The RS CVn–type Star GT Mus Shows Most Energetic X-Ray Flares Throughout the 2010s

Abstract

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 the Monitor of All-sky X-ray Image (MAXI) from 2009 August to 2017 August. The detected flare peak luminosities were 1–4 × 1033 erg s−1 in the 2.0–20.0 keV band for its distance of 109.6 pc. Our timing analysis showed long durations (τ r + τ d) of 2–6 days with long decay times (τ 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 × 1038 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 × 1038 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 × 1012 cm (or 60 R ☉). The electron density is derived to be 1 × 1010 cm−3, which is consistent with the typical value of solar and stellar flares (1010–13 cm−3). The ratio of the cooling timescales between radiative (τ rad) and conductive (τ cond) cooling is estimated to be τ rad ∼ 0.1 τ cond from the temperature; thus, radiative cooling was dominant in this flare.