A rising tide of high-energy X-rays was recently detected by NASA’s Swift satellite from a source toward the center of our Milky Way galaxy, and according to the space agency, that outburst produced by the rare X-ray nova announced the presence of a stellar-mass black hole that had remained unknown until now.
An X-ray nova is a short-lived X-ray source that appears suddenly. It reaches its emission peak in just a few days and then disappears 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, said scientists.
On the morning of Sept. 16, Swift’s Burst Alert Telescope was triggered twice by the rapidly brightening source. The same thing happened again the next day.
"Bright X-ray novae are so rare that they're essentially once-a-mission events and this is the first one Swift has seen," said Neil Gehrels, the mission's principal investigator, at NASA's Goddard Space Flight Center in Greenbelt, Md. "This is really something we've been waiting for."
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 exact distance, they think the object resides about 20,000 to 30,000 light-years away in the galaxy's inner region.
Although ground-based observatories detected infrared and radio emissions, astronomers couldn’t catch Swift J1745-26 in visible light due to the thick clouds of obscuring dust, according to the scientists.
On Sept. 18, the nova emitted hard X-rays with energies above 10,000 electron volts or several thousand times that of visible light 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.
"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.
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.
According to the scientists, 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.
However, under certain conditions, a stable flow within the disk depends on the rate of matter flowing into it from the companion star. At certain rates, the disk fails to maintain a steady internal flow and instead flips between two dramatically different conditions - a cooler, less ionized state, where gas simply collects in the outer portion of the disk like water behind a dam, and a hotter, more ionized state that sends a tidal wave of gas surging toward the center.
"Each outburst clears out the inner disk, and with little or no matter falling toward the black hole, the system ceases to be a bright source of X-rays," said John Cannizzo, a Goddard astrophysicist. "Decades later, after enough gas has accumulated in the outer disk, it switches again to its hot state and sends a deluge of gas toward the black hole, resulting in a new X-ray outburst."