Chandra finds Milky Way's black hole grazing on asteroids
X-ray flares have been detected about once a day from the supermassive black hole known as Sagittarius A* that last a few hours with brightness ranging from a few times to nearly 100 times that of the black hole’s regular output.
The giant black hole at the center of the Milky Way may be vaporizing and devouring asteroids, which could explain the frequent flares observed, according to astronomers using data from NASA’s Chandra X-ray Observatory.
For several years, Chandra has detected X-ray flares about once a day from the supermassive black hole known as Sagittarius A* (Sgr A*). The flares last a few hours with brightness ranging from a few times to nearly 100 times that of the black hole’s regular output. The flares also have been seen in infrared data from the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile.
“People have had doubts about whether asteroids could form at all in the harsh environment near a supermassive black hole,” said Kastytis Zubovas from the University of Leicester in the United Kingdom. “It’s exciting because our study suggests that a huge number of them are needed to produce these flares.”
Zubovas and his colleagues suggest there is a cloud around Sgr A* containing trillions of asteroids and comets, stripped from their parent stars. Asteroids passing within about 100 million miles (160 million kilometers) of the black hole, roughly the distance between Earth and the Sun, would be torn into pieces by the tidal forces from the black hole.
These fragments then would be vaporized by friction as they pass through the hot, thin gas flowing onto Sgr A* – similar to a meteor heating up and glowing as it falls through Earth’s atmosphere — a flare is produced, and the remains of the asteroid are eventually swallowed by the black hole.
“An asteroid’s orbit can change if it ventures too close to a star or planet near Sgr A*,” said Sergei Nayakshin, also of the University of Leicester. “If it’s thrown toward the black hole, it’s doomed.”
The scientists estimate that it would take asteroids larger than about 6 miles (10km) in radius to generate the flares observed by Chandra. Meanwhile, Sgr A* also may be consuming smaller asteroids, but these would be difficult to spot because the flares they generate would be fainter.
These results reasonably agree with models estimating how many asteroids are likely to be in this region, assuming that the number around stars near Earth is similar to the number surrounding stars near the center of the Milky Way.
“As a reality check, we worked out that a few trillion asteroids should have been removed by the black hole over the 10-billion-year lifetime of the galaxy,” said Sera Markoff from the University of Amsterdam in the Netherlands. “Only a small fraction of the total would have been consumed, so the supply of asteroids would hardly be depleted.”
Planets thrown into orbits too close to Sgr A* also should be disrupted by tidal forces, although this would happen much less frequently than the disruption of asteroids because planets are not as common. Such a scenario may have been responsible for a previous X-ray brightening of Sgr A* by about a factor of a million about a century ago. While this event happened many decades before X-ray telescopes existed, Chandra and other X-ray missions have seen evidence of an X-ray “light echo” reflecting off nearby clouds, providing a measure of the brightness and timing of the flare.
“This would be a sudden end to the planet’s life, a much more dramatic fate than the planets in our solar system ever will experience,” Zubovas said.
Long observations of Sgr A* will be made with Chandra later in 2012 that will give valuable new information about the frequency and brightness of flares and should help test the model proposed here to explain them. This work could improve understanding about the formation of asteroids and planets in the harsh environment of Sgr A*.