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Einstein Probe Discovery of an X-ray Flare from K-type Star PM J23221-0301

Guoying Zhao, WeiKang Zheng, Rong-Feng Shen, Qingcang Shui, Dongyue Li, Chang Zhou, Tianci Zheng, Weimin Yuan, HeYang Liu, Chong Ge, Junfeng Wang, Alexei V. Filippenko, Thomas G. Brink, Jordan Forman, Mayra Gutierrez, Isabelle Jones, Ravjit Kaur, Naunet Leonhardes-Barboza, Petra Mengistu, Avi Patel, Andrew Skemer, Anavi Uppal, Nicole Wolff, Michele N. Woodland

TL;DR

The study reports the discovery of a luminous X-ray flare EP J2322.1-0301 from the nearby K-type star PM J23221-0301 by the Einstein Probe, followed by rapid X-ray focusing and ground-based optical observations. The X-ray emission exhibits a fast-rise, exponential-decay (FRED) profile with a rise time of about 1.4 ks and a decay time of about 5.7 ks, reaching a peak luminosity of ≈1.3×10^31 erg s^-1 and a total radiated energy of ≈9.1×10^34 erg in the 0.5–4.0 keV band; time-resolved spectroscopy requires a multitemperature plasma (kT1≈4.7 keV, kT2≈1.1 keV, kT3≈0.27 keV). Optical spectroscopy reveals transient Hα emission that disappears about 1.3 days later, enabling an X-ray–Hα energy ratio of ≈15.2, consistent with empirical flare relations. The results support magnetic reconnection as the flare driver and demonstrate EP's capability for rapid discovery and precise localization of stellar transients, providing new constraints on flare energetics, plasma structure, and the star–planet radiation environment in late-type stars.

Abstract

Stellar flares are an intense stellar activity that can significantly impact the atmospheric composition of the surrounding planets and even the possible existence of life. During such events, the radiative energy of the star is primarily concentrated in the optical and X-ray bands, with the X-ray flux potentially increasing by tens or even hundreds of times. Einstein Probe (EP) detected a new X-ray transient EP J2322.1-0301 on 27 September 2024. Its spatial localization shows a high positional coincidence with the nearby high proper motion K-type star PM J23221-0301. Follow-up X-ray observations confirmed the flux enhancement of the source, while optical spectroscopic monitoring revealed time-variable features, particularly the disappearance of the H-alpha emission line. This X-ray flare is consistent with a characteristic fast-rise-exponential-decay (FRED) light curve, with a rise timescale of 1.4 ks, a decay timescale of 5.7 ks, and a total duration of about 7.1 ks. The peak luminosity in the 0.5-4.0 keV energy band reached about 1.3 x 10^31 erg s^-1, with a total energy release of about 9.1 x 10^34 erg, consistent with the empirical energy correlations observed in magnetic-reconnection-driven stellar flares, as inferred from the multitemperature plasma structure and H-alpha-X-ray energy correlation. This discovery underscores EP's capability in understanding stellar magnetic activity via observing stellar transients.

Einstein Probe Discovery of an X-ray Flare from K-type Star PM J23221-0301

TL;DR

The study reports the discovery of a luminous X-ray flare EP J2322.1-0301 from the nearby K-type star PM J23221-0301 by the Einstein Probe, followed by rapid X-ray focusing and ground-based optical observations. The X-ray emission exhibits a fast-rise, exponential-decay (FRED) profile with a rise time of about 1.4 ks and a decay time of about 5.7 ks, reaching a peak luminosity of ≈1.3×10^31 erg s^-1 and a total radiated energy of ≈9.1×10^34 erg in the 0.5–4.0 keV band; time-resolved spectroscopy requires a multitemperature plasma (kT1≈4.7 keV, kT2≈1.1 keV, kT3≈0.27 keV). Optical spectroscopy reveals transient Hα emission that disappears about 1.3 days later, enabling an X-ray–Hα energy ratio of ≈15.2, consistent with empirical flare relations. The results support magnetic reconnection as the flare driver and demonstrate EP's capability for rapid discovery and precise localization of stellar transients, providing new constraints on flare energetics, plasma structure, and the star–planet radiation environment in late-type stars.

