Astronomers are shining new light on a stellar odd couple, one partner being "consumed" by the other, nestled among thousands of suns at the center of a star cluster in the Milky Way. The strange pair are emitting intense radiation in both ultraviolet light and in X-rays.
New observations from the Hubble Space Telescope provide "definitive confirmation" that both sources of energy are the same binary system: a dead white dwarf star orbiting an even more exotic neutron star. The neutron star is five miles across with a mass 1 1/2 times that of the Sun. The orbital period of just 11 minutes is the shortest of any known binary system, indeed so short that the white dwarf is moving at a speed of nearly three million miles an hour.
Using the Faint Object Spectrograph aboard the Hubble telescope, a team led by Scott Anderson and Bruce Margon, associate research professor and professor of astronomy at the University of Washington, have obtained the first ultraviolet spectra of the powerful energy source. The observations confirm that a powerful X-ray source and a previously observed ultraviolet radiation source are one and the same binary star. The binary is located in NGC6624, a globular cluster of stars bound together by gravity in the constellation Saggitarius, about 26,000 light years from Earth.
The latest discoveries of this much-studied astronomical puzzle are being presented in a poster-paper at the American Astronomical Society's national meeting today in Toronto. The findings are being submitted to the Astrophysical Journal.
The NGC6624 binary consists of a neutron star, the highly dense and compact remnant of a massive exploded star, whose intense gravity is slowly sucking material from the surface of its companion, the white dwarf. This burned out corpse of a Sun-like star is orbiting only 80,000 miles distant from the neutron star.
Previous researchers had recorded 11-minute changes in X-ray emission from the binary, corresponding to one orbit of the binary pair. Astronomers predicted that there would be a corresponding variation in ultraviolet light, with the ultraviolet emission brightening then dimming every 11 minutes. The Anderson team used the Hubble telescope to detect this predicted change in the ultraviolet brightness.
Anderson's team found, however, that the ultraviolet radiation has peaks five to six times stronger than those previously seen in X- rays. Scientists speculate that as gas is sucked from the white dwarf, it accumulates in a disc, and then spirals toward the neutron star. The intense heat creates strong X-rays, some of which are radiated back to strike the side of the white dwarf that faces toward the neutron star.
This X-ray "heated" side, Anderson explains, is alternately hidden and then visible to the Hubble telescope as the white dwarf orbits the neutron star, giving the effect of peaks of ultraviolet radiation every 11 minutes. It is this 'dimmer switch' effect that gives confirmation of the presence of the white dwarf.
This vast energy source in the globular cluster NGC6624 first attracted attention in mid-1970s when it was identified as one of the first star clusters emitting X-rays. "These X-rays were a clear signature of a neutron star," says Margon.
However, says Anderson, it proved "frustratingly difficult" to detect radiation from the binary system from Earth-based telescopes. The Earth's atmosphere screens out ultraviolet light and also blurs images of astronomical objects. This blurring creates particularly serious problems in the case of globular clusters because they are so crowded with stars.
In 1993, researchers using the orbiting Hubble telescope identified an intensely ultraviolet object in the core of NGC6624. This radiation, they theorized, was coming from the same X-ray source. "The ultraviolet emission was so intense," says Anderson, "it would be hard to imagine it was not the right identification for the X-ray source." However, in 1993 astronomers did not have enough data to look for the 11-minute orbital period in the ultraviolet light.
Anderson believes that such research may yield clues to the evolution of binary star systems in globular clusters. He asks: "Were they formed when the globular cluster was forming, or through gravitational capture of one star by another star after the cluster formed?" One of the major questions to be answered about the NGC6624 binary, Anderson says, is whether the two stars are about to coalesce, and in doing so turn into some other sort of exotic object.
The research was supported by a grant from NASA. Other researchers on the team are Ronald Downes of the Space Telescope Science Institute, Baltimore; Richard Allen of the University of Arizona, and Eric Deutsch, a graduate student at the University of Washington.