Disk, Corona, Jet Connection in the Intermediate State of MAXI J1820+070 Revealed by NICER Spectral-Timing Analysis

TL;DR

This study uses NICER spectral-timing analysis to explore the connection between the disk, corona, and jet in MAXI J1820+070 during its state transition, revealing how the corona's size and jet activity are linked.

Contribution

It provides new insights into the evolution of the X-ray corona and its relationship with jet launching during the state transition of a black hole binary.

Findings

Reverberation lags increase as the corona expands during the transition.

Corona contraction occurs in the hard state, indicating dynamic spatial changes.

Corona expansion precedes a radio flare by about 5 days, linking it to jet activity.

Abstract

We analyze 5 epochs of 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.