Tandem superflare observations reveal origin of the stellar Fe Kα line
edited by Sadie Harley, reviewed by Robert Egan
The Fe Kα line, or iron Kα line, is often used in astronomical research to understand the physical composition of astronomical objects. This line is produced when a K-shell electron of an iron ion in the photosphere—the gas on the stellar surface—is ejected by an external process, and has been detected in X-ray spectra of solar and stellar flares. Yet the dominant mechanism behind this ionization process has remained an open question for many years.
Astronomers have proposed two possible mechanisms: photoionization by X-ray photons emitting from hot flare plasma, or collisional ionization by high-energy electrons accelerating at the onset of the flare.
With these two possibilities in mind, a team of researchers at Kyoto University set out to uncover the truth behind the iron Kα line. The findings have been published in The Astrophysical Journal.
Watching a stellar flare unfold
The team focused on the triple star system UX Arietis, conducting several days of simultaneous ultraviolet and X-ray observations using NICER, NASA's X-ray telescope aboard the International Space Station, and Hisaki, JAXA's ultraviolet space telescope. While Hisaki was developed primarily for observations of planets in the solar system, the researchers demonstrated that it can also be used to study distant stars.
When the team detected a superflare, they discovered that the ultraviolet emission peaked about 1.4 hours earlier than the X-ray emission. They also observed that the iron Kα line peak coincided with the peak of the thermal X-ray continuum emitted by the hot fire plasma, and not with the ultraviolet emission, which is associated with high-energy electrons.
This is very clear evidence of photoionization as the dominant emission mechanism of the iron Kα line during a stellar flare. Specifically, the researchers found that hot plasma generated in the stellar flare loop emits X-ray photons, causing the ionization of iron atoms at the stellar surface and producing the iron Kα line.
"We are intrigued that a long-standing, unresolved problem in solar and stellar flare research was solved through coordinated observations with Hisaki and NICER, even though Hisaki was not originally designed to study the sun or stars," says first author Shun Inoue.
What the results mean for astronomy
This study represents the first instance in which time-resolved observations led to the clear demonstration of this mechanism in a stellar flare. As a result of the team's efforts, other astronomers can now use the iron Kα line as a diagnostic tool to infer where stellar flares occur on the surfaces of stars.
In the near future, the team plans to use XRISM, a telescope with high energy resolution and the ability to more accurately measure the iron Kα line, which will enable them to investigate the flare structure and location in greater detail. They hope that their findings will contribute to future research on stellar flares and exoplanets.
Publication details
Origin of the Stellar Fe Kα Line Clarified with Far-ultraviolet and X-Ray Observations of a Superflare on the RS Canum Venaticorum–type Star UX Arietis, The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae2be0
Journal information: Astrophysical Journal
Key concepts
flare Space & astrophysical plasma X-ray astronomy
Provided by Kyoto University
Citation: Tandem superflare observations reveal origin of the stellar Fe Kα line (2026, April 27) retrieved 27 April 2026 from https://phys.org/news/2026-04-tandem-superflare-reveal-stellar-fe.htmlThis document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
