The reason most black holes are difficult to detect, including the one at the center of the Milky Way, is that unless there’s a whole lot of stuff being pulled into them to create an accretion disk, they’re just silent and nearly invisible. If it weren’t the fact that our own black hole is so close, we probably wouldn’t be able to detect it at all. But in 2013, Sgr A*, our friendly neighborhood black hole will awaken when a massive gas cloud passes nearby.
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.
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