JWST/MIRI Spectroscopy of the Quiescent Black Hole X-ray Binary XTE J1118+480: Constraints on Jet and Circumbinary Dust Contributions to the Mid-infrared Emission

Abstract

We report on JWST/MIRI low-resolution spectroscopy ($5$-$12 \mu$m) and imaging ($15$-$25 \mu$m) of the black hole X-ray binary XTE~J1118+480 in quiescence, at an X-ray luminosity nine orders of magnitude sub-Eddington, together with contemporaneous optical monitoring and a radio upper limit of $4.5~\mu$Jy at 6 GHz. With the donor subtracted, the $5$-$12 \mu$m continuum is curved, turning over near $6~\mu$m; a single power law is rejected in favor of a $764\pm 9$ K blackbody. Longward of the turnover the spectral index is $\alpha = +1.21\pm0.36$, steeper at the $2\sigma$ level than the flat to slightly inverted spectrum of a compact jet. Interpreted as synchrotron, the turnover implies equipartition fields of $\sim$$10^{4}$ G on scales of tens of gravitational radii, but the MIRI photometry at $15$-$21~\mu$m-taken $2.6$ hr before the spectrum-lies $30$-$90$ per cent above its extrapolation, suggesting a second self-absorbed zone. A single partially self-absorbed jet spectrum can reach the photometry only at the expense of the spectral curvature and the $25.5~\mu$m upper limit, so a steady compact jet is disfavored, although not excluded. Representative self-consistent irradiated-disk models instead reproduce the principal features of the $5$-$25~\mu$m spectral energy distribution, with a directly illuminated dust wall at $\simeq 5$ binary separations, outside the tidal-truncation radius, and a minimum dust mass of order $10^{22}$ g. This is the first spectroscopic evidence, though not yet conclusive, favoring circumbinary dust over a jet in a quiescent black hole X-ray binary.