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Bibliographic Details
Main Authors:	Gandhi, P., Russell, D. M., Baglio, M. C., Bhargava, Y., Duncan, R., Gúrpide, A., Heinke, C. O., Knigge, C., Long, K. S., Maccarone, T. J., Mastroserio, G., Russell, T. D., Shaw, A. W., Tetarenko, A. J., Vincentelli, F. M., Borowski, E. S., Buckley, D. A. H., Casella, P., Brown, C. Dashwood, Dewangan, G. C., Hynes, R. I., Markoff, S., Tomsick, J. A., Alabarta, K., Carotenuto, F., Carver, E., Castro-Segura, N., Charles, P., Lewis, F., Paice, J. A., Pawar, D., Ressler, M. E., Rout, S. K., Saikia, P., Shahbaz, T., Sivakoff, G. R.
Format:	Preprint
Published:	2025
Subjects:	High Energy Astrophysical Phenomena
Online Access:	https://arxiv.org/abs/2510.01338
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Table of Contents:

Progress in understanding the growth of accreting black holes remains hampered by a lack of sensitive coordinated multiwavelength observations. In particular, the mid-infrared (MIR) regime remains ill-explored except for jet-dominant states. Here, we present comprehensive follow-up of the black hole X-ray binary GX 339-4 during a disc-dominated state in its 2023/24 outburst as part of a multi-wavelength campaign coordinated around JWST/MIRI. The X-ray properties are fairly typical of soft accretion states, with a high-energy Comptonised tail. The source is significantly detected between 5-10$μ$m, albeit at a faint flux level requiring MIR compact jet emission to be quenched by a factor of $\sim$300 or more relative to previous hard-state detections. The MIRI spectrum can be described as a simple power-law with slope $α$ = +0.39$\pm$0.07 ($F_ν$ $\propto$ $ν^α$), but surprisingly matches neither the radio/sub-mm nor the optical broadband slopes. Significant MIR stochastic variability is detected. Synchrotron radiation from the same medium responsible for high-energy Comptonisation can self-consistently account for the observed MIRI spectral-timing behaviour, offering new constraints on the physical conditions in the soft-state accretion disc atmosphere/corona. Alternative explanations, including a circumbinary disc or emission from a warm wind, fail to cleanly explain either the spectral properties or the variability. Multiwavelength timing cross-correlations show a puzzlingly long MIR lag relative to the optical, though at limited significance. We compile archival MIR and X-ray luminosities of transient black hole systems, including previously unreported detections of GX 339-4. These trace the evolution of the MIR-to-X-ray flux ratio with accretion state, and also reveal high MIR luminosities for GX 339-4 across all states. (abridged)

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