We present an in-depth study of two superflares (F1 and F2) detected on an active binary star CC Eridani by the Swift observatory. These superflares triggered the Burst Alert Telescope (BAT) in the hard X-ray band on 2008 October 16 and 2012 February 24. The rise phases of both the flares were observed only with BAT, whereas the decay phases were observed simultaneously with the X-ray Telescope. It has been found that the flares decay faster in the hard X-ray band than in the soft X-ray band. Both flares F1 and F2 are highly energetic with respective peak X-ray luminosities of $\sim$$10^{32.2}$ and $\sim$$10^{31.8}$ erg s$^{-1}$ in 0.3--50 keV energy band, which are larger than any other flares previously observed on CC Eri. The time-resolved spectral analysis during the flares shows the variation in the coronal temperature, emission measure, and abundances. The elemental abundances are enhanced by a factor of $\sim$8 to the minimum observed in the post-flare phase for the flare F1. The observed peak temperatures in these two flares are found to be 174 MK and 128 MK. Using the hydrodynamic loop modeling, we derive loop lengths for both the flares as 1.2$\pm$0.1$\times$10$^{10}$ cm and 2.2$\pm$0.6$\times$10$^{10}$ cm, respectively. The Fe K$α$ emission at 6.4 keV is also detected in the X-ray spectra and we model the K$α$ emission feature as fluorescence from the hot flare source irradiating the photospheric iron. These superflares are the brightest, hottest, and shortest in duration observed thus far on CC Eri.
Spectroscopic investigation of the southern late-K/early-M dwarf binary CC Eri
University of Graz（格拉茨大学）；RG Science（RG科学）；The Czech Academy of Sciences（捷克科学院）；Charles University（查理大学）；University of Wrocław（弗罗茨瓦夫大学）；Thüringer Landessternwarte, TLS（图林根州立天文台）
We present a comprehensive study of three large long-duration flares detected on an active M-dwarf binary EQ Peg using the Soft X-Ray Telescope of the AstroSat observatory. The peak X-ray luminosities of the flares in the 0.3 - 7 keV band are found to be within ~5 - 10 x 10^{30} erg s^{-1}. The e-folding rise- and decay-times of the flares are derived to be in the range of 3.4 - 11 and 1.6 - 24 ks, respectively. Spectral analysis indicates the presence of three temperature corona with the first two plasma temperatures remain constant during all the flares and the post-flare observation at ~3 a…
Multi-wavelength Study of A Superflare on RS CVn-type Star HD22468 Triggered at Hard X-ray by SVOM
National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Universite Paris Cite, CNRS, Astroparticule et Cosmologie（巴黎大学，法国国家科学研究中心，粒子与宇宙学实验室）；Institute de Recherche en Astrophysique & Planetologie（天体物理与行星学研究所在）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）
We present the first multi-wavelength simultaneous detection of QPP in a superflare (more than a thousand times stronger than known solar flares) on a cool star, in soft X-rays (SXR, with XMM-Newton) and white light (WL, with Kepler). It allowed for the first-ever analysis of oscillatory processes in a stellar flare simultaneously in thermal and non-thermal emissions, conventionally considered to come from the corona and chromosphere of the star, respectively. The observed QPP have periods $1.5 \pm 0.15$ hours (SXR) and $3 \pm 0.6$ hours (WL), and correlate well with each other. The unique rel…
Spectroscopic observations of active solar-analog stars having high X-ray luminosity, as a proxy of superflare stars
