| Apr 27, 2026 |
Tandem superflare observations uncover the mechanism behind an astronomical mystery.
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(Nanowerk News) 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.
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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.
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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.
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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.
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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.
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“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.
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This study (The Astrophysical Journal, “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”) 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.
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In the near future the team plans to use XRISM, a telescope with a 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.
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