Astronomers combing through archival data from NASA’s Chandra X-ray Observatory have identified a new class of cosmic objects that behave unlike anything catalogued before – and which could help resolve two long-standing mysteries in astrophysics.
The objects were uncovered in a trove of data collected by Chandra between 1999 and 2017, suggesting more may yet be found.
‘Hypersoft X-ray sources’ emit unusually low-energy X-rays and intense levels of ultraviolet radiation.
They were identified in six galaxies, including the Andromeda and Pinwheel galaxies.
In total, 84 were discovered by comparing images taken at different X-ray energies; the new objects appeared only in those taken at the lowest energies.
More amazing Chandra observations
What these objects could be – and why they're so weird
"We’ve never encountered a group of objects that act like this," says lead researcher Mustafa Muhibullah of the University of Alabama.
"Of course, the next step was to try to figure out what these things are."
Researchers believe they may be binary systems in which a compact object – potentially a white dwarf, neutron star or black hole – is pulling material from a companion star.
As this material falls towards the compact object, it heats up and produces radiation.
Their unusual behaviour could help solve two puzzles. The first is Type Ia supernovae, which are used to measure the Universe’s expansion.
The scenarios that trigger them remain uncertain; these new hypersoft X-ray sources could be a candidate.
"If we could find a way to spot these Type Ia supernova explosions before they go off, that would be really important," says co-author Jimmy Irwin.
The second puzzle is the source of radiation that strips electrons from diffuse gas in and around galaxies, a process that helps shape star formation.
The new objects’ intense ultraviolet output may provide part of that missing radiation.
There are good reasons these objects remained hidden for so long.
Low-energy X-rays are difficult to detect, while ultraviolet radiation is readily absorbed by hydrogen and helium gas between the stars.
"By combing through the Chandra archive, we were able to eliminate what used to be a blind spot for telescopes," says co-author Rosanne Di Stefano.
The team now hope to identify more hypersoft X-ray sources and determine what powers them.
Why this discovery is so exciting
Comment by astronomer Chris Lintott
Dian Triani in Harvard will be looking at these new discoveries with some excitement, having led work only last year that pointed out that many galaxies show signs of harbouring excited gas, despite the absence of a nice, bright active galactic nucleus to do the job.
Objects like these seem to provide enough ‘hard’ or high-energy radiation to have an impact on their surroundings.
More than that, they’re a useful reminder that, even in a time with new telescopes arriving rapidly, there is scientific gold hidden in the archives.
With astronomers getting better at sifting through the data, who knows what might turn up next?
