Observations of the Disk/Jet Coupling of MAXI J1820+070 During Its Descent to Quiescence

Black hole X-ray binaries in the quiescent state (Eddington ratios typically less than or similar to 10(-5)) display softer X-ray spectra (photon indices Gamma similar to 2) compared to higher-luminosity black hole X-ray binaries in the hard state (Gamma similar to 1.7). 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 reflare 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-X similar to 4 x 10(32) erg s(-1), equivalent to Eddington fractions of similar to 4 x 10(-7). During its decay toward quiescence, the X-ray spectrum of MAXI J1820+070 softens from Gamma similar to 1.7 to Gamma similar to 2, with the softening taking similar to 30 days and completing at L-X 10(34) erg s(-1) (10(-5) L-Edd). While the X-ray spectrum softens, the radio spectrum generally remains flat or inverted throughout the decay. We also find that MAXI J1820+070 follows a radio (L-R)-X-ray luminosity correlation of the form L-R proportional to L-X(0.520.07), making it the fourth black hole system to follow the so-called "standard track" unbroken over several (in this case, four) decades in L-X. Comparing the radio/X-ray spectral evolution(s) with the L-R-L-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.