A long-duration superflare on the K giant HD 251108

Abstract: Many giant stars are magnetically active, which causes rotational variability, chromospheric emission lines, and X-ray emission. Large outbursts in these emission features can set limits on the magnetic field strength and thus constrain the mechanism of the underlying dynamo. HD~251108 is a Li-rich active K-type giant. We find a rotational period of 21.3~d with color changes and additional long-term photometric variability. Both can be explained with very stable stellar spots. We followed the decay phase of a superflare for 28 days with NICER and from the ground. We track the flare decay in unprecedented detail in several coronal temperature components. With a peak flux around $10^{34}$~erg~s$^{-1}$ (0.5-4.0~keV) and an exponential decay time of 2.2~days in the early decay phase, this is one of the strongest flares ever observed; yet it follows trends established from samples of smaller flares, for example for the relations between H$\alpha$ and X-ray flux, indicating that the physical process that powers the flare emission is consistent over a large range of flare energies. We estimate a flare loop length about 2-4 times the stellar radius. No evidence is seen for abundance changes during the flare.

• The paper presents a detailed analysis of an extraordinary long-duration superflare observed on the K-type giant star HD 251108 using multi-wavelength observational data.
• Key findings include the flare's peak X-ray intensity of ~10^34 erg/s, a decay phase tracked over 28 days with an initial 2.2-day e-folding time, and a stable IFIP abundance pattern.
• The research suggests immense magnetic structures (2-4 times stellar radius) driving the flare and provides insights into magnetic dynamo mechanisms in evolved stars, guiding future studies on stellar activity.

An Examination of a Long-Duration Superflare on the K Giant HD 251108

The recent study conducted by G{\"u}nther et al. addresses a significant event in the astrophysical community: the observation of an extraordinary long-duration superflare on the K-type giant star HD 251108. By leveraging multi-wavelength observational data over an extended period, this research provides a comprehensive analysis of the stellar flare, contributing valuable insights into the magnetic activity of evolved cool stars.

Stellar Characteristics and Observational Insights

HD 251108 is categorized as a Li-rich active K giant, marked by prominent magnetic activity often associated with such evolved stars. The star exhibits a rotational period of approximately 21.3 days, and extensive monitoring has identified both short-term rotational variability and long-term photometric variability. These variations are attributed to stable stellar spots affecting light emission consistency.

The observational data utilized includes optical photometry from ASAS-SN, TESS, and ground-based optical surveys, alongside X-ray observations from eROSITA, NICER, and Swift. This combination allows for detailed observation of both pre-flare quiescence and the superflare event itself.

Superflare Characteristics

During the superflare, the X-ray flux reached a peak intensity of approximately 10^34 erg/s within the 0.5-4.0 keV range, making the event one of the most luminescent flare occurrences documented in K giants. The decay phase was meticulously tracked over 28 days, revealing an initial exponential decay with a 2.2-day e-folding time followed by a complex decay pattern.

The flare's analysis indicates highly energetic processes consistent across a broad range of flare energies, as evident in the Hα and X-ray flux relation. The observed emission measure evolution and lack of significant abundance changes challenge the contrasting results seen in other stellar flares, such as those documented in RS CVn stars or Algol-type binaries.

Implications and Theoretical Considerations

The study deduces flare loop lengths in the range of 2-4 times the stellar radius, indicating immense magnetic structures capable of sustaining such energetic activity. The stable IFIP (Inverse First Ionization Potential) abundance pattern observed throughout the flare aligns with the established data on active stars, suggesting a complex interplay of magnetic reconnection processes that warrant further investigation.

Theoretical implications of this research extend to our understanding of magnetic dynamo mechanisms in evolved stars, possibly linked to their internal structures and rotation periods. The distinct dynamical activity could offer insights into planet-hosting potential and habitability considerations around such stellar types.

Future Directions

This research paves the way for future endeavors into understanding the magnetic activity and flare dynamics of K giants. Expanding the sample size with other similar flaring giants, integrating spectropolarimetric data, and employing advanced 3D hydro-magnetic simulations could refine our knowledge of these phenomena. Additionally, continued multi-wavelength monitoring could shed light on potential interactions between magnetic activity and circumstellar environments, possibly influencing exoplanetary atmospheric studies.

In conclusion, the work of G{\"u}nther et al. provides significant contributions to stellar astrophysics by detailing a comprehensive case study of an extraordinary superflare, thus setting the foundation for future exploration into the magnetic phenomena of evolved giant stars.