RS CVn型恒星GT Mus在整个2010年代显示出最具活力的X射线耀斑
The RS CVn type star GT Mus shows most energetic X-ray flares throughout the 2010s
摘要：
我们报告说，RS CVn型恒星GT Mus（HR 4492，HD 101379 + HD 101380）是最活跃的恒星在X射线天空中在过去十年的周期性能量耀斑的规模。利用全天X射线图像监测仪（MAXI），从2009年8月到2017年8月，共观测到11次耀斑。在2.0-20.0 keV波段，探测到的耀斑峰值亮度为1-4 $\times$ 10 $^{33}$ erg s $^{-1}$，距离为109.6 pc。我们的时间分析显示2-6天的长持续时间（$τ_{\rm r} + τ_{\rm d}$）和1-4天的长衰减时间（$τ_{\rm d}$）。在0.1-100 keV波段，耀斑衰变阶段释放的能量范围为1-11 $\times$ 10 $^{38}$尔格，处于观测到的恒星耀斑的上端。在整个持续时间内释放的能量范围为2-13 $\times$ 10 $^{38}$ erg。我们在2017年7月18日用中子星内部成分探测器（NICER）对11个耀斑中的一个进行了X射线跟踪观测，发现耀斑准静态冷却。在准静态冷却模型的基础上，导出耀斑环长为4 $\times$ 10 $^{12}$ cm（或60 R $_{\odot}$）。电子密度为1 $\times$ 10 $^{10}$ cm $^{-3}$，与太阳耀斑和恒星耀斑的典型值（10 $^{10-13}$ cm $^{-3}$）一致。辐射冷却（$τ_{\rm rad}$）和传导冷却（$τ_{\rm cond}$）之间的冷却时间尺度之比估计为$τ_{\rm rad}$ $\sim$ 0.1 $τ_{\rm cond}$，因此辐射冷却在这次耀斑中占主导地位。
英文摘要：
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 Monitor of All-sky X-ray Image (MAXI) from 2009 August to 2017 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 timing analysis showed long durations ($τ_{\rm r} + τ_{\rm d}$) of 2-6 days with long decay times ($τ_{\rm d}$) of 1-4 days. The released energies during the decay 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. The released energies during whole duration time ranged 2-13 $\times$ 10$^{38}$ erg in the same band. We carried out X-ray follow-up observations for one of the 11 flares with 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 $\times$ 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 of solar and stellar flares (10$^{10-13}$ cm$^{-3}$). The ratio of the cooling timescales between radiative cooling ($τ_{\rm rad}$) and conductive cooling ($τ_{\rm cond}$) is estimated to be $τ_{\rm rad}$ $\sim$ 0.1$τ_{\rm cond}$ from the temperature; thus radiative cooling was dominant in this flare.
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