The powerlaw X-ray spectra of active galactic nuclei at moderate to high accretion rates normally appear softer when they brighten, for which the underlying mechanisms are yet unclear. Utilizing XMM-Newton observations and excluding photons < 2 keV to avoid contamination from the soft excess, in this work we scrutinize the powerlaw spectral variability of NCG 4051 from two new aspects. We first find that a best-fit “softer-when-brighter” relation is statistically insufficient to explain the observed spectral variabilities, and intervals deviated from the empirical relation are clearly visible in the light curve of 2–4 keV/4–10 keV count rate ratio. The deviations are seen not only between but also within individual XMM-Newton exposures, consistent with random variations of the corona geometry or inner structure (with timescales as short as ∼1 ks), in addition to those behind the smooth “softer-when-brighter” trend. We further find the “softer-when-brighter” trend gradually weakens with the decreasing timescale (from ∼ 100 ks down to 0.5 ks). These findings indicate that the powerlaw spectral slope is not solely determined by its brightness. We propose a two-tier geometry, including flares/nano-flares on top of the inner disc and an embedding extended corona (heated by the flares, in analogy to solar corona) to explain the observations together with other observational clues in literature. Rapid spectral variabilities could be due to individual flares/nano-flares, while slow ones are driven by the variations in the global activity of inner disc region (akin to the variation of solar activity, but not the accretion rate) accompanied with heating/cooling and inflation/contraction of the extended corona.

活跃的银河核的幂律X射线光谱在增亮时通常显得较柔和，其潜在机理尚不清楚。利用XMM-Newton观测值并排除小于2 keV的光子，以避免来自软过量的污染，在这项工作中，我们从两个新方面详细研究了NCG 4051的幂律光谱变异性。我们首先发现，最合适的“更柔和更亮”的关系在统计上不足以解释观察到的光谱变异性，并且偏离经验关系的区间在2–4 keV / 4–10 keV的光曲线中清晰可见。计数率比。不仅在各个XMM-Newton曝光之间而且在各个XMM-Newton曝光中都可以看到偏差，与电晕的几何形状或内部结构的随机变化（时间尺度短至约1 ks）相一致，此外还有平滑的“变软-变亮”趋势。我们进一步发现，随着时间的减少（从约100 ks下降到0.5 ks），“更柔和更亮”的趋势逐渐减弱。这些发现表明幂律光谱斜率不仅仅由其亮度决定。我们提出了两层几何结构，包括内盘顶部的耀斑/纳米耀斑和嵌入的扩展电晕（类似于太阳日冕，由耀斑加热）来解释这些观察结果以及其他文献中的观察线索。快速的光谱变化可能是由于单个耀斑/纳米耀斑，