Quasars, the brightest objects we're aware of, are powered by the supermassive black holes that are thought to reside at the center of every galaxy. But many galaxies fail to feed their black holes enough matter, leading to a body that's quiet and difficult to detect. Our own galaxy's central black hole, called Sgr A*, falls into the latter category. We can detect it at wavelengths up to the X-ray range, but it's dim enough that we'd have a hard time spotting it if it weren't so close.
That may be about to change, however. Astronomers have spotted a cloud of gas with a mass about three times that of Earth that's on a trajectory that will have it pass close to Sgr A* in 2013. When it does, it may feed matter into the black hole's accretion disk, powering a sudden surge in Sgr A*'s output.
Since Sgr A* doesn't emit much in the way of radiation, a lot of what we've learned about it comes from tracking the stars that orbit it at close range; many of these have eccentric orbits that take them very close to the black hole. The Very Large Telescope has a program set up to perform periodic observations of the stars in order to track their orbits closely. It was during these observations that the team "discovered an object moving at about 1,700 km/s along a trajectory almost straight towards Sgr A*."
Observations of its emissions indicated that the object was a gas cloud that was much more dense than the material that's typically found in the area, and cooler as well. It's not quite heading straight at Sgr A*, but it's on a highly eccentric orbit that will take it extremely close to the body—36 light hours by the summer of 2013 (for comparison, the Voyager spacecraft are over a dozen light hours from the Earth). As a result of this plunge, the black hole's gravity has been accelerating the gas within the time we've been observing it; its total velocity (including some motion that's not towards the black hole) has increased from 1,200 km/s to nearly double that speed over the last seven years.
In the nearly 20 years we've been observing Sgr A*, only two stars have ever come closer to it. But stars are held together by gravity; this cloud is too diffuse to have that sort of coherence. As a result, the authors expect that it will undergo dramatic changes as it blasts in to the neighborhood of the black hole. The shock of hitting the low-density, high-temperature gas will compress the cloud even as the black hole's gravity starts to stretch it out along the direction of its orbit. This could eventually split the cloud into multiple fragments, each of which may take a slightly different path around the black hole.
As these fragments reach the point in the orbit closest to black hole, its temperatures may reach 106K, hot enough for it to start emitting X-rays.
But that may not be the only fireworks. If the cloud does end up fragmenting, then there's a chance that one of the fragments will end up feeding into the accretion disk surrounding the black hole. "This could in principle release up to around 1048 erg over the next decade," the authors estimate. We'll have to wait and see, but you can be sure lots of electronic eyes will be watching.
In the meantime, when nothing in particular happens in 2012, the cloud's destruction may end up providing a new bit of excitement for doomsday aficionados.