12-26 GHz宽带射电辐射揭示AU微星上存在微耀斑的证据
Broadband 12-26 GHz Radio Radiation Reveals Evidence for Micro-flares on AU Mic
中文总结 AI 辅助
研究通过VLA对AU微星的12-26 GHz射电观测，发现其存在微耀斑，揭示活跃M型矮星日冕非热射电辐射由连续微耀斑和磁场捕获主导。
中文摘要 AI 辅助
我们呈现了对年龄22 Myr的dM1e型恒星AU微星（AU Mic）的12-18 GHz和18-26 GHz射电波段（Ku、K波段）的VLA序列观测。在总共3小时的连续源观测中，我们探测到两次耀斑和两次边缘事件，得出射电耀斑发生率约为1次/小时。该发生率与之前的Ku波段观测结果一致，但耀斑（<1 mJy）和宁静态（~0.4 mJy）的流量密度均显著更低。此外，此处的宁静态谱形独特，可对AU Mic的射电辐射源施加独特约束。时间平均的宁静态谱最适合用回旋同步辐射描述，其峰值约在17 GHz，光学薄谱指数α≈-0.6。我们估计源区的磁场强度约为1 kG，覆盖恒星表面的比例<0.5%，但瞬时总电子动能约为10²⁸ erg。由谱指数推导的电子能量分布幂律指数δ≈2，意味着电子近乎连续注入，这可能源于AU Mic表面发生的维持射电辐射的微耀斑。每个波段各出现一次清晰耀斑，其衰减与上升的e折叠时间比为3-4，表明电子被磁场捕获。Ku波段耀斑在上升和峰值时段为光学厚，说明其峰值频率高于18 GHz。这些宁静态和耀斑特征共同表明，活跃M型矮星日冕的非热射电辐射由连续、未分辨的微耀斑和磁场捕获主导。
英文摘要
We present sequential 12-18 and 18-26 GHz radio-band ($Ku$, $K$) VLA observations of the 22 Myr dM1e star AU Mic. We detect two flares and two marginal events over a total of 3 contiguous hours on source, resulting in a radio flare rate of $\sim$1 flare hour$^{-1}$. While this rate is consistent with previous $Ku$-band observations, both flaring ($<$1 mJy) and quiescent ($\sim$0.4 mJy) flux densities are significantly lower. Furthermore, the quiescent spectral shape here is distinct, allowing for unique constraints on the radio-emitting sources of AU Mic. The time-averaged quiescent spectrum is best described by gyrosynchrotron radiation with a peak around 17 GHz and an optically thin spectral index of $α\approx -0.6$. We estimate that the source regions have magnetic field strengths of $\sim$1 kG and cover a fraction of $<$0.5% of the stellar surface, yet the instantaneous total electron kinetic energies are $\sim$10$^{28}$ erg. The power-law index describing the distribution of electrons with energy derived from the spectral index, $δ\approx 2$, implies a near-continuous injection of electrons. This could arise from micro-flares that occur over the surface of AU Mic that sustain the radio radiation. One clear flare per band occurs, with decay-to-rise $e$-folding time ratios of $3 - 4$, indicating magnetic trapping of the electrons. The $Ku$-band flare is optically thick during the rise and peak times, indicating a peak frequency above 18 GHz. Together, these quiescent and flaring characteristics suggest that continuous, unresolved micro-flaring and magnetic trapping dominate the non-thermal radio emission of active M-dwarf coronae.
