UNIVERSITY PARK, Pa. -- Two studies in the Aug. 25 issue of the journal Nature provide new insights into a cosmic accident that has been streaming X-rays toward Earth since late March. The X-rays are being produced by a distant black hole as it slowly devours a Sun-like star. "Incredibly, this source is still producing X-rays and may remain bright enough for the Swift satellite to observe into next year. It behaves unlike anything we've seen before," said David Burrows, professor of astronomy at Penn State University and the lead scientist for the Swift observatory's X-Ray Telescope instrument.
NASA's Swift satellite, which is controlled by Penn State from the Mission Operations Center near the University Park campus, was the first to alert astronomers to the intense and unusual high-energy flares from this new source in the constellation Draco. Astronomers soon realized that the source, which is now known as Swift J1644+57, was the result of a truly extraordinary event -- the awakening of a distant galaxy's dormant black hole as it shredded and consumed an errant star. The galaxy is so far away that the light from the blast has traveled 3.9 billion years before reaching Earth.
Burrows led the team that produced one of the Nature papers about the unusual object. This paper highlights the X-ray and gamma-ray observations from Swift and other detectors, including the Japan-led Monitor of All-sky X-ray Image (MAXI) instrument aboard the International Space Station. The team led by Burrows includes scientists from NASA and other institutions in the United States, United Kingdom, Italy, Japan, Taiwan, and the Republic of Korea.
The second study, led by Ashley Zauderer, a post-doctoral fellow at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., examines the unprecedented outburst through observations from numerous ground-based radio observatories, including the National Radio Astronomy Observatory's Expanded Very Large Array (EVLA), a Y-shaped configuration of telescopes near Socorro, N.M.
Most galaxies, including our own, possess a central, super-sized black hole that is millions of times the Sun's mass. According to the new studies, the black hole in the galaxy hosting Swift J1644+57 may be twice the mass of the black hole lurking at the center of our own Milky Way galaxy.
As a star falls toward a black hole, it is ripped apart by intense tides. The gas is corralled into an accretion disk that swirls around the black hole and becomes rapidly heated to temperatures of millions of degrees. The innermost gas in the disk spirals toward the black hole, where rapid motion and magnetism create dual, oppositely directed "funnels" through which some particles may escape. Jets driving matter at velocities greater than 90 percent the speed of light form along the black hole's spin axis. In the case of Swift J1644+57, one of these jets happened to point straight at Earth.
When first detected March 28, the flares initially were assumed to signal a gamma-ray burst, one of the nearly daily short blasts of high-energy radiation often associated with the death of a massive star and the birth of a black hole in the distant universe. But as the emission continued to brighten and flare, astronomers realized that the most plausible explanation was the tidal disruption of a Sun-like star seen as emission beamed toward Earth.
The brightness and energy of a black hole's jet is greatly enhanced when viewed head-on. The phenomenon, called relativistic beaming, explains why Swift J1644+57 was seen at X-ray energies and appeared so strikingly luminous, even though theoretical studies of tidally disrupted stars suggested they would appear as flares at the comparatively lower optical and ultraviolet energies.
By March 30, EVLA observations by Zauderer's team showed a brightening radio source centered on a faint galaxy near Swift's position for the X-ray flares. These data provided the first conclusive evidence that the galaxy, the radio source, and the Swift event were linked. "The radio emission occurs when the outgoing jet slams into the interstellar environment," Zauderer explained. "By contrast, the X-rays arise much closer to the black hole, likely near the base of the jet."
"Our observations show that the radio-emitting region is still expanding at more than half the speed of light," said Edo Berger, an associate professor of astrophysics at Harvard and a coauthor of the radio paper. "By tracking this expansion backward in time, we can confirm that the outflow formed at the same time as the Swift X-ray source."
Swift, launched in November 2004, is managed by NASA's Goddard Space Flight Center in Maryland. It is operated in collaboration with Penn State University in Pennsylvania, the Los Alamos National Laboratory in New Mexico, and Orbital Sciences Corporation in Virginia, with international collaborators in the United Kingdom., Italy, Germany and Japan. MAXI is operated by the Japan Aerospace Exploration Agency as an external experiment attached to the Kibo module of the space station.
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