Disk, Corona, Jet Connection in the Intermediate State of MAXI J1820+070 Revealed by NICER Spectral-timing Analysis
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.
Additional Information© 2021. The American Astronomical Society. Received 2021 February 19; revised 2021 March 5; accepted 2021 March 7; published 2021 March 19. While this work was being finalized, we became aware of De Marco et al. (2021), showing the decrease in the observed soft lag frequency during the state transition, corroborating this aspect of our work. J.W., G.M., E.K., and J.A.G. acknowledge support from NASA grant 80NSSC17K0515, and thank the International Space Science Institute (ISSI) and participants of the ISSI Workshop "Sombreros and lampposts: The Geometry of Accretion onto Black Holes" for fruitful discussions. J.A.G. thanks the Alexander von Humboldt Foundation. A.I. and D.B. acknowledge support from the Royal Society.
Published - Wang_2021_ApJL_910_L3.pdf
Accepted Version - 2103.05616.pdf