The Rs Cvn-Type Star Gt Mus Shows Most Energetic X-Ray Flares Throughout The 2010s

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 the Monitor of All-sky X-ray Image (MAXI) from 2009 August to 2017 August. The detected flare peak luminosities were 1-4 x 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(r) + tau(d)) of 2-6 days with long decay times (tau(d)) of 1-4 days. The released energies during the decay phases of the flares in the 0.1-100 keV band were in the range of 1-11 x 10(38) erg, which are at the upper end of the observed stellar flare. The released energies during the whole duration were in the range of 2-13 x 10(38) erg in the same band. We carried out X-ray follow-up observations for one of the 11 flares with the 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 x 10(12) cm (or 60 R-circle dot). The electron density is derived to be 1 x 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 (tau(rad)) and conductive (tau(cond)) cooling is estimated to be tau(rad) similar to 0.1 tau(cond) from the temperature; thus, radiative cooling was dominant in this flare.