whatever this disruption is all about and we may have to wait for another such event to take place to learn more. However, Pasham said, “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."