Unexplained X-Ray Blasts from Space Might Just Solve a Cosmic Mystery, Scientists Say

Nobody knows what causes quasi-periodic eruptions (QPEs), but a new study has found a much more efficient way of detecting these mysterious explosions.
​Image: Cavan Images via Getty
Image: Cavan Images via Getty
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Over the past few years, scientists have been puzzled by observations of energetic X-ray bursts in distant galaxies that seem to defy explanation. 

Now, researchers have pioneered a novel technique for tracking down these mystifying events that will not only shed light on their origin, but may open up an entirely new window into the nature of the universe, according to a study published on Wednesday in Nature.

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Up until this point, these bizarre X-ray bursts, known as quasi-periodic eruptions (QPEs), have been spotted either serendipitously or in archival images inside “active” galaxies that shine brightly. Much of this approach relied on luck, but Riccardo Arcodia, a PhD student at the Max-Planck Institute for Extraterrestrial Physics (MPE) who led the new study, realized that the Spektrum-Röntgen-Gamma (SRG) mission, a German-Russian space observatory launched in 2019, might have a secret QPE-spotting weapon that could more efficiently spot these events.

“After the discovery of the first QPE source was published and presented in a conference I was attending in September 2019, I discussed with my colleagues/supervisors how we could use our telescope to systematically detect more of them,” Arcodia said in an email. 

SRG carries an instrument called eROSITA, developed by MPE, that is imaging the entire sky at X-ray wavelengths. This huge collecting area made eROSITA a promising potential detector of QPEs, so Arcodia and his colleagues began searching for them in the instrument’s surveys. Lo and behold, the researchers spotted not just “the most luminous and the most distant QPE discovered so far, and the most extreme in terms of timescales,” confirming eROSITA’s abilities, but also another while they were still scrambling to write up their findings about the first. 

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Concept art of eROSITA on SRG. Image: DLR

Follow-up observations with the European Space Agency's X-ray Multi-Mirror Mission (XMM-Newton) mission and NASA's Neutron star Interior Composition Explorer (NICER) telescope confirmed both detections, but also revealed something stranger: the observations showed that unlike previous QPEs, the eROSITA eruptions originated in “quiescent” galaxies that do not have active cores, a discovery that challenges one of the leading models of QPE formation. 

"It is currently unknown what triggers these events, how long they last and how they are connected to the physical properties of the inner accretion flows,” the paper notes.

With the encouragement of colleagues and co-authors, in particular Gabriele Ponti and Erin Kara, Arcodia decided to compile all of the exciting new findings—and their implications— in the new Nature paper.

“We were already preparing a Letter (a shorter paper) with the first results, but when we received all the data, we realized we had something much bigger that we thought in our hands,” he said.

The discovery of previous QPEs—those in active galaxies—inspired a model that relies on the existence of a flow of accreted material around a black hole. If the inner parts of this flow get hot and dense enough, they flare up into X-ray bursts that might produce the type of signatures seen in QPEs.

A second model proposes that the QPEs are triggered by the presence of extremely dense objects, such as white dwarfs (a type of dead star), that orbit large black holes. The signatures of the two new events, called eRO-QPE1 and eRO-QPE2, along with their location in quiet galaxies, seems to match this model better. However, Arcodia cautioned that the actual dynamics of these interactions are a “billion dollar question.”

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“It could be the gravitational pull of the black hole partially stripping the smaller companion at each closest approach of their orbit,” he explained, noting that this should create short-lived QPEs. “QPEs could be caused by the compact object interacting with the small accretion flow formed around the main bigger black hole after the first interaction between the two objects.”

“We do not know so far and this is also what we aim to understand with future research,” Arcodia noted. “Hopefully this paper will spark new ideas from the community!”

The team is confident that eROSITA can spot about three to four QPE candidates every year, given the success of the initial search. As these detections mount, scientists should be able to constrain which model best accounts for the bizarre eruptions.

In addition to solving the mystery of their origins, Arcodia and his colleagues think QPEs could play an important role in the new era of so-called “multi-messenger” astronomy, in which scientists can observe two different types of information from the same cosmic event. In fact, they could help to solve fundamental questions in the universe, such as the nature of events that create mysterious ripples in spacetime known as gravitational waves. 

Since the advent of gravitational wave astronomy in 2015, scientists have been able to utilize a completely different source of data about the universe beyond traditional light-based astronomy. Multi-messenger astronomy was revolutionized in 2017, when scientists detected the gravitational waves emitted by the collision of two neutron stars and were able to track down the light produced by the event, known as the “optical counterpart” or “electromagnetic messenger.”

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If QPEs are caused by dense objects orbiting black holes, a scenario called an “extreme-mass-ratio inspiral,” they may well be electromagnetic messengers of corresponding gravitational wave events. No observatory can currently detect gravitational waves at the low frequencies that these inspiral events would produce, but future instruments, such as the Laser Interferometer Space Antenna (LISA), might be able to detect them.

“Extreme-mass-ratio inspirals were systems defined and planned to be observed via gravitational waves signals with LISA in the future,” Arcodia said, noting that waves would be produced by the objects orbiting close to the black hole before they are eaten by it. 

“If QPEs are indeed their electromagnetic counterpart (which is still a suggestion), we do not know if they would be appearing before their gravitational wave counterparts, during, or at the final stages of the gravitational interaction,” he added.

Assuming that scientists in the future are able to capture both gravitational waves and X-ray light from these interactions, it would reveal abundant information about the exotic events and could even provide broader insights, such as new measurements of the expansion of the universe. 

But given that LISA is not slated for launch until the 2030s, this tantalizing prospect will not come to fruition anytime soon. In the near-term, scientists will focus on finding more QPEs, and learning more about their possible sources.

“We have ongoing observations in X-rays to monitor the possible period evolution over the next year,” Arcodia concluded. “This would help in testing more quantitatively the scenario of the orbiting object which is otherwise so far a suggested scenario.”