Nasa has discovered a previously unknown stellar-mass black hole in our Milky Way galaxy.

The Swift satellite recently detected a rising tide of high energy X-rays from a source a few degrees from the centre of the galaxy, towards the constellation Sagittarius.

While astronomers do not know its precise distance, they think it resides about 20,000 to 30,000 light-years away in the Milky Way's inner region.

"Bright X-ray novae are so rare that they're essentially once-a-mission events and this is the first one Swift has seen," the Nasa mission's principal investigator Neil Gehrels said.

"This is really something we've been waiting for."

An X-ray nova is a short-lived 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.

The rapidly brightening source triggered Swift's burst alert telescope twice on the morning of September 16, and once again the next day.

"The pattern we're seeing is observed in X-ray novae where the central object is a black hole.

"Once the X-rays fade away, we hope to measure its mass and confirm its black hole status," Boris Sbarufatti, an astrophysicist at Brera Observatory  who is working with other Swift team members, said.

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 LMXB, the gas in the disk spirals inward, heats up as it heads toward the black hole, and produces a steady stream of X-rays.

- © Fairfax NZ News