An X-ray burster's visible trail.

Searching a crowded cluster of stars with the Hubble Space Telescope, astronomers have found the visible-light counterpart to perhaps the most baffling X-ray-emitting source in the Milky Way. The discovery may provide new clues about the orientation and composition of this double star, which has intrigued astronomers for two decades.

This stellar system made headlines in 1975 when the Astronomical Netherlands Satellite revealed it as the first known X-ray burster in the heavens. Subsequent studies with the orbiting Einstein Observatory brought a second claim to fame. That work revealed that the two components of this double star - a neutron star, the collapsed core of a massive star, and a white dwarf - lie only 160,000 kilometers apart, less than half the Earth-moon distance. The tiny separation qualifies this duo as the most compact binary ever found.

Despite these properties, the search foravisible counterpart to the X-ray signal proved elusive because the binary resides in the globular cluster NGC 6624, a dense knot of stars 28,000 lightyears from Earth. Ground-based telescopes couldn't pick out the star's dim optical emission from the crowd.

While using Hubble's Faint Object Camera to search for visible-light emissions, Ivan R. King of the University of California, Berkeley, and his colleagues also happened to take an ultraviolet image of the cluster. That proved fortunate, because the binary, though dim in visible light, far outshines neighboring stars in the ultraviolet. Comparing the ultraviolet and visible-light images, the researchers easily identified the source. Kingand hiscolleaguesreporttheirfindingsintheaug. 20 ASTROPHYSICAL JOURNAL LETTERS.

According to King and other astronomers, the double star emits X-rays because its neutron star gravitationally rips helium from the white dwarf partner. The helium forms a swirling disk around the neutron star and some of it spirals onto the star's surface. The energy unleashed by the infalling matter produces a steady stream of X-rays occasionally interrupted by X-ray bursts. Scientists conjecture that this radiation heats the surrounding disk, prompting it to emit the ultraviolet and visible light detected by Hubble.

Preliminary measurements of the intensity of this light indicate that the disk has a tilt of perhaps 45' relative to the line of sight of Earthbound observers, King says.

He adds that the Hubble images may solve a long-standing puzzle about the double star. As the neutron star steals more mass from the white dwarf, the dwarf recedes. Thus the time it takes for the two partners to orbit each other should slightly lengthen. But X-ray astronomers found just the opposite.

Hubble shows that the binary lies only about 0.1 light-year from the dense core of the globular cluster, much closer than thought. The core thus exerts a stronger tug on the burster, and if it pulls the burster toward Earth, the motion would make it appear as if the double star's period is decreasing rather than increasing, King speculates.