Black holes devour stars in gulps and nibbles


IT'S like a buffet where no one agrees on table manners. When a black hole encounters a star, it seems there is more than one way for this cosmic enigma to chow down.


Stars can safely orbit a black hole if they keep their distanceMovie Camera, but if they cross a line called the Roche limit, they get torn apart in a so-called tidal disruption event (TDE). This is when the black hole loads up its plate by stretching the star in one direction and squeezing it in the other, gradually distributing hot gas from the star in a disc around itself. Once it has finished the task, the black hole can gorge on the star's remains until nothing is left.


Normally, forming the disc and eating it both take years, but Pablo Laguna of the Georgia Institute of Technology in Atlanta and his colleagues have discovered that some black holes positively race through these stages (http://ift.tt/1zZtDzQ).


The team simulated what happens when a black hole a million times the mass of the sun encounters a sunlike star at close range – so close that they are almost touching.



In this situation, the star's orbit twists slightly as it rotates, due to an effect called relativistic precession. Now the black hole can load its plate in an instant, tearing the star apart and spreading it out into a disc in a couple of minutes. "The precession effect helps to spread the debris and wrap it around the black hole," Laguna says. The black hole then takes a few days to swallow the star.


Meanwhile, another team has spotted what appears to be a black hole eating at a snail's pace, returning to the star just once a decade to load up a single spoon. Deborah Mainetti of the University of Milano-Bicocca in Milan, Italy, and her colleagues used data from three space telescopes to track a galaxy called IC 3599, around 300 million light years from Earth.


In the 1990s, a German space telescope called Rosat saw the galaxy flaring up. The way it faded matched a traditional TDE, so could have been caused by a central black hole ripping up a star. But the flaring was also much less bright than expected, making some doubt whether it really was a black hole mid-meal.


NASA's Chandra space telescope saw a brief flare in 2002, and NASA's Swift has seen more in recent years. Together, the observations suggest that IC 3599 experiences a TDE-like event every nine and a half years, says Mainetti (http://ift.tt/1BehZqm).


"If this was a disruption of three different stars, we wouldn't have seen such a similarity," she says. That means it must be the same star on a very eccentric orbit, passing just close enough once a decade for the black hole to have a little taste – a process Mainetti calls spoon-feeding. At this rate, it could take 10,000 years to consume the entire star.


Spoon-feeding could unmask black holes that wouldn't otherwise reveal themselves, says Mainetti. "This is a very important way to observe black holes that are generally quiescent."


This article appeared in print under the headline "Black holes both gobble and nibble"


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