The RS CVn-type Star GT Mus Shows Most Energetic X-Ray Flares Throughout the 2010s
Hamaguchi, Kenji; Corcoran, Michael F.; Maeda, Yoshitomo; Enoto, Teruaki; Arzoumanian, Zaven; Gendreau, Keith; Shidatsu, Megumi; Tsuboi, Yohko; Iwakiri, Wataru; Mihara, Tatehiro; Nakahira, Satoshi; Negoro, Hitoshi; Serino, Motoko; Markwardt, Craig B.; Sasaki, Ryo; Sato, Tatsuki; Kawai, Hiroki
Japan, United States
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 (τr + τd) of 2-6 days with long decay times (τd) 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 (τrad) and conductive (τcond) cooling is estimated to be τrad ∼ 0.1 τcond from the temperature; thus, radiative cooling was dominant in this flare.