MAXI J1820 +070下降到静止期间的盘/喷流耦合观测
Observations of the Disk/Jet Coupling of MAXI J1820+070 During its Descent to Quiescence
摘要：
静态黑洞X射线双星（爱丁顿比通常为$\lesssim$ 10 $^{-5}$）显示较软的X射线光谱（光子指数$Γ\sim2$）相比，高亮度的黑洞X射线双星在硬态（$Γ\sim1.7$）。然而，这种软化的原因，以及它对潜在的吸积流的影响，仍然是不确定的。在这里，我们在2018年爆发和随后的2019年重新耀斑的衰减期间，对黑洞X射线双星MAXI J1820 $+$ 070进行了准同步X射线和射电光谱监测，提供了一个监测黑洞X射线双星的机会，因为它积极地过渡到静止状态。我们探测1-10 keV的X射线光度低至$L_{\rm X}\sim4\times10^{32}$ erg s $^{-1}$，相当于$\sim4\times10^{-7}$的爱丁顿分数。在其向静止衰减的过程中，MAXI J1820 $+$ 070的X射线光谱从$Γ\sim1.7$软化为$Γ\sim2$，软化过程以$\sim30$ d进行，并在$L_{\rm X}\approx10^{34}$ erg s $^{-1}$（$\approx10^{-5} L_{\rm Edd}$）处完成。虽然X射线光谱软化，但无线电频谱在整个衰变过程中通常保持平坦/倒置。我们还发现MAXI J1820 $+$ 070遵循$L_{\rm R} \propto L_{\rm X}^{0.52\pm0.07}$形式的无线电（$L_{\rm R}$）-X射线光度相关，使其成为第四个遵循$L_{\rm X}$数十年来不间断的所谓“标准轨道”的黑洞系统。通过比较射电/X射线谱演化与$L_{\rm R}$ - $L_{\rm X}$平面的关系，发现X射线的软化与无辐射效率的吸积流中康普顿化过程产生的X射线是一致的。我们通常不赞成X射线发射仅来自喷流内部，可能的例外是通过同步加速器自康普顿过程产生的X射线。
英文摘要：
Black hole X-ray binaries in the quiescent state (Eddington ratios typically $\lesssim$10$^{-5}$) display softer X-ray spectra (photon indices $Γ\sim2$) compared to higher-luminosity black hole X-ray binaries in the hard state ($Γ\sim1.7$). However, the cause of this softening, and its implications for the underlying accretion flow, are still uncertain. Here, we present quasi-simultaneous X-ray and radio spectral monitoring of the black hole X-ray binary MAXI J1820$+$070 during the decay of its 2018 outburst and of a subsequent re-flare in 2019, providing an opportunity to monitor a black hole X-ray binary as it actively transitions into quiescence. We probe 1-10 keV X-ray luminosities as low as $L_{\rm X}\sim4\times10^{32}$ erg s$^{-1}$, equivalent to Eddington fractions of $\sim4\times10^{-7}$. During its decay towards quiescence, the X-ray spectrum of MAXI J1820$+$070 softens from $Γ\sim1.7$ to $Γ\sim2$, with the softening taking $\sim30$d, and completing at $L_{\rm X}\approx10^{34}$ erg s$^{-1}$ ($\approx10^{-5} L_{\rm Edd}$). While the X-ray spectrum softens, the radio spectrum generally remains flat/inverted throughout the decay. We also find that MAXI J1820$+$070 follows a radio ($L_{\rm R}$) -- X-ray luminosity correlation of the form $L_{\rm R} \propto L_{\rm X}^{0.52\pm0.07}$, making it the fourth black hole system to follow the so-called `standard track' unbroken over several (in this case, four) decades in $L_{\rm X}$. Comparing the radio/X-ray spectral evolution(s) with the $L_{\rm R}$ -- $L_{\rm X}$ plane, we find that the X-ray softening is consistent with X-rays produced by Comptonization processes in a radiatively inefficient accretion flow. We generally disfavor X-ray emission originating solely from within the jet, with the possible exception of X-rays produced via synchrotron self-Compton processes.
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