Chandra Finds Hidden X-Ray Sources

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NASA's Chandra X-ray Observatory has helped astronomers find a new group of unusual objects in other galaxies. Scientists call them hypersoft X-ray sources because they produce many low-energy X-rays. They also produce strong ultraviolet light. Researchers say they have not seen a group of objects behaving in exactly this way before.

The team searched public data from Chandra and found 84 of these sources in six galaxies. Two of the galaxies, Andromeda and the Pinwheel Galaxy, are spiral galaxies. The other four are elliptical galaxies. The objects appeared in areas with young stars and also in places with older stars. This suggests the sources may exist in different kinds of stellar environments.

Scientists do not yet know exactly what creates the signals. One idea is that a black hole, neutron star, or white dwarf is pulling material from a companion star. As the material becomes hot, it can release X-rays before falling onto the compact object. Similar binary systems are already known, but these newly identified sources have an unusual combination of very soft X-rays and bright ultraviolet radiation.

The discovery may help with two important questions. Some white dwarf systems may later become a Type Ia supernova, an explosion that astronomers use to measure the expansion of the universe. Researchers have long tried to identify the systems before they explode. The strong ultraviolet light may also help explain how gas between stars becomes ionized gas, meaning electrons are removed from atoms.

The researchers found the objects by studying old Chandra observations in a new way. Low-energy X-rays are difficult to detect, and ultraviolet light can be absorbed by gas between stars. By looking carefully at the archive, the team found a population that had remained hidden. The next step is to learn what these objects really are and how common they may be.