Separating signals in the soft x-ray sky

X rays are a valuable source of information about distant, hot celestial bodies, such as supernova remnants and active galactic nuclei. But measurements of soft x rays—which have photon energies of no more than a few kiloelectron volts—have been clouded by local contamination. Gas throughout the solar system emits soft x rays, and so does Earth’s outermost atmosphere, which reaches as far as the Moon. And that contamination varies in both space and time.

Far beyond the Moon’s orbit, the eROSITA telescope measured x rays outside the influence of Earth’s atmosphere. And now, using data from its four full-sky surveys of the western galactic hemisphere, Konrad Dennerl of the Max Planck Institute for Extraterrestrial Physics and colleagues have separated the signals of x rays generated in the heliosphere from those generated beyond it. ¹ The resulting map provides a clearer picture of the x rays generated by solar wind and will help x-ray astronomers better account for them when analyzing distant x-ray signals.

The issue of soft x-ray contamination was identified three decades ago. Though it had long been known that x rays are emitted by ultrahot gas, it came as a surprise in 1996 when they were found coming from a comet. ² The observation led to recognition of a process known as solar wind charge exchange: When heavy ions in solar wind interact with neutral matter, such as that found in comets, planetary atmospheres, and interstellar matter, the ions capture electrons and emit x rays at the same wavelengths as the ionized gases in ultrahot astrophysical objects. ³ “All observations are contaminated with this extra emission,” says Kip Kuntz of Johns Hopkins University, who studies x-ray emissions.

For bright sources under study, the impact is insignificant. But for faint sources, the contamination can make up a significant fraction of the measured emissions. Estimates for the pressure, mass, and other physical parameters of astrophysical objects are affected by the difference. “That has consequences for [studying] dark matter and even for cosmology,” says Dennerl.

X-ray measurements by eROSITA began in 2019, when the Sun’s activity was at a minimum, and extended over a two-year period. The data, illustrated in the figure, provide insights into the spatial, temporal, and compositional variability of solar wind. Solar wind has fast and slow components—slow wind is more ionized and is thus the more dominant source of x rays. As solar activity increased over the observation period, slow wind expanded out from low latitudes. X-ray spectroscopy can reveal the wind’s heavy-ion composition and highest charge states, both of which are otherwise difficult to measure remotely and typically require in situ sampling.

The new data confirm previous observations that as interstellar matter flows into the solar system, the Sun’s gravity sends helium from the matter into a ballistic trajectory that produces a concentrated cone of the gas. The measurements also verified the presence of a hydrogen cavity around the Sun that is caused by the interaction of ionized hydrogen with solar wind.

The eROSITA instrument was built by the Max Planck Institute for Extraterrestrial Physics in Germany and is carried on the Spektr-RG, a Russian–German satellite launched in 2019. The collaboration included an agreement that data from the western galactic hemisphere would go to the German eROSITA consortium and the Russian eROSITA consortium would retain data from the eastern hemisphere. The instrument stopped collecting data in 2022 because of a breakdown in cooperation between the consortia following Russia’s invasion of Ukraine.