最近对太阳型超级耀斑恒星的研究表明，即使是像太阳一样缓慢旋转的老恒星也可能有大的恒星黑子和超级耀斑。我们进行了高色散光谱的49个附近的太阳类似星（G型主序星与$T_{\rm{eff}}\approx5,600\sim6,000$ K）确定为ROSAT软X射线源，这不是从以前的研究双星。我们期望这些恒星可以被用作明亮的太阳类超耀斑恒星的替代物，因为超耀斑恒星被期望显示出强烈的X射线光度。在49颗目标星中，超过一半（37颗）没有双星的证据，它们的大气参数（$T_{\rm{eff}}$、$\log g$和[Fe/H]）都在普通太阳类星的范围内。我们测量了Ca II 8542和H $α$线，这是色球活动的良好指标。这些线的强度表明，所有的目标恒星都有大的星斑。我们还测量了目标恒星的$v\sin i$（投影旋转速度）和锂丰度。Li丰度是理解恒星对流带演化的关键，它反映了恒星的年龄、质量和自转历史。我们证实，许多目标恒星快速旋转，与太阳相比，锂丰度很高，正如许多以前的研究所表明的那样。然而，也有一些目标星旋转缓慢（$v\sin i$ =2 $\sim$ 3 km s $^{-1}$），像太阳一样锂丰度低。这些结果支持了类似太阳的古老而缓慢旋转的恒星可能具有高活动水平和大的星点。这与我们以前对太阳型超耀斑恒星的研究结果是一致的。今后，为了从恒星发电机理论的角度研究大星点的详细性质，对这些活跃的太…
Solar and stellar flares are powerful events which produce intense radiation across the electromagnetic spectrum. Multiwavelength observations are highly important for understanding the nature of flares, because different flare-related processes reveal themselves in different spectral ranges. To study the correlation between thermal and nonthermal processes in stellar flares, we have searched the databases of Kepler (optical observations) and XMM-Newton (soft X-rays) for the flares observed simultaneously with both instruments; nine distinctive flares (with energies exceeding $10^{33}$ erg) on…
We present an in-depth study of a large and long duration ($>$1.3 days) X-ray flare observed on an RS CVn type eclipsing binary system SZ Psc using observations from Swift observatory. In the 0.35$-$10 keV energy band, the peak luminosity is estimated to be 4.2$\times$10$^{33}$ erg s$^{-1}$. The quiescent corona of SZ Psc was observed $\sim$5.67 d after the flare using Swift observatory, and also $\sim$1.4 yr after the flare using the XMM-Newton satellite. The quiescent corona is found to consist of three temperature plasma: 4, 13, and 48 MK. High-resolution X-ray spectral analysis of the quie…
X-ray and Hα superflare on an RS CVn-type star, UX Arietis: Constraint on the flare location from radial velocity change during the flare
Chuo University（中央大学）；NASA Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；The Catholic University of America（美利坚天主教大学）；Kyoto University（京都大学）；Subaru Telescope, National Astronomical Observatory of Japan（日本国立天文台昴星望远镜）；University of Colorado Boulder（科罗拉多大学博尔德分校）；National Solar Observatory（美国国家太阳观测台）；Chiba University（千叶大学）
A superflare of BP Tau simultaneously caught by EP X-ray and TESS optical observations
Polar Research Institute of China（中国极地研究中心）；Key Laboratory for Polar Science, MNR（自然资源部极地科学重点实验室）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学科维理天文与天体物理研究所）；Department of Astronomy, Peking University（北京大学天文学系）；Hamburger Sternwarte（汉堡天文台）；Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel（基尔大学理论与天体物理研究所）；INAF—Osservatorio Astronomico di Palermo（意大利国家天体物理学院巴勒莫天文台）；Department of Physics, Imperial College London（伦敦帝国理工学院物理系）
In 2015 we started the XMM-Newton monitoring of the young solar-like star Epsilon Eridani (440 Myr), one of the youngest solar-like stars with a known chromospheric CaII cycle. By analyzing the most recent Mount Wilson S-index CaII data of this star, we found that the chromospheric cycle lasts 2.92 +/- 0.02 yr, in agreement with past results. From the long-term X-ray lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric CaII cycle. The average X-ray luminosity results to be 2 x 10^28 erg/s, with an amplitude that is only a factor 2 througho…
We present X-ray observations of novae V2491 Cyg and KT Eri about 9 years post-outburst, of the dwarf nova and post-nova candidate EY Cyg, and of a VY Scl variable. The first three objects were observed with XMM-Newton, KT Eri also with the Chandra ACIS-S camera, V794 Aql with the Chandra ACIS-S camera and High Energy Transmission Gratings. The two recent novae, similar in outburst amplitude and light curve, appear very different at quiescence. Assuming half of the gravitational energy is irradiated in X-rays, V2491 Cyg is accreting at $\dot{m}=1.4\times10^{-9}-10^{-8}M_\odot/yr$, while for KT…
