Using the Einstein Probe (EP), astronomers have detected a new X-ray transient event, which turned out to be an X-ray flare from the star PM J23221-0301 located about 150 light years away. The finding was reported in a research paper published December 18 on the arXiv preprint server.
Stellar flares are energetic and impulsive releases of large amounts of energy from a star. They occur when a shift in the star’s magnetic field accelerates electrons to speeds approaching that of light, which results in eruptions producing emission across the entire electromagnetic spectrum.
Looking for stellar flares
PM J23221-0301 is a K-type star located some 150.7 light years away from Earth. The star is about 30% smaller and less massive than the sun. It has an effective temperature of 4,055 K and its age is estimated to be 1.2 billion years.
Previous observations identified PM J23221-0301 as an X-ray source with a quiescent flux, while optical monitoring revealed sporadic brightening episodes. That is why a team of astronomers led by Guoying Zhao of Sun Yat-Sen University in China, decided to monitor PM J23221-0301 using various ground-based telescopes and space observatories, including EP. The data collected with EP in September 2024 revealed the presence of an X-ray transient coincident with the star, which received the designation EP J2322.1-0301.
“In this work, we present the discovery and multiwavelength characterization of the X-ray transient EP J2322.1-0301, discovered by the Einstein Probe on September 27, 2024. We identify it as a stellar flare from the high proper motion K-type star PM J23221-0301,” the researchers wrote in the paper.
Mysterious X-ray transient turns out to be a typical flare
The collected data allowed the team to confirm the association of the transient EP J2322.1-0301 with a stellar flare from PM J23221-0301. In particular, they noted that there is a spatial coincidence between the X-ray transient and the star (within 20-arcsecond position uncertainty) and the X-ray light curve shape is consistent with a fast-rise-exponential-decay (FRED) profile. The association is further confirmed by the presence of transient hydrogen-alpha emission line in optical spectra and a consistency of flare energetics with known stellar-flare properties.
According to the paper, the flare had a total duration of about two hours, with a rise time of approximately 0.4 hours, and decay time of 1.6 hours. These timescales are within the typical range of known stellar flares reported by other studies.
The acquired data indicate that the flare from PM J23221-0301 reached a peak luminosity (in the 0.5–4.0 keV energy band) of 13 nonillion erg/s. The total energy release was estimated to be 91 decillion ergs. These values are also generally consistent with those measured for other observed stellar flares.
The study also revealed a multitemperature plasma of the flare, which suggests a possible stratification of the flaring plasma. The astronomers concluded that this finding is in agreement with the standard flare-loop models, where chromospheric evaporation generates hotter plasma in the upper regions of the loop and cooler material at lower altitudes.
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More information:
Guoying Zhao et al, Einstein Probe Discovery of an X-ray Flare from K-type Star PM J23221-0301, arXiv (2025). DOI: 10.48550/arxiv.2512.16679
Journal information:
arXiv
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