ROSAT eyes the extreme-ultraviolet universe.

Since the research satellite ROSAT began surveying the heavens in July 1990, its ultraviolet telescope, known as the Wide-Field Camera, has spotted some 1,000 sources of extreme-ultraviolet radiation. Researchers are now focusing on 385 of the brightest of these sources, which will serve as guideposts for the EUVE mission, reports ROSAT investigator Kenneth A. Pounds of the University of Leicester in England.

The camera, he notes, has revealed several surprises about white dwarfs, the dense remnants of stars up to a few times the mass of the sun. Though the instrument has detected more than 100 white dwarfs in the extreme ultraviolet, it has found only 15 -- half as many as expected -- with surface temperatures greater than 20,000 kelvins. Moreover, the hottest dwarfs imaged by the camera appear surprisingly faint.

Puzzling over the data, theorists now speculate that the energetic radiation from a hot dwarf thrusts into the star's lower atmosphere atoms of carbon, nitrogen and oxygen that normally reside below the visible surface of the star. The raised atoms act as a screen, preventing little, if any, of the dwarf's extreme-ultraviolet radiation from reaching ROSAT. Hydrogen and helium make up the bulk of a white dwarf's lower atmosphere, while carbon and heavier atoms contribute less than one ten-thousandth of its mass. Nonetheless, the findings suggest that these heavier atoms may dim or completely mask the light of a hot dwarf, Pounds says.

White dwarfs also seem to star in another ROSAT mystery. Pounds and his colleagues found that some 20 hot young stars, called A stars, appear to have energetic, ultraviolet-emitting upper atmospheres, or coronas, even though astronomers believe such stars lack the ability to heat these regions. Pounds and his co-workers examined several of the A stars with a high-resolution, visible-light telescope and found indications that an unseen companion orbited each of the stars. He suspects that the hidden stars, rather than the A stars, are the source of the puzzling ultraviolet radiation.

Pounds believes the hidden companions are white dwarfs, since they glow brilliantly only in the extreme ultraviolet. "There's a whole population of white dwarfs hiding away that we were not aware of," he says. One intriguing possibility, notes Pounds, is that white dwarfs locked in orbit with another star might evolve differently from dwarfs that lack a partner.

ROSAT has also glimpsed a dozen extreme-ultraviolet sources far beyond the Milky Way -- possibly quasars and active galactic nuclei. The mere fact that ROSAT can detect such distant sources supports the notion that hydrogen atoms, which readily absorb extreme-ultraviolet light, do not blanket our galaxy uniformly. In addition, says Pounds, the observations suggest that extreme-ultraviolet sources at the core of distant galaxies have enough energy to burn off most of the hydrogen and other light-absorbing gases surrounding them, enabling the radiation to reach the Milky Way.

But the biggest surprises, Pounds says, may lie among the 15 percent of extreme-ultraviolet sources detected by ROSAT that have no known counterpart at any other wavelength. Some researchers have theorized that the sources are elderly, isolated neutron stars -- superdense, burned-out stars that represent the final stage of evolution for stars with three to about eight times the mass of the sun. Young, rotating neutron stars that have large magnetic fields typically emit beams of radio waves, but as these stars age they slow down and radiate far less at these wavelengths.

Galaxies probably serve as burial grounds for countless of these elderly neutron stars, and emissions in the extreme ultraviolet may represent a key way to map them in the Milky Way, Pounds speculates. Though this older population would appear silent at radio wavelengths, these dense stars would still suck in material around them, emitting ultraviolet radiation in the process.

Recently, Pounds notes, some of his colleagues have begun to consider an intriguing idea that could provide an answer to another astronomical puzzle. Some of the unidentified sources detected by ROSAT may coincide with the location of gamma ray bursters, objects that unleash flashes of high-energy photons and then vanish. The highly uniform distribution of gamma ray bursts detected last year by the Compton Gamma Ray Observatory has surprised researchers because one likely source of the emissions -- known, radio-emitting neutron stars in our Milky Way -- clusters along the plane of our galaxy and could not produce such a radiation pattern (SN: 9/28/91, p.196). But an old, never-before-detected population of neutron stars scattered throughout the Milky Way might account for the uniform distribution of bursters -- as well as some of the unidentified sources found by ROSAT.