Check the X-rays

A tsunami of high-energy X-rays streaming toward Earth from a rare X-ray nova has revealed the presence of a previously unknown black hole near the center of our Milky Way galaxy. A team of scientists including Penn State astronomers detected the X-ray event with NASA’s Swift satellite, whose science and flight operations are controlled by Penn State from the Mission Operations Center near the University Park campus.

“Many unknown potential X-ray novae lurk in our galaxy undetected until they undergo extremely dramatic bright outbursts in X-ray,” says Jamie Kennea, science operations team leader for NASA’s Swift Mission and a researcher at Penn State, “Swift waits patiently for these sources to reveal themselves, and announces them to the world within minutes of their detection.”

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. In this case, the rapidly brightening source triggered Swift’s Burst Alert Telescope twice on the morning of Sept. 16 and once again the next day. 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.

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 is about 20,000 to 30,000 light-years from Earth 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 the nova in visible light.

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. Soon, 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,” says Boris Sbarufatti, an astrophysicist at Brera Observatory in Milan, Italy, who currently is working with other Swift team members at Penn State. “Once the X-rays fade away, we hope to measure its mass and confirm its black-hole status.”

Black sky with red objects, with labels showing objects of significance.
NASA/Goddard Space Flight Center/S. Immler and H. Krimm

Swift J1745-26 with labels and scale of moon as it would appear in the field of view from Earth. [High-resolution version]

The black hole must be a member of a low-mass X-ray binary (LMXB) system, which includes a normal, Sun-like star, its discoverers say. 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 conditions, 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,” says John Cannizzo, a NASA 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.” This phenomenon, called the thermal-viscous limit cycle, helps astronomers explain transient outbursts across a wide range of systems, from protoplanetary disks around young stars, to dwarf novae—where the central object is a white dwarf star—and even bright emission from supermassive black holes in the hearts of distant galaxies.

Jamie Kennea, Ph.D., is science operations team leader for NASA's Swift Mission and a research associate at Penn State, jak51@psu.edu.

The Swift Observatory, launched in November 2004, is managed by Goddard Space Flight Center, and operated in collaboration with Penn State, the Los Alamos National Laboratory in New Mexico, and Orbital Sciences Corporation, with international collaborators in the United Kingdom and Italy and including contributions from Germany and Japan. The science and flight operations of the Swift Observatory are controlled by Penn State. Swift's X-Ray Telescope and UV/Optical Telescope were developed and built by international teams led by Penn State.

Last Updated November 16, 2012