We analyze five epochs of Neutron star Interior Composition Explorer (NICER) data of the black hole X-ray binary MAXI J1820+070 during the bright hard-to-soft state transition in its 2018 outburst with both reflection spectroscopy and Fourier-resolved timing analysis. We confirm the previous discovery of reverberation lags in the hard state, and find that the frequency range where the (soft) reverberation lag dominates decreases with the reverberation lag amplitude increasing during the transition, suggesting an increasing X-ray emitting region, possibly due to an expanding corona. By jointly fitting the lag-energy spectra in a number of broad frequency ranges with the reverberation model reltrans, we find the increase in reverberation lag is best described by an increase in the X-ray coronal height. This result, along with the finding that the corona contracts in the hard state, suggests a close relationship between spatial extent of the X-ray corona and the radio jet. We find the corona expansion (as probed by reverberation) precedes a radio flare by ∼5 days, which may suggest that the hard-to-soft transition is marked by the corona expanding vertically and launching a jet knot that propagates along the jet stream at relativistic velocities.

我们分析了黑洞 X 射线双星 MAXI J1820+070 在其 2018 年爆发的明亮硬到软状态转变期间的五个中子星内部成分探测器 (NICER) 数据，同时使用反射光谱和傅里叶分辨时间分析. 我们证实了之前在硬状态下混响滞后的发现，并发现（软）混响滞后占主导地位的频率范围随着过渡期间混响滞后幅度的增加而减小，表明 X 射线发射区域增加，可能是由于不断扩大的日冕。通过使用混响模型reltrans联合拟合多个宽频率范围内的滞后能谱，我们发现混响滞后的增加最好通过 X 射线冠状高度的增加来描述。这一结果以及日冕在硬状态下收缩的发现表明 X 射线日冕的空间范围与射电射流之间存在密切关系。我们发现日冕扩张（通过混响探测）在射电耀斑之前约 5 天，这可能表明硬到软转变的标志是日冕垂直扩张并发射一个喷射结，沿着喷射流传播相对论速度。