Scientists found 84 strange objects hiding inside six galaxies. Their unusual X-ray emissions could change what we know about the universe

Scientists have discovered 84 unusual objects spread across six nearby galaxies that do not behave like the X-ray sources astronomers normally see. The objects produce unusually low-energy X-rays while apparently giving off large amounts of energetic ultraviolet radiation, a combination that has left researchers trying to work out what exactly they are. The newly identified objects have been named “hypersoft X-ray sources” because most of their detectable X-ray emission comes at the lowest energies that NASA’s Chandra X-ray Observatory can observe.

The discovery, described in a paper published in Nature Astronomy, could also help scientists investigate two long-standing problems in astrophysics.

“We’ve never encountered a group of objects that act like this,” said Mustafa Muhibullah of the University of Alabama, who led the study. “Of course, the next step was to try to figure out what these things are.”

84 objects were found in six galaxies

The researchers went through publicly available observations stored in the Chandra archive rather than looking for a completely new signal with fresh observations. They searched for sources that could be seen in Chandra images at the lowest X-ray energies but disappeared when the telescope looked at higher energies.

That unusual pattern became the key to finding the objects.

The team identified 84 hypersoft X-ray sources in six galaxies. The group included M101, also known as the Pinwheel Galaxy, and Messier 31, or the Andromeda Galaxy. The other four galaxies are elliptical galaxies.

Interestingly, the objects were not limited to one particular type of stellar environment. Some were found in areas where new stars are actively forming, while others appeared in regions dominated by much older stars.

According to NASA, the sources are extremely luminous despite being difficult to detect. The most powerful examples can produce energy comparable to some of the brightest non-nuclear X-ray sources known in galaxies.

They are bright, but their X-rays are unusually soft

X-ray binaries are already known to be powerful cosmic objects. They can contain a black hole, neutron star or white dwarf that pulls material away from a companion star. As this material falls towards the compact object, it heats up and produces radiation.

But the newly discovered sources appear to be different.

The researchers think hypersoft X-ray sources could involve similar binary systems, but their radiation is concentrated at much lower X-ray energies than normally expected. Their spectra also suggest that a large portion of their energy may come out as extreme ultraviolet, or EUV, radiation.

This is an important part of why the objects remained hidden for so long.

The extreme ultraviolet region of the electromagnetic spectrum is particularly difficult for astronomers to observe. Hydrogen and helium gas can absorb this radiation before it travels very far, while Chandra itself becomes less sensitive at the lowest X-ray energies.

In other words, the objects may have been producing enormous amounts of radiation without giving astronomers an easy way to see it.

What could these mysterious objects be?

Scientists do not yet have a single confirmed explanation for the sources.

The leading idea is that they are different kinds of X-ray binary systems. A black hole, neutron star or white dwarf could be pulling material from a nearby companion star. The material becomes extremely hot as it moves towards the compact object and produces X-rays and ultraviolet radiation.

The researchers also point out that systems containing accreting white dwarfs or objects that have recently undergone a nova could be among the possibilities.

What makes the discovery unusual is not that astronomers have never seen binary systems before. They have found many of them. The difference is the combination of extremely soft X-rays and strong ultraviolet emission seen in these 84 sources.

The researchers therefore suggest that a much larger population of similar systems may have gone unnoticed.

The discovery could help explain Type Ia supernovae

One of the bigger questions these objects could help answer involves Type Ia supernovae.

These powerful stellar explosions are important in astronomy because they have been used to measure the expansion of the universe. Observations of Type Ia supernovae played a major role in the discovery that the expansion of the universe is accelerating.

Scientists have long been interested in finding the systems that eventually produce these explosions. Some white dwarfs that pull material from companion stars are considered possible Type Ia supernova progenitors, but astronomers have not been able to identify the complete population of systems that eventually explode.


The newly discovered hypersoft sources could provide another piece of that puzzle.

“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.”

That does not mean all 84 objects will eventually become supernovae. Instead, the discovery gives researchers a new group of objects to study when looking for possible links between accreting white dwarfs and Type Ia explosions.

Another cosmic mystery involves gas between stars

The sources could also have a role in another astrophysical problem.

Astronomers know that energetic radiation can strip electrons from gas between stars. This process, called ionisation, affects the material from which stars and galaxies develop and can influence how galaxies evolve.

Hot and massive stars are already known to contribute to this process. However, scientists do not think they account for all of the energy involved in some environments.

The strong ultraviolet output suggested by hypersoft X-ray sources could provide another source of that energy.

Because these objects may exist in much larger numbers than previously recognised, their combined ultraviolet radiation could potentially have an important effect on the gas surrounding stars.

Why did astronomers miss them for so long?

The strange part of the discovery is that these objects are not necessarily faint.

Their problem is that much of their radiation comes from a part of the electromagnetic spectrum that is extremely difficult to observe. Their low-energy X-rays also require long observations, and Chandra's sensitivity decreases towards the lowest X-ray energies.

On top of that, the energetic ultraviolet radiation they produce can be absorbed by hydrogen and helium gas between the stars. That material acts almost like a wall, preventing much of the radiation from reaching telescopes.

“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.”

For now, astronomers are still working out what these objects actually are. But finding 84 of them across six different galaxies gives researchers a much larger sample to study.