The RS CVn type star GT Mus shows most energetic X-ray flares throughout the 2010s

Abstract

We report that the RS CVn-type star GT Mus (HR 4492, HD 101379 + HD 101380) was the most active star in the X-ray sky in the last decade in terms of the scale of recurrent energetic flares. We detected 11 flares from GT Mus in 8 yr of observations with Monitor of All-sky X-ray Image (MAXI) from 2009 August to 2017 August. The detected flare peak luminosities were 1-4 $\times$ 10${}^{33}$ erg s${}^{-1}$ in the 2.0-20.0 keV band for its distance of 109.6 pc. Our timing analysis showed long durations (${\tau }_{\mathrm{r}}+{\tau }_{\mathrm{d}}$) of 2-6 days with long decay times (${\tau }_{\mathrm{d}}$) of 1-4 days. The released energies during the decay phases of the flares in the 0.1-100 keV band ranged 1-11 $\times$ 10${}^{38}$ erg, which are at the upper end of the observed stellar flare. The released energies during whole duration time ranged 2-13 $\times$ 10${}^{38}$ erg in the same band. We carried out X-ray follow-up observations for one of the 11 flares with Neutron star Interior Composition Explorer (NICER) on 2017 July 18 and found that the flare cooled quasi-statically. On the basis of a quasi-static cooling model, the flare loop length is derived to be 4 $\times$ 10${}^{12}$ cm (or 60 R${}_{\odot }$). The electron density is derived to be 1 $\times$ 10${}^{10}$ cm${}^{-3}$, which is consistent with the typical value of solar and stellar flares (10${}^{10-13}$ cm${}^{-3}$). The ratio of the cooling timescales between radiative cooling (${\tau }_{\mathrm{rad}}$) and conductive cooling (${\tau }_{\mathrm{cond}}$) is estimated to be ${\tau }_{\mathrm{rad}}$ $\sim$ 0.1${\tau }_{\mathrm{cond}}$ from the temperature; thus radiative cooling was dominant in this flare.

17 pages, 9 figures, 4 tables, Published for publication in The Astrophysical Journal (23-Mar-2021)

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Cited by in corpus (1)

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