A Large X-ray Stellar Flare from the RS CVn type Star Sigma Gem Observed with MAXI and NICER
Abstract
We report on a property of an abundance variation during a huge X-ray flare from the RS CVn type star Sigma Gem observed with MAXI and NICER. At 2019-02-04UT06:30, all-sky X-ray monitor MAXI detected the flare. After 10 hours from that, NICER started to observe it intermittently for nine days. So far, MAXI has been detecting more than 80 giant flares with an energy of 1e+36-39 erg from RS CVn binaries for 10 years. Nevertheless, because MAXI has limited photon statistics, spectral time-evolutions have not been revealed. Therefore, we aimed to obtain the time-resolved spectra of such an energetic flare with a NICER follow-up observation. In practice, we analyzed the NICER spectra in the 0.6-8.0 keV, which were well reproduced by an absorbed three-temperature optically thin thermal plasma model with elemental abundance of Fe, Si, S, O and Ne. This is the first time to obtain the time-dependence of each elemental composition during an X-ray flare. Generally, coronae of active stars (e.g. RS CVn system) show the inverse first ionized potential (FIP) effect (e.g. Laming et al. 2015). This effect shows that an elements of high-FIP (> 10 eV) are over-abundant relative to elements of lower FIP. When flares occur on these stars, enhancement of the low-FIP elemental abundances are often seen (e.g. Nordon and Behar 2008). This is considered to reflect the chemical composition of photosphere/chromosphere, which are evaporated by flare. From our analysis, we found abundance enhancements of the low-FIP elements (Fe, Si and S), which was 2-5 times larger than the quiescent level. On the other hand, the high-FIP elements (O and Ne) did not show enhancements during observation. Therefore, it possibly shows the photospheric/chromospheric elemental composition by the evaporated plasma.
- Publication:
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American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23514807S