Two competing mechanisms have been proposed by astronomers. The first is photoionization, in which X-ray photons emitted by hot flare plasma eject the iron electrons. The second is collisional ionization, in which high-energy electrons accelerated at the onset of a flare collide with and dislodge the K-shell electrons. Distinguishing between them has proven difficult because both processes can occur during a flare event.
To settle the question, a research team at Kyoto University targeted the triple star system UX Arietis. The researchers conducted simultaneous ultraviolet and X-ray observations spanning several days, combining data from two space telescopes: NICER, NASA's X-ray instrument aboard the International Space Station, and Hisaki, JAXA's ultraviolet space telescope originally built to study Solar System planets.
When the team detected a superflare from UX Arietis, the timing of the emissions proved decisive. The ultraviolet emission peaked approximately 1.4 hours before the X-ray emission. Because ultraviolet emission is associated with high-energy electrons, a peak in the iron Kalpha line coinciding with the ultraviolet peak would have pointed to collisional ionization. Instead, the team found that the iron Kalpha line peak coincided with the peak of the thermal X-ray continuum produced by the hot flare plasma.
That timing offset provides clear evidence that photoionization is the dominant mechanism driving the iron Kalpha emission during stellar flares. The sequence established by the observations shows that hot plasma generated within the flare loop emits X-ray photons, which then travel to the stellar photosphere and ionize iron atoms there, producing the iron Kalpha line.
This is the first time that time-resolved observations have conclusively demonstrated this mechanism operating in a stellar flare. "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," said first author Shun Inoue.
The result has practical consequences for the broader astronomical community. Now that photoionization is confirmed as the dominant mechanism, other researchers can use the iron Kalpha line as a reliable diagnostic tool to determine where on a stellar surface a flare originates. Knowing the flare location is important not only for understanding stellar activity but also for assessing the radiation environment experienced by orbiting exoplanets.
Looking ahead, the team plans to use XRISM, a telescope with high energy resolution capable of measuring the iron Kalpha line with greater precision. Those observations are expected to allow detailed investigation of flare structure and location. The researchers hope the findings will advance both stellar flare science and exoplanet research.
Related Links
Kyoto University
Solar Science News at SpaceDaily
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