We perform two- and three-dimensional particle-in-cell simulations of reconnection in magnetically-dominated pair plasmas subject to strong Compton cooling. Reconnection under such conditions operates in accretion disk coronae around black holes, which produce hard X-rays through Comptonization. Our simulations show that most of the plasma in the reconnection layer is kept cold by Compton losses and locked in magnetically-dominated plasmoids with small thermal pressure. Compton drag clears cavities inside plasmoids and affects their bulk motions. These effects, however, weakly change the reconnection rate and the plasmoid size distribution from those in non-radiative reconnection. This demonstrates that the reconnection dynamics is governed by similar magnetic stresses in both cases and weakly affected by thermal pressure. We examine the energy distribution of particles energized by radiative reconnection and observe two components. (1) A mildly-relativistic peak, which results from bulk motions of cooled plasmoids. This component receives most of the dissipated reconnection power and dominates the output X-ray emission. The peak has a quasi-Maxwellian shape with an effective temperature of 100 keV. Thus, it mimics thermal Comptonization used previously to fit hard-state spectra of accreting black holes. (2) A high-energy tail, which receives 20% of dissipated reconnection power. It is populated by particles accelerated impulsively at X-points or "picked up" by fast outflows from X-points. The high-energy particles immediately cool, and their inverse Compton emission explains the MeV spectral tail detected in the hard state of Cyg X-1. Our first-principle simulations support reconnection as a mechanism powering hard X-ray emission from accreting black holes.

中文翻译：

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吸积黑洞日冕中辐射磁重联的动力学模拟

我们对受强康普顿冷却的磁主导对等离子体中的重新连接进行二维和三维粒子内粒子模拟。在这种条件下重新连接在黑洞周围的吸积盘日冕中运行，通过康普顿化产生硬 X 射线。我们的模拟表明，重联层中的大部分等离子体通过康普顿损失保持低温，并锁定在具有较小热压的磁主导等离子体团中。康普顿阻力清除等离子体团内部的空腔并影响它们的整体运动。然而，这些影响微弱地改变了非辐射重联中的重联率和等离子体团大小分布。这表明，在两种情况下，重新连接动力学都受相似的磁应力控制，并且受热压力的影响很小。我们检查了由辐射重联激发的粒子的能量分布并观察了两个分量。(1) 一个温和的相对论峰，它是由冷却的等离子体团的整体运动引起的。该组件接收大部分耗散的重新连接功率，并在输出 X 射线发射中占主导地位。该峰具有准麦克斯韦形状，有效温度为 100 keV。因此，它模拟了先前用于拟合吸积黑洞的硬态光谱的热补偿。(2) 高能尾，接收耗散重连功率的 20%。它由在 X 点脉冲加速或从 X 点快速流出“拾取”的粒子填充。高能粒子立即冷却，它们的逆康普顿发射解释了在 Cyg X-1 的硬态中检测到的 MeV 光谱尾部。