Observations from the spaceborne eROSITA X-ray telescope have enabled researchers to distinguish the soft X-ray glow emanating from within our Solar System from that of more distant sources, providing a clearer picture of the true cosmic soft X-ray background, according to a new study. The findings also show that this once obfuscating foreground contamination can be used to trace solar wind activity and map material flowing through the heliosphere. Soft X-ray surveys of the distant cosmos are greatly obscured by a diffuse and fluctuating foreground X-ray “glow” that originates not from deep space, but from within our own Solar System. This emission is driven by a process called solar wind charge exchange (SWCE), in which solar wind particles interact with diffuse gas within the heliosphere, producing abundant soft X-ray photons. Because the composition of heavy ions in the solar wind fluctuates, the resulting soft X-ray emission varies over time, across different regions of the sky, and in its spectral characteristics. This makes isolating this foreground contamination a challenge and has complicated efforts to observe the true cosmic X-ray background. Here, Konrad Dennerl and colleagues used data collected by the Extended Roentgen Survey with an Imaging Telescope Array (eROSITA) instrument on the Spectrum-Roentgen-Gamma (SRG) spacecraft, whose position far beyond Earth’s immediate atmospheric influence provides a clearer signal of soft X-ray emission. Dennerl et al. analyzed repeated observations of the same sky regions from four successive all-sky surveys, taken at six-month intervals during a period of minimal solar activity. This allowed the authors to constrain and quantify heliospheric soft X-ray emissions and create a dark sky map that was largely free of this foreground contamination. According to Dennerl et al., more than 94% of the soft X-ray flux observed in the new dark sky map is from beyond the Solar System. The authors also demonstrate that X-ray observations can be used to map where within the heliosphere these emissions originate, as well as trace the flow of interstellar matter through the Solar System.
For reporters interested in topics related to research integrity, author Konrad Dennerl notes, “Open data and reproducible analysis practices are essential for research integrity. However, immediate public access requirements for the data underlying a publication can delay the timely release of new results, when the same datasets could be used by others in ways that undermine the work of the original teams. Developing policies that better balance rapid openness with adequate time for thorough analysis by project team members should be an important priority.”
