New black hole discovered in Milky Way galaxy by NASA’s Swift

     NASA’s Swift satellite has found evidence of the presence of a previously unknown stellar-mass black hole in our Milky Way galaxy. The satellite recently detected a rising tide of high-energy X-rays from a source toward the centre of our galaxy. The outburst, produced by a rare X-ray nova, announced the presence of a new black hole.

“Bright X-ray novae are so rare that they’re essentially once-a-mission events and this is the first one Swift has seen. This is really something we’ve been waiting for,” said Neil Gehrels, the mission’s principal investigator, at NASA’s Goddard Space Flight Center in Greenbelt, Md. 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. The rapidly brightening source triggered Swift’s Burst Alert Telescope twice on the morning of Sept. 16, and once again the next day. 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 20,000 to 30,000 light-years away in the galaxy’s inner region. 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 nova peaked in hard X-rays - energies above 10,000 electron volts, or several thousand times that of visible light - on Sept. 18, when it reached an intensity equivalent to that of the famous Crab Nebula, a supernova remnant that serves as a calibration target for high-energy observatories and is considered one of the brightest sources beyond the solar system at these energies. Even as it dimmed at higher energies, the nova brightened in the lower-energy, or softer, emissions detected by Swift’s X-ray Telescope, a behavior typical of X-ray novae. By Wednesday, Swift J1745-26 was 30 times brighter in soft X-rays than when it was discovered and it continued to brighten. “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,” said Boris Sbarufatti, an astrophysicist at Brera Observatory in Milan, Italy, who currently is working with other Swift team members at Penn State in University Park, Pa.

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