A new window for magnetic activity on Ultracool dwarfs: eROSITA & a synergistic multiwavelength approach

超冷矮星（UCD）被定义为光谱类型为M7或更高的天体。这一群跨越了氢燃烧的质量极限（约0.07太阳质量），它包括非常低质量的恒星和棕矮星。术语UCD避免了误称，因为在大多数情况下，这些物体的质量是未知的，因此，对于给定的UCD，不清楚它是否能够燃烧氢，因此它是一颗星星还是褐矮星。由于它们的模糊，对UCD的磁活动的研究仅在过去的15年中取得了进展。这揭示了一个令人困惑的行为：UCD活动的特征可能是从类恒星（X射线闪耀和射电微弱）到类行星（射电爆发和X射线微弱）的磁过程的转变。这些差异的物理起源尚未确定，但磁场强度和可能由旋转速率决定的形态可能起着主要作用。该项目的目的是驳斥以前关于亚恒星边界附近磁活动二分法的证据，并通过全面的多波长研究系统地寻找其起源。eROSITA和TESS（几乎）全天空空间飞行任务的出现为UCD打开了一个新的窗口，首次提供了日冕X射线发射和光球光学活动测量以及表面旋转周期的统计样本。 eROSITA将使X射线检测到的UCD数量增加至少10倍，并且由于其多时期数据（4年内8次访问），它将能够对耀斑变异性进行系统研究。 TESS提供了光学耀斑和旋转周期，根据以前的研究，这似乎对确定UCD的活动类型至关重要。这将得到对UCD的深入研究的补充，为此，本资金申请的申请人收集了所有关键活动参数（X射线，无线电和旋转）的多波长观测结果。使用XMM-牛顿和JVLA对精心挑选的具有TESS旋转周期的UCD样本进行的极高灵敏度观测对于揭示活动行为转变的起源至关重要。

Ultracool dwarfs (UCDs) are defined as objects with spectral type M7 and later. This group spans the hydrogen-burning mass limit (at ~ 0.07 solar masses) and it comprises both very low-mass stars and brown dwarfs. The term UCD avoids misnoming as the mass of such objects is unknown in most cases and, therefore, for a given UCD it is unclear if it is able to burn hydrogen and thus whether it is a star or a brown dwarf.As a result of their faintness the study of magnetic activity on UCDs has advanced only duringthe last ~15 yrs. This uncovered a puzzling behavior: the activity of UCDs might be characterized by a transition from stellar-like (X-ray flaring and radio-faint) to planetary-like (radio-bursting and X-ray faint) magnetic processes. The physical origin of these differences has not yet been identified but the magnetic field strength and morphology that is probably ruled by the rotation rate likely plays a major role. Verifying or refuting the previous evidence for the dichotomy of magnetic activity near the substellar boundary and systematically searching for its origin through a comprehensive multi-wavelength study is the aim of this project. The advent of the (nearly) all-sky space missions eROSITA and TESS opens a new window to UCDs, providing for the first time statistical samples of coronal X-ray emission and photospheric optical activity measures and surface rotation periods. eROSITA will increase the number of X-ray detected UCDs by at least a factor of 10 and it will enable a systematic study of flare variability thanks to its multi-epoch data (8 visits over 4 years). TESS provides optical flares and rotation periods that -- according to previous studies -- appear to be crucial for determining the type of activity in UCDs. This will be complemented by an in-depth study of UCDs for which the applicant of this funding request has collected multi-wavelength observations of all critical parameters for activity (X-rays, radio and rotation). Such extremely high sensitivity observations with XMM-Newton and the JVLA for a carefully selected sample of UCDs with TESS rotation periods is crucial for unvealing the origin of the transition in the activity behavior.