During a period of strong γ-ray flaring activity from BL Lacertae, we organized Swift, Neutron star Interior Composition Explorer (NICER), and Nuclear Spectroscopic Telescope Array (NuSTAR) follow-up observations. The source has been monitored by Swift-XRT (X-ray Telescope) between 2020 August 11 and October 16, showing a variability amplitude of 65, with a flux varying between 1.0 × 10−11 and 65.3 × 10−11 erg cm−2 s−1. On 2020 October 6, Swift-XRT has observed the source during its historical maximum X-ray flux. A softer-when-brighter behaviour has been observed by XRT, suggesting an increasing importance of the synchrotron emission in the X-ray part of the spectrum covered by XRT during this bright state. Rapid variability in soft X-rays has been observed with both the Swift-XRT and NICER observations with a minimum variability time-scale of 60 and 240 s, and a doubling time-scale of 274 and 1008 s, respectively, suggesting very compact emitting regions (1.1 × 1014 and 4.0 × 1014 cm). At hard X-rays, a minimum variability time-scale of ∼5.5 ks has been observed by NuSTAR. We report the first simultaneous NICER and NuSTAR observations of BL Lacertae during 2020 October 11–12. The joint NICER and NuSTAR spectra are well fitted by a broken power law with a significant difference of the photon index below (2.10) and above (1.60) an energy break at ∼2.7 keV, indicating the presence of two different emission components (i.e. synchrotron and inverse Compton) in the broad-band X-ray spectrum. Leaving the total hydrogen column density towards BL Lacertae free to vary, a value of NH,tot = (2.58 ± 0.09) × 1021 cm−2 has been estimated.

中文翻译：

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NICER、NuSTAR 和 Swift 对 2020 年 8 月至 10 月 γ 射线耀斑 BL Lacertae 的后续观测

在 BL Lacertae 强烈的 γ 射线耀斑活动期间，我们组织了 Swift、中子星内部成分探测器 (NICER) 和核光谱望远镜阵列 (NuSTAR) 后续观测。Swift-XRT（X 射线望远镜）在 2020 年 8 月 11 日至 10 月 16 日期间对源进行了监测，显示变化幅度为 65，通量在 1.0 × 10−11 和 65.3 × 10−11 erg cm−2 之间变化s-1。2020 年 10 月 6 日，Swift-XRT 在其历史最大 X 射线通量期间对源进行了观测。XRT 观察到更亮时更柔和的行为，这表明在这种明亮状态下 XRT 覆盖的光谱的 X 射线部分中同步加速器发射的重要性日益增加。Swift-XRT 和 NICER 观测均观察到软 X 射线的快速变化，最小变化时间尺度分别为 60 和 240 秒，时间尺度加倍为 274 和 1008 秒，表明发射非常紧凑区域（1.1 × 1014 和 4.0 × 1014 cm）。在硬 X 射线中，NuSTAR 观察到最小变异时间尺度约为 5.5 ks。我们报告了 2020 年 10 月 11 日至 12 日期间对 BL Lacertae 的首次同时 NICER 和 NuSTAR 观测。NICER 和 NuSTAR 的联合光谱很好地符合破幂律，光子指数在低于 (2.10) 和高于 (1.60) 时存在显着差异，能量在 ∼2.7 keV 处发生断裂，表明存在两种不同的发射成分（即同步加速器）和逆康普顿）在宽带 X 射线光谱中。