标题： 通过XMM-Newton观测到的Mrk 421在极端耀发期间的快速变化 显示英文标题

标题： Rapid Variability of Mrk 421 During Extreme Flaring as Seen Through the Eyes of XMM-Newton

摘要： 通过对耀变星在电磁波谱中的变化性研究，可以揭示导致快速流量增加的潜在过程，即所谓的耀发。 我们报告了在高能（VHE）能段由FACT于2019年6月9日探测到增强的$\gamma$射线活动的同时，在高偏峰BL Lac天体Mrk 421中发生的极其明亮的X射线耀发。 我们触发了一次XMM-Newton观测，该仪器连续观察了源25小时。 我们发现，该源处于有史以来使用XMM-Newton观测到的最亮状态，0.3至10 keV能量范围内达到了$2.8\times10^{-9}$erg cm$^{-2}$s$^{-1}$的通量。 我们进行了光谱和时序分析以揭示粒子加速机制，并寻找最短的源内固有时标。 在整个观测过程中，Mrk 421表现出典型的亮度越高越硬的行为，并显示出顺时针滞后回路模式，这表明冷却过程占主导地位而超过加速过程。 虽然不同子频带的X射线辐射高度相关，但我们排除了大时间延迟的可能性，因为计算得到的zDCFs与零延迟一致。 我们在1 ks的时间尺度上发现了0.3至10 keV频段以及硬X射线频段（4至10 keV）内的快速变化，时间缩短至300秒。 考虑到这些时间尺度，我们讨论了解释所观测到的X射线耀发的不同模型，并发现等离子体团主导的磁场重联过程最能描述我们的观测结果。 显示英文摘要

摘要： By studying the variability of blazars across the electromagnetic spectrum, it is possible to resolve the underlying processes responsible for rapid flux increases, so-called flares. We report on an extremely bright X-ray flare in the high-peaked BL Lacertae object Mrk 421 that occurred simultaneously with enhanced $\gamma$-ray activity detected at very high energies (VHE) by FACT on 2019 June 9. We triggered an observation with XMM-Newton, which observed the source quasi-continuously for 25 hours. We find that the source was in the brightest state ever observed using XMM-Newton, reaching a flux of $2.8\times10^{-9}$ erg cm$^{-2}$ s$^{-1}$ over an energy range of 0.3 - 10 keV. We perform a spectral and timing analysis to reveal the mechanisms of particle acceleration and to search for the shortest source-intrinsic time scales. Mrk 421 exhibits the typical harder-when-brighter behaviour throughout the observation and shows a clock-wise hysteresis pattern, which indicates that the cooling dominates over the acceleration process. While the X-ray emission in different sub-bands is highly correlated, we can exclude large time lags as the computed zDCFs are consistent with a zero lag. We find rapid variability on time scales of 1 ks for the 0.3 - 10 keV band and down to 300s in the hard X-ray band (4 - 10 keV). Taking these time scales into account, we discuss different models to explain the observed X-ray flare, and find that a plasmoid-dominated magnetic reconnection process is able to describe our observation best.