A team of scientists from the Max Planck Institute for Extraterrestrial Physics says they have finally managed to “disentangle the X-ray glow” originating in our own Solar System from deep space emissions that can appear similar, offering researchers the clearest view to date of the soft X-ray sky below 1 keV.
The research team behind the ‘illuminating’ data captured by the SRG/eROSITA space telescope said that separating the local signal, called the SWCX glow, from deep space emissions redefines the nature of the previously dismissed X-ray glow as an observational tool that could open up a pathway to “studies of the heavy ion content of the solar wind across all latitudes, its variation with solar activity, and its interaction with the interstellar medium.”
Solar System’s “Ubiquitous” X-ray Glow Adversely Affects a Wide Range of Studies
In an email to The Debrief, MPE researchers explained that the Solar System’s X-ray glow is caused by high-energy solar wind ions, such as carbon and oxygen, capturing neutral atoms present in Earth’s upper atmosphere. According to the team behind the new discovery, the resulting X-ray glow from the high-energy interaction is a “ubiquitous foreground signal” that affects virtually every effort to study the diffuse soft X-ray sky.
For example, scientists attempting to study the Local Hot Bubble, which is a region of hot plasma surrounding the solar neighborhood, experienced slewed readings due to the SWCX glow. This same effect has similarly hampered efforts to study the soft X-ray sky out to the outskirts of distant galaxy clusters, data that the MP researchers described as “key” for cosmological models.
“An accurate determination of the SWCX glow is therefore critical, and until now has been only partly successful,” the MPE email explained.
To finish the job, the MPE team tapped into data collected by the SRG/eROSITA space telescope. Unlike ground-based observatories or telescopes that operate in Low Earth Orbit, SRG/eROSITA is located at Lagrange point L2. Approximately 1.5 million kilometers from Earth, which is over four times the distance to the Moon, this location lets the observatory avoid the X-ray glow that has stymied previous efforts.
Specifically, the team used four sky surveys obtained by the observatory between 2019 and 2021, yielding detailed sky maps. Notably, these measurements were captured from solar minimum to rising solar activity, allowing the team to track changes in the X-ray sky over time.
Successful Isolation of the Heliospheric Component from Competing Signals
After comparing observations spanning various solar activity levels, team leader Konrad Dennerl and colleagues were able to isolate the “heliospheric component” of the signal from the deep space emissions. According to the team, this isolated local signal allowed them to reconstruct the Solar System’s soft X-ray sky “as it would appear when observed from outside the Solar System.”
“The data reveal a clear evolution of heliospheric X-ray emission across the solar cycle,” they told The Debrief.
The new map showed that emissions were weak and confined to lower latitudes during the solar minimum, then intensified and spread to higher latitudes as solar activity increased. The new map also confirmed a polar “hole” of reduced X-ray emissions observed by previous spacecraft that should be present near the solar minimum, and then gradually close.”
“This is like watching the Solar System breathe in X-rays,” explained researcher Gabriele Ponti, who had first noticed the time-varying emission in a specific sky field.
A closer analysis of the data also revealed some unexpected details. For example, the team spotted a localized region of enhanced X-ray emissions near Earth’s orbit that does not orbit the Sun. Although the team initially said this appeared to defy orbital mechanics, they ultimately determined it was caused by an interstellar “breeze” of gas filled with helium atoms flowing through the Solar System.
“The Sun’s gravity bends their trajectories, creating a concentrated stream on the downwind side—the helium focusing cone, predicted since the 1970s,” they explained.
Unprecedented Insights Into Solar Physics and Heliospheric Dynamics
When discussing the implications of disentangling the SWCX glow from competing emissions, the MPE team said their findings represent a paradigm shift in soft X-ray astronomy, “redefining what was considered a contaminating nuisance into a powerful diagnostic tool.”
Konrad Dennerl, one of the discoverers of cometary X-ray emission in 1996, which provided the explanation of the soft X-ray glow, said tracking how the stream of high-energy particles emitted by the sun modifies the appearance offers significant scientific value.
“(It) not only allows us to clean up observations of the distant universe but also provides unprecedented insights into solar physics and heliospheric dynamics,” the researcher explained.
The research team also noted that the discovery highlights eROSITA’s role in advancing both heliophysics and astrophysics and underscores the importance of factoring X-ray data related to Solar System processes.
“Understanding our Solar System’s X-ray emission is the key to properly interpreting observations of the diffuse X-ray sky,” Dennerl said.
The study “Determination of the Solar System contribution to the soft X-ray sky” was published in Science.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.