TL;DR: Scientists using NASA's Chandra X-ray Observatory found 84 hypersoft X-ray sources in six nearby galaxies that shine brightly in very low-energy X-rays and extreme ultraviolet light.

Scientists have identified a new class of cosmic objects that behave unlike any group seen before, and the find may bear on two long-standing questions in astrophysics. Using data openly available in the Chandra archive, researchers located 84 objects called hypersoft X-ray sources in six galaxies, according to NASA Science.

The announcement was featured in a paper published Wednesday in Nature Astronomy, according to the Chandra Press Room. The lead author is Mustafa Muhibullah of the University of Alabama, with co-authors Jimmy Irwin, also of the University of Alabama, and Rosanne Di Stefano of the Center for Astrophysics | Harvard & Smithsonian.

84 sources found in six galaxies

The team searched six galaxies and found hypersoft sources in each search area. Two are spiral galaxies, M31, the Andromeda galaxy, and M101, the Pinwheel galaxy, while the other four are elliptical galaxies, according to NASA Science.

The objects were present in both regions of active star formation and areas containing older stars. That spread matters because it shows the phenomenon is not confined to one type of stellar neighborhood.

In M101, seven of the newly discovered objects are called out in an annotated composite image. The underlying view combines X-ray data from Chandra with optical data, showing spiral arms in shades of purple around a golden yellow core, with scores of tiny white and purple specks scattered along and between the arms.

To a casual observer, the mystery specks look like the other specks of light in the galaxy. In the annotated version, red circles mark the unusual objects for easy identification, according to NASA Science.

What makes hypersoft sources different

The label hypersoft refers to unusually low-energy X-ray output. The team spotted the sources by finding objects that appeared in Chandra images taken at the lowest X-ray energies but vanished in higher-energy images.

That pattern means these objects give off far more low-energy X-rays than high-energy ones. Because low-energy X-rays border energetic ultraviolet radiation on the electromagnetic spectrum, the researchers determined that the sources are also producing large amounts of energetic ultraviolet radiation, according to NASA Science.

The abstract in Nature Astronomy describes them as luminous, point-like, non-nuclear X-ray objects detected primarily or exclusively below 0.3 keV. Typical X-ray binaries emit primarily above 0.3 keV, whereas cooler sources peaking in the extreme ultraviolet have remained challenging to detect because of a long-standing observational blind spot.

The most luminous examples radiate near 10^38 erg s-1 in the narrow X-ray band, with spectral models suggesting even higher bolometric luminosities, most of which emerge in the extreme ultraviolet, according to Nature Astronomy. The paper ranks them among the most energetic sources in galaxies, even though their spectra, which peak in the extreme ultraviolet, evaded earlier detections.

“We’ve never encountered a group of objects that act like this,” said Muhibullah, according to NASA Science. “Of course, the next step was to try to figure out what these things are.”

Likely binaries, but unusually bright in ultraviolet

It remains unclear what types of objects are responsible for the low-energy X-rays and intense ultraviolet radiation. The team thinks they most likely involve a black hole, neutron star, or white dwarf pulling material from a companion star.

In that picture, material pulled from the companion star is heated up to produce X-rays before falling onto the white dwarf or neutron star, or into the black hole. Such binary systems have been seen before, but not with such bright ultraviolet radiation and low-energy X-rays, according to NASA Science.

X-ray binaries powered by accretion onto black holes, neutron stars or white dwarfs from companion stars are among the brightest non-explosive beacons in galaxies, outshining the Sun by millions of times in luminosity, according to Nature Astronomy. The new paper proposes that hypersoft sources represent X-ray binaries spanning several physical classes, including accreting white dwarfs or post-nova systems and systems hosting accreting black holes.

The discovery suggests that there may be large populations of binary systems with energetic ultraviolet radiation that have been undetected until now.

“These clandestine X-ray sources are actually among the most energetic objects in galaxies, and they could be solving two cosmic mysteries at once,” said Muhibullah, according to the Chandra Press Room.

Mystery 1: The stars behind Type Ia supernovae

One mystery concerns the origins of Type Ia supernovae. Scientists think that some white dwarf systems pulling material from companion stars may eventually explode as a supernova known as Type Ia.

Those explosions are critical for measuring the expansion of the universe. These supernovae played a key role in discovering that this expansion is accelerating. Astronomers have been looking for the stars that turn into Type Ia supernovae for many years, so far without success, according to NASA Science.

Accreting white dwarfs or post-nova systems in the hypersoft population are potential progenitors of Type Ia supernovae, according to Nature Astronomy.

“If we could find a way to spot these Type Ia supernova explosions before they go off, that would be really important,” said Irwin, according to NASA Science. “Right now, we study them after they’ve exploded, and astronomers have struggled to understand what is actually ignited.”

The statement reflects the current research challenge: the explosions are studied after they occur, while the pre-explosion systems have been difficult to identify. The new sources do not by themselves establish which objects explode, but they add a population that researchers can compare with progenitor models.

Mystery 2: What ionizes gas between stars

The other mystery is what strips electrons from gas between the stars in some galaxies. This stripping of electrons is important to probe because it can affect how quickly stars form and influence the life cycles of galaxies.

Hot, massive stars play a role, but they do not completely explain what is causing this stripping. The intense levels of ultraviolet radiation from the hypersoft X-ray sources may play a vital role, according to NASA Science.

The Nature Astronomy abstract similarly states that beyond their elusive nature, the sources may play a crucial role in ionizing gas within galaxies.

This is not a claim that the ionization question is closed. The researchers frame the ultraviolet output as a potentially important contribution that needs further study alongside the light from known hot stars.

Why telescopes missed them until now

Two effects combined to hide the population. In addition to the low-energy X-ray output, which is very difficult for X-ray telescopes to detect, the high-energy ultraviolet radiation is readily absorbed by helium and hydrogen gas that fills the space between the stars, creating a nearly impenetrable barrier to look through, according to the Chandra Press Room.

The Nature Astronomy abstract describes cooler sources peaking in the extreme ultraviolet as challenging to detect because of a long-standing observational blind spot. The new work attacked that blind spot by re-examining archival Chandra observations sensitive to the lowest X-ray energies.

“By combing through the Chandra archive, we were able to eliminate what used to be a blind spot for telescopes,” said Di Stefano, according to NASA Science. “That’s how we found what appears to be a new class of cosmic objects with remarkable qualities.”

The Chandra data supporting the findings are available through the Chandra Data Archive, with a minimum dataset for reproducing the analyses posted through Zenodo, according to Nature Astronomy.

How the mission and archive made the search possible

NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts, according to NASA Science.

For this study, the key resource was not a new pointing alone but data openly available to the public in the Chandra archive. The researchers used those holdings to search six galaxies for objects visible only at the softest energies.

Media contacts listed for the release are Megan Watzke at the Chandra X-ray Center in Cambridge and Joel Wallace at Marshall Space Flight Center in Huntsville, according to the Chandra Press Room. More information about the mission is posted at https://www.nasa.gov/chandra.

The next research step is classification: determining which hypersoft sources are white dwarfs, neutron stars or black holes, how many belong to each group, and whether any subset can be tied directly to future supernovae or to the ionization of nearby interstellar gas.

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