NASA Goddard Space Flight Center
December 14, 2005
Scientists using NASA's Swift satellite have found evidence of a black hole swallowing a neutron star. The discovery is reported in the December 15 issue of Nature.
This rare event, seen on July 24, created a gamma-ray burst that lasted only for a few milliseconds. However, observations of the lingering afterglow provided evidence of what could have been the demise of a neutron star orbiting a black hole.
The black hole may have first stretched the dense neutron star into a crescent and broke off pieces in the process. The black hole could have then swallowed the star largely in one gulp, feeding on the broken off pieces in the minutes and hours that followed. Such a black hole would grow more massive.
"For billions of years this black hole and neutron star orbited each other in a gravitational tug-of-war," said Dr. Scott Barthelmy of NASA's Goddard Space Flight Center in Greenbelt, Md., lead author on one of three Nature articles on this subject. "The neutron star lost."
In recent months, the Swift team has reported that "short" gamma-ray bursts arise from a merger either between two neutron stars or a neutron star and black hole. The specific scenario was not clear. Although not definitive, this latest analysis of the July burst is the best evidence of a black hole - neutron star merger, said Barthelmy.
A neutron star is the core remains of an exploded star once about 10 to 25 times more massive than our sun. It contains about a sun's worth of mass crammed into a sphere only about 12 miles across. A black hole is the core remains of an even larger exploded star, over 25 times the mass of the sun.
The July burst, called GRB 050724, was one of the most thoroughly observed short gamma-ray bursts to date. Swift, NASA's Chandra telescope and the Keck Observatory in Hawaii, along with other observatories, captured the burst afterglow in detail. The combined data enabled scientists to speculate on the nature of the merging objects.
If GRB 050724 were a neutron star merger, according to current models, there would not be many pieces falling into a black hole later. The two objects would smash, instantly form a black hole, and after a modest afterglow no more light would be seen. Similarly, two black holes would smash and release very little residual light. But GRB 050724 had a long, flaring afterglow.
Prof. Peter Meszaros of Penn State University, University Park, Pa., and Dr. Bing Zhang of the University of Nevada, Las Vegas, co-authors on the Barthelmy Nature article, theorize that smaller flares of X-ray light and optical light, detected in the first tens of seconds after GRB 050724, could have resulted from pieces of the neutron star falling into the black hole. Flares occurring later might be from magnetized pieces of gas, which would behave differently. Supporting this merger scenario is the fact that the GRB 050724 burst took place in the outskirts of an old, elliptical galaxy filled with neutron stars and black holes.
"There's only one thing I know of that could rip apart a neutron star with bits flying out, and that's a black hole. Now we have the first evidence that this might actually be occurring," said Meszaros.
Numerical simulations by Drs. Melvyn Davies and Andrew King and others at Leicester University, England, have provided evidence for such a disruption of a neutron star by a black hole, including the late infall of pieces of matter. Other simulations elsewhere indicate conversely that neutron star mergers would leave no flaring afterglow.
Drs. Nial Tanvir of the University of Hertfordshire in Hatfield, England, and Edo Berger of the Carnegie Observatories in Pasadena, Calif., are lead authors on the two accompanying Nature articles, which describe follow-up observations after Swift's detection of GRB 050724.
Swift, launched in November 2004, is a NASA mission in partnership with the Italian Space Agency and the Particle Physics and Astronomy Research Council, United Kingdom, and is managed by Goddard Space Flight Center. Penn State University personnel control science and flight operations from the Mission Operations Center.
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