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 $\lesssim$10${}^{-5}$) display softer X-ray spectra (photon indices $\mathrm{\Gamma }\sim 2$) compared to higher-luminosity black hole X-ray binaries in the hard state ($\mathrm{\Gamma }\sim 1.7$). 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 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 $L_{\mathrm{X}}\sim 4\times {10}^{32}$ erg s${}^{-1}$, equivalent to Eddington fractions of $\sim 4\times {10}^{-7}$. During its decay towards quiescence, the X-ray spectrum of MAXI J1820$+$070 softens from $\mathrm{\Gamma }\sim 1.7$ to $\mathrm{\Gamma }\sim 2$, with the softening taking $\sim 30$d, and completing at $L_{\mathrm{X}}\approx {10}^{34}$ 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_{\mathrm{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 $L_{\mathrm{X}}$. Comparing the radio/X-ray spectral evolution(s) with the $L_{\mathrm{R}}$ – $L_{\mathrm{X}}$ 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.