KT Eridani was a very fast nova in 2009 peaking at V=5.42 mag. We marshal large data sets of photometry to finally work out the nature of KT Eri. From the TESS light curve, as confirmed with our radial velocity curve, we find an orbital period of 2.61595 days. With our 272 spectral energy distributions from simultaneous BVRIJHK measures, the companion star has a temperature of 6200$\pm$500 K. Our century-long average in quiescence has V=14.5. With the Gaia distance (5110$^{+920}_{-430}$ parsecs), the absolute magnitude is +0.7$\pm$0.3. We converted this absolute magnitude (corrected to the dis…
We present a detailed analysis of a superflare on the active M dwarf star AD Leonis. The event presents a rare case of a stellar flare observed simultaneously in X-rays (with XMM-Newton) and in optical (with the Transiting Exoplanet Survey Satellite, TESS). The radiated energy both in the 0.2-12 keV X-ray band ($1.26 \pm 0.01 \cdot 10^{33}$ erg) and the bolometric value ($E_{F,bol} = 5.57 \pm 0.03 \cdot 10^{33}$ erg) put this event at the lower end of the superflare class. The exceptional photon statistics deriving from the proximity of AD Leo has enabled measurements in the 1-8 AA GOES band f…
The Neutron star Interior Composition ExploreR (NICER) X-ray observatory observed two powerful X-ray flares equivalent to superflares from the nearby young solar-like star, kappa1 Ceti, in 2019. NICER follows each flare from the onset through the early decay, collecting over 30 cts s-1 near the peak, enabling a detailed spectral variation study of the flare rise. The flare in September varies quickly in ~800 sec, while the flare in December has a few times longer timescale. In both flares, the hard band (2-4 keV) light curves show typical stellar X-ray flare variations with a rapid rise and sl…
X-ray Activity of the RS CVn-type Star σ Gem with the First-Year Observations of Einstein Probe
National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（中国科学院大学天文学院）；INAF - Osservatorio Astronomico di Palermo（意大利国家天体物理研究所巴勒莫天文台）；Department of Physics, Imperial College London（伦敦帝国理工学院物理系）
The cascading 3.21 MeV and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in $^{12}$C, excited by the $^{12}$C(p,p$^{\prime}$) reaction at 10.7 MeV proton energy. From the proton-$γ$-$γ$ triple coincidence data, a value of ${Γ_{\rm rad}}/Γ=6.2(6) \times 10^{-4}$ was obtained for the radiative branching ratio. Using our results, together with ${Γ_π^{E0}}/Γ$ from Eriksen et al., Phys. Rev. C 102, 024320 and the currently adopted $Γ_π(E0)$ values, the radiative width of the Hoyle state is determined as $Γ_{\rm rad}=5.1(6) \times 10^{…
Spectral diagnostics of cool flare loops observed by SST: I. Inversion of the Ca II 8542 Å and H$β$ lines
耀斑环是爆发事件的一个组成部分，在从X射线到冷的色球等离子体的温度范围内被检测到。虽然太阳动力学天文台的大气成像组件（SDO/AIA）定期观测到热环，但在边缘外看到的冷环是罕见的。在本文中，我们采用了SOL 2017 -09- 10 T16：06 X8.2级耀斑的独特观测结果，该耀斑产生了一个扩展的弧线。瑞典1米太阳望远镜（SST）拍摄了一系列Ca II 8542射电和氢H $β$谱线的冷离边环光谱图像。我们的重点是循环顶点。非LTE光谱反演是通过构建覆盖等离子体参数的实际范围的扩展模型网格来实现的。多级加速λ迭代（MALI）代码解决了非LTE的辐射传输问题，在一个一维的外部照明板，近似研究的环路段。Ca II 8542和H $β$线的反演产生了两个类似的解，都表明在$2 \times 10^{12}$ cm $^{-3}$附近有高电子密度，在25 km $^{-1}$附近有相对较大的微湍流。这些研究与相同或类似事件的其他独立研究具有合理的一致性。特别是，在$10^{12} - 10^{13}$ cm $^{-3}$范围内的高电子密度与SDO的日震和磁成像仪白光观测结果一致。在太阳爆发耀斑中存在如此高的密度，支持了对表现出超级耀斑的恒星的光学连续辐射的循环解释。
