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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 thescale of recurrent energetic flares. We detected 11 flares from GT Mus in 8 yrof observations with Monitor of All-sky X-ray Image (MAXI) from 2009 August to2017 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 timinganalysis showed long durations ($\tau_{\rm r} + \tau_{\rm d}$) of 2-6 days withlong decay times ($\tau_{\rm d}$) of 1-4 days. The released energies during thedecay 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. Thereleased energies during whole duration time ranged 2-13 $\times$ 10$^{38}$ ergin the same band. We carried out X-ray follow-up observations for one of the 11flares with Neutron star Interior Composition Explorer (NICER) on 2017 July 18and found that the flare cooled quasi-statically. On the basis of aquasi-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 ofsolar and stellar flares (10$^{10-13}$ cm$^{-3}$). The ratio of the coolingtimescales between radiative cooling ($\tau_{\rm rad}$) and conductive cooling($\tau_{\rm cond}$) is estimated to be $\tau_{\rm rad}$ $\sim$ 0.1$\tau_{\rmcond}$ from the temperature; thus radiative cooling was dominant in this flare.

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