Friday, 11 September 2015

Astronomers Detect Tidal Disruptions By Dormant Supermassive Black Holes


In a tidal disruption event, an unfortunate star passes too close to a dormant supermassive black hole and gets torn apart by tidal forces, feeding the black hole for a short time. Astronomers use distinctive observational signatures to detect these events, but they are not seeing nearly as many tidal disruption events as theory says they should.

A recent study by UC Santa Cruz researchers suggests that astronomers might be missing many of these events because of how the streams of shredded stars fall onto the black hole. James Guillochon, who earned his Ph.D. at UC Santa Cruz and is now at the Harvard-Smithsonian Center for Astrophysics, and Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics, based their analysis on a series of computer simulations of tidal disruption events. They reported their findings in a paper published August 20 in the Astrophysical Journal.

When a black hole tears a star apart, the star's material is stretched out into what's known as a tidal stream. That stream continues on a trajectory around the black hole, with roughly half the material eventually falling back on the black hole, whipping around it in a series of orbits. Where those orbits intersect each other, the material smashes together and circularizes, forming a disk that then accretes onto the black hole.

Astronomers don't observe anything until after the tidal streams collide and the material begins to accrete onto the black hole. At that point, they observe a sudden peak in luminosity, which then gradually decreases as the tail end of what's left of the star accretes and the black hole's food source eventually runs out.

So why have astronomers only been observing about a tenth as many tidal disruption events (TDEs) as theory predicts they should see? By studying the structure of tidal streams in TDEs, Guillochon and Ramirez-Ruiz have found a potential reason, and the culprit is general relativity.

"It is an effect of general relativity that is modulating the digestion process of the black hole, so the digestion rate depends strongly on the mass of the black hole," Ramirez-Ruiz said.

The researchers ran a series of simulations of tidal disruption events around black holes of varying masses and spins to see what form the resulting tidal streams take over time. They found that precession of the tidal stream due to the black hole's gravitational effects changes how the stream interacts with itself, and therefore what astronomers observe. Some cases behave as expected for what's currently considered a "typical" event, but some do not.

For cases where the relativistic effects are small (such as black holes with masses less than a few million solar masses), the tidal stream collides with itself after only a few windings around the black hole, quickly forming a disk — but the disk forms far from the black hole, so it takes a long time to accrete. As a result, the observed flare can take 100 times longer to peak than typically expected, so these sources may not be identified as tidal disruption events.

Furthermore, for cases where the black hole is both massive and has a spin greater than a certain value (about 20 percent of its maximum allowed spin), the tidal stream doesn't collide with itself right away. Instead, it can take many windings around the black hole before the first intersection. In these cases, it may potentially be years after a star gets ripped apart before the material accretes and astronomers are able to observe the event.

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