New Swift monitoring observations of the variable, radio-quiet quasar, PDS 456, are presented. A bright X-ray flare was captured in September 2018, the flux increasing by a factor of 4 and with a doubling time-scale of 2 days. From the light crossing argument, the coronal size is inferred to be about 30 gravitational radii for a black hole mass of $10^{9} {\rm M}_{\odot}$ and the total flare energy exceeds $10^{51}$ erg. A hardening of the X-ray emission accompanied the flare, with the photon index decreasing from $\Gamma=2.2$ to $\Gamma=1.7$ and back again. The flare is produced in the X-ray corona, the lack of any optical or UV variability being consistent with a constant accretion rate. Simultaneous XMM-Newton and NuSTAR observations were performed, $1-3$ days after the flare peak and during the decline phase. These caught PDS 456 in a bright, bare state, where no disc wind absorption features are apparent. The hard X-ray spectrum shows a high energy roll-over, with an e-folding energy of $E_{\rm fold}=51^{+11}_{-8}$ keV. The deduced coronal temperature, of $kT=13$ keV, is one of the coolest measured in any AGN and PDS 456 lies well below the predicted pair annihilation line in X-ray corona. The spectral variability, becoming softer when fainter following the flare, is consistent with models of cooling X-ray coronae. Alternatively, an increase in a non-thermal component could contribute towards the hard X-ray flare spectrum.

中文翻译：

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类星体 PDS 456 中燃烧的 X 射线日冕

介绍了对可变射电静音类星体 PDS 456 的新 Swift 监测观测。2018 年 9 月捕获了一次明亮的 X 射线耀斑，通量增加了 4 倍，时间尺度加倍，为 2 天。从光交叉论据推断，对于质量为 $10^{9} {\rm M}_{\odot}$ 且总耀斑能量超过 $10^{51} 的黑洞，日冕大小被推断为大约 30 个引力半径$ 尔格。X 射线发射的硬化伴随着耀斑，光子指数从 $\Gamma=2.2$ 下降到 $\Gamma=1.7$ 并再次返回。耀斑是在 X 射线日冕中产生的，没有任何光学或紫外线变化与恒定的吸积率一致。在耀斑峰值后 1-3 天和下降阶段同时进行 XMM-Newton 和 NuSTAR 观测。这些以明亮的方式捕捉到 PDS 456，裸露状态，没有明显的盘风吸收特征。硬X射线光谱显示出高能量翻转，e折叠能量为$E_{\rm fold}=51^{+11}_{-8}$ keV。推导出的日冕温度，$kT=13$keV，是任何 AGN 中测得的最冷温度之一，PDS 456 远低于预测的 X 射线日冕对湮灭线。光谱变异性在耀斑后变暗时变得更柔和，这与冷却 X 射线日冕的模型一致。或者，非热成分的增加可能有助于硬 X 射线耀斑光谱。是任何 AGN 中测得的最冷之一，而 PDS 456 远低于预测的 X 射线日冕对湮灭线。光谱变异性在耀斑后变暗时变得更柔和，这与冷却 X 射线日冕的模型一致。或者，非热成分的增加可能有助于硬 X 射线耀斑光谱。是任何 AGN 中测得的最冷之一，而 PDS 456 远低于预测的 X 射线日冕对湮灭线。光谱变异性在耀斑后变暗时变得更柔和，这与冷却 X 射线日冕的模型一致。或者，非热成分的增加可能有助于硬 X 射线耀斑光谱。