Chandra Found 84 Objects That Shine in the Softest X-Rays and Vanish in the Hard Ones. Nobody Knows What They Are.

Astronomers combing through two decades of archived data from NASA's Chandra X-ray Observatory have identified a new class of cosmic objects that behave unlike anything catalogued before: they blaze in the lowest-energy X-rays Chandra can detect, disappear entirely in higher-energy images, and by inference pump out enormous amounts of extreme ultraviolet light. The team calls them hypersoft X-ray sources, and it says they may resolve two of astrophysics' longest-standing puzzles at once.

"We've never encountered a group of objects that act like this," said Mustafa Muhibullah of the University of Alabama, who led the study published Wednesday in Nature Astronomy. "Of course, the next step was to try to figure out what these things are."

The researchers found 84 of the sources across six galaxies: the two big nearby spirals, M31 in Andromeda and M101, the Pinwheel, plus four elliptical galaxies. The method was simple in principle. They looked for point sources that appeared in Chandra's softest energy band and vanished in the harder bands. Because soft X-rays sit right next to energetic ultraviolet on the spectrum, an object that is bright only in soft X-rays is almost certainly a prodigious ultraviolet emitter too. The sources turned up in regions of active star formation and in older stellar populations alike, which argues against any single exotic explanation tied to young stars.

The team's best guess is that each is a binary system in which a compact object, a white dwarf, neutron star or black hole, is pulling gas off a companion star. That gas heats up as it spirals inward and radiates X-rays before it lands. Such accreting binaries are common, but none seen before combine this much ultraviolet output with so little hard X-ray emission. "These clandestine X-ray sources are actually among the most energetic objects in galaxies, and they could be solving two cosmic mysteries at once," Muhibullah said.

The first mystery is the origin of Type Ia supernovae, the exploding white dwarfs whose uniform brightness let astronomers measure cosmic distances and, in 1998, discover that the expansion of the universe is accelerating. Theory says a white dwarf that steals enough mass from a companion should detonate, but decades of searching have failed to catch such a system in the act before it blows. "If we could find a way to spot these Type Ia supernova explosions before they go off, that would be really important," said co-author Jimmy Irwin of the University of Alabama. "Right now, we study them after they've exploded, and astronomers have struggled to understand what is actually ignited." A population of ultraviolet-bright, mass-gaining white dwarfs is exactly what that search has been missing.

The second mystery is what ionizes the diffuse gas between stars in some galaxies, stripping electrons from atoms in a way that regulates how fast new stars can form. Hot, massive stars do part of the job but not all of it. The extreme ultraviolet flood from dozens of hypersoft sources per galaxy could account for the shortfall.

The reason they went unnoticed is a double blind spot. Soft X-rays are the hardest for X-ray telescopes to register cleanly, and the ultraviolet the sources emit is soaked up almost completely by the hydrogen and helium that fills interstellar space, so it never reaches Earth directly. "By combing through the Chandra archive, we were able to eliminate what used to be a blind spot for telescopes," said co-author Rosanne Di Stefano of the Center for Astrophysics | Harvard & Smithsonian. "That's how we found what appears to be a new class of cosmic objects with remarkable qualities." The 84 objects came from public data anyone could have downloaded; the discovery was in knowing which energy band to subtract from which.