in the constellation Sagittarius
Roughly 10 times the mass of the Sun
Diameter roughly equal to the size of a large city
Most black holes are too feeble to draw attention to themselves. Many of them swim through space alone, so there is no companion star or disk of hot gas for astronomers to detect. And in most cases, even those with companions are just too faint to stand out.
Every once in a while, though, they shout their existence to the universe by flaring to hundreds or even thousands of times their normal brightness -- bright enough to attract the attention of astronomers and their eyes in space.
Such is the case with a possible black hole known as IGR J17497-2821.
On September 17, 2006, a European space telescope known as Integral detected a bright flare of gamma rays from near the center of the Milky Way galaxy. Other telescopes in space and on the ground quickly turned toward the outburst.
With the observations from these instruments, astronomers categorized the outburst as an X-ray nova.
Such a nova occurs in a system in which a black hole or neutron star is stealing hot gas from the surface of a nearby, usually bloated companion star. The gas spirals toward the compact object, forming a superhot accretion disk.
Sometimes, the disk becomes unstable and its material crashes toward the black hole. The gas piles up as it gets closer to the black hole, eventually triggering an explosion that produces a torrent of gamma rays and X-rays.
By watching the intensity of the outburst at different wavelengths, measuring how quickly it faded, and keeping an eye open for follow-up outbursts, astronomers determined that the outburst almost certainly came from a black hole and not a less-dense neutron star. Follow-up observations also identified the companion star is an orange giant, which is a bloated star that is nearing the end of its life.
Models of X-ray novae suggest the black hole could be about 10 times as massive as the Sun. Additional observations of the orbit of the black hole and the companion star will allow astronomers to refine that estimate. And they will continue to watch IGR J17497 to see if it gives another shout-out.
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This document was last modified: September 8, 2011.