Revisiting Multizone Jet Models Applied to the Quiescent Black Hole X-Ray Binary XTE J1118+480

Black hole X-ray binaries are capable of launching compact radio jets, even in their quiescent state where they spend the majority of their time accreting at low Eddington ratios L_X/L_Edd ≲ 10⁻⁵. There are still several open questions regarding the physical properties of quiescent black hole jets and accretion flows. Here, we apply a steady-state multizone jet model (BHJet) to the broadband spectrum of the highly quiescent black hole X-ray binary XTE J1118+480 (L_X/L_Edd ∼ 10^−8.5), and we compare our results to previous work that applied a predecessor model to the same broadband spectrum. We find two statistically comparable model fits that explain the X-ray emission as either synchrotron self-Compton (from a thermal distribution of particles in the jet base) or as synchrotron-cooled radiation (from an accelerated tail of nonthermal particles). However, we argue that the synchrotron self-Compton fit is more realistic. Finally, we confirm previous suggestions that the jet base becomes cooler and more compact in quiescence, compared to hard state black hole X-ray binaries.