On Sept.
16, NASA's Swift satellite detected a rising tide of high-energy X-rays from a source toward the center of our Milky Way galaxy.
The outburst, produced by a rare X-ray nova, announced the presence of a previously unknown stellar-mass black hole.
An X-ray nova is a short-lived X-ray source that appears suddenly, reaches its emission peak in a few days and then fades out over a period of months.
The outburst arises when a torrent of stored gas suddenly rushes toward one of the most compact objects known, either a neutron star or a black hole.
Named Swift J1745-26 after the coordinates of its sky position, the nova is located a few degrees from the center of our galaxy toward the constellation Sagittarius.
While astronomers do not know its precise distance, they think the object resides about 20000 to 30000 light-years away in the galaxy's inner region.
The pattern of X-rays from the nova signals that the central object is a black hole.
Ground-based observatories detected infrared and radio emissions, but thick clouds of obscuring dust have prevented astronomers from catching Swift J1745-26 in visible light.
The black hole must be a member of a low-mass X-ray binary (LMXB) system, which includes a normal, sun-like star.
A stream of gas flows from the normal star and enters into a storage disk around the black hole.
In most LMXBs, the gas in the disk spirals inward, heats up as it heads toward the black hole, and produces a steady stream of X-rays.
But under certain ...
