Chandra Finds 84 'Hypersoft' X-ray Objects in Six Galaxies

Chandra spotted the 84 suspicious objects emitting very low-energy X-rays in six galaxies, including four evolved elliptical galaxies and two spiral galaxies: the Andromeda Galaxy, or M31, and the Pinwheel Galaxy, or M101.

"We've never encountered a group of objects that act like this," Mustafa Muhibullah of the University of Alabama said in a statement, according to Space.com.

Compact objects—black holes, neutron stars, and white dwarfs—are the remains of dead stars. Their small size and powerful gravity allow them to approach orbiting companion stars and pull away the stars' outer layers of gas. The gas then swirls around the compact object and falls onto its surface, a process that usually produces high-energy X-rays because the captured gas becomes superheated by the compact object's gravitational pull.

Because the newly found objects produce weak X-rays, Muhibullah's team calls them hypersoft X-ray sources. Low-energy X-rays and ultraviolet light are adjacent on the electromagnetic spectrum, so the team suspects the low-energy X-rays are spillover from objects emitting large amounts of ultraviolet. The researchers also think these objects are lower-level compact binaries.

Such objects are difficult to find because the interstellar medium—hydrogen and helium gas between stars—absorbs ultraviolet light. That may explain why no hypersoft X-ray sources have been found in our own galaxy: observers must look through the interstellar medium along the plane of the Milky Way. As a result, a large population of hypersoft X-ray sources could remain undiscovered and could have an important effect on the universe around them, according to the report.

The findings have been published in the journal Nature Astronomy, Space.com reported.