2021ApJ...907...34S

Black hole X-ray binaries in the quiescent state (Eddington ratios typically <=10^–5) display softer X-ray spectra (photon indices Γ ∼ 2) compared to higher-luminosity black hole X-ray binaries in the hard state (Γ ∼ 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 ∼ 4 x 10³² erg s^–1, equivalent to Eddington fractions of ∼4 x 10^–7. During its decay toward quiescence, the X-ray spectrum of MAXI J1820+070 softens from Γ ∼ 1.7 to Γ ∼ 2, with the softening taking ∼30 days and completing at L_X ≃ 10³⁴ 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 ∝ L_X^0.52±0.07, 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.