Abstract

Stellar flares are an intense stellar activity that can significantly impact the atmospheric composition of the surrounding planets and even the possible existence of life. During such events, the radiative energy of the star is primarily concentrated in the optical and X-ray bands, with the X-ray flux potentially increasing by tens or even hundreds of times. Einstein Probe (EP) detected a new X-ray transient EP J2322.1-0301 on 27 September 2024. Its spatial localization shows a high positional coincidence with the nearby high proper motion K-type star PM J23221-0301. Follow-up X-ray observations confirmed the flux enhancement of the source, while optical spectroscopic monitoring revealed time-variable features, particularly the disappearance of the H-alpha emission line. This X-ray flare is consistent with a characteristic fast-rise-exponential-decay (FRED) light curve, with a rise timescale of 1.4 ks, a decay timescale of 5.7 ks, and a total duration of about 7.1 ks. The peak luminosity in the 0.5-4.0 keV energy band reached about 1.3 x 10^31 erg s^-1, with a total energy release of about 9.1 x 10^34 erg, consistent with the empirical energy correlations observed in magnetic-reconnection-driven stellar flares, as inferred from the multitemperature plasma structure and H-alpha-X-ray energy correlation. This discovery underscores EP's capability in understanding stellar magnetic activity via observing stellar transients.

Paper Structure

This paper contains 12 sections, 3 equations, 8 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. Einstein Probe Discovery
  4. Lick Observatory KAIT Photometry and Kast Spectroscopy
  5. ASAS-SN
  6. Spectral and Temporal Analysis
  7. X-ray Spectral Fitting
  8. X-ray Light Curve
  9. Flare Luminosity and Energy
  10. H$\alpha$ Emission
  11. Stellar Rotation
  12. Conclusion

Figures (8)

  • Figure 1: The 60 s time-binned light curves of EP J2322.1-0301, as observed by WXT in the 0.5--4.0 keV band and by the FXT in two energy bands, along with the hardness ratio defined as C$_{\mathrm{2.0-10~keV}}$/C$_{\mathrm{0.5-2.0~keV}}$ (exhibiting only slight variations). Here, $t_0$ is the WXT trigger time 2024-09-27T01:03:48 (UTC). WXT/CMOS36 and WXT/CMOS37 represent two adjacent CMOS detection units of EP/WXT, and the target is located in the overlapping region of the fields of view of the two units.
  • Figure 2: The EP/WXT detection localization (red circle, 2.46$^\prime$ error radius), the subsequent EP/FXT refinement (cyan circle, 20$^{\prime\prime}$ radius), and the ROSAT cataloged source localization (blue circle) overlaid on the Digital Sky Survey (DSS) optical image. Potential candidate multiwavelength counterparts within the WXT error circle, including Gaia DR3, 2MASS, WISE, and USNO are also labeled. The high proper motion K-type star PM J23221-0301 is the centrally brighter star indicated by the aquamarine arrow, showing $<6^{\prime\prime}$ offset from the FXT centroid.
  • Figure 3: Short-term KAIT optical light curves of PM J23221-0301. The "S" notations and dash lines mark the commencement of Kast's two spectral observations, corresponding to 2.79 hr and 30.53 hr post-WXT trigger.
  • Figure 4: Optical spectra of PM J23221-0301, obtained using Kast on two consecutive nights: 2.79 hr (red) and 30.53 hr (black) post-WXT trigger. Notably, as shown in the zoomed-in panel, the 2.79 hr spectrum features a distinct H$\alpha$ emission line ($\lambda= 6563$$\AA$); however, this emission line is absent the next night.
  • Figure 5: Long-term ASAS-SN optical light curves of PM J23221-0301, with data obtained from https://asas-sn.osu.edu/. The red dashed vertical lines demarcate the EP observation window.
  • ...and 3 more figures