Evolution of NICER spectra during the eruption; shown are four spectra of one orbit each. Spectra are grouped to a minimum of 25 counts per bin. The colors correspond to the time of observation. Credit: arXiv (2024). DOI: 10.48550/arxiv.2410.03616
Using the Neutron Star Interior Composition Explorer (NICER) and various ground-based telescopes, an international team of astronomers made observations of a strong X-ray superflare that occurred in 2022 on a known giant star under the name HD 251108. Results of the observation campaign, published on October 4 on the preprint server arXivprovide more information about this star’s flaring activity.
Superflares are massive bursts of energy from a stellar surface. Detecting new flares of this type and studying them in detail is essential to better understand the origin of these events and the interaction between magnetic fields and star surfaces.
Located approximately 1,646 light years away, HD 251108 is an evolved, magnetically active K-type giant star, approximately seven times larger than the Sun. The star is relatively cool, with an effective temperature of 4,460 K, and its mass is comparable to that of the sun.
In late 2022, HD 251108 experienced a powerful X-ray superflare, and a group of astronomers led by Hans Moritz Gunther of the MIT Kavli Institute for Astrophysics and Space Research in Cambridge, Massachusetts, began monitoring this event to better understand flaring activity on single giant stars.
“We tracked the decay phase of a super-flare for 28 days with NICER and from the ground. We track the decay of the flare in unprecedented detail in several components of the coronal temperature,” the researchers wrote in the article.
Observations revealed that the 2022 superflare on HD 251108 had a peak flux of about 10 decillion erg/s in the 0.5–4.0 keV band and an exponential decay time of 2.2 days at beginning of the disintegration phase. This makes it one of the most powerful eruptions ever observed.
Based on the data collected, the length of the eruption loop was estimated to be two to four times larger than the radius of HD 251108. Additionally, approximately 10 days after the peak of the eruption, the eruption underwent a short phase of limited reactivity. -the heating and the light curve began to deviate from the initial decay.
The study found that the chemical abundances of HD 251108 are stable throughout the flare and consistent with typical active stars with the first inverse ionization potential (IFIP) effect. Astronomers noted that during the initial decay, the X-ray light curve corresponds to a decay in the alpha hydrogen flux, while the plasma shows some heating.
According to the document, HD 251108 exhibits rotational modulation with a period of 21.3 days. Such behavior can be explained by large star spots, stable for several years, but rotating in and out of view.
Observations also revealed that the star exhibits photometric variability of about 0.5 mag, on time scales of one or more decades of order 0.5 mag. This is consistent with these large, very stable star spots.
More information:
Hans Moritz Günther et al, A long-lasting superflare on the giant K HD 251108, arXiv (2024). DOI: 10.48550/arxiv.2410.03616
Journal information:
arXiv
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