We report that the RS CVn-type star GT Mus (HR4492, HD101379+HD101380) 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 eleven flares from GT Mus in eight years of observations
with MAXI (Monitor of All-sky X-ray Image) from 2009 August to 2017 August. The detected
flare peak luminosities were 1-4x10^33 erg s^-1 in the 2.0-20.0 keV band for its
distance of 109.6 pc. The durations of the flares ranged from 1-4 days. The flare
energies are calculated to be 1-8x10^38 erg, which are at the upper end of the
observed stellar flare. We carried out X-ray follow-up observations for one of the
eleven flares with NICER (Neutron star Interior Composition Explorer) 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 5 ¡ß 1012 cm (or 70 R ). The electron
density is derived to be 6 ¡ß 109 cm¡Ý3 , which is not inconsistent with the typical
value of solar and stellar flares (1010¡Ý12 cm¡Ý3 ). The ratio of the cooling timescales
between radiative cooling (¦Órad ) and conductive cooling (¦Ócond ) is estimated to be
¦Órad ~ 0.1¦Ócond . Therefore, radiative cooling was dominant in this flare.