Infrared Synchrotron Emission in the Soft State of GX 339-4 and the Mid-Infrared/X-ray Luminosity Plane of Black Hole X-ray Binaries
Authors
P. Gandhi
D. M. Russell
M. C. Baglio
Y. Bhargava
R. Duncan
A. Gúrpide
C. O. Heinke
C. Knigge
K. S. Long
T. J. Maccarone
G. Mastroserio
T. D. Russell
A. W. Shaw
A. J. Tetarenko
F. M. Vincentelli
E. S. Borowski
D. A. H. Buckley
P. Casella
C. Dashwood Brown
G. C. Dewangan
R. I. Hynes
S. Markoff
J. A. Tomsick
K. Alabarta
F. Carotenuto
E. Carver
N. Castro-Segura
P. Charles
F. Lewis
J. A. Paice
D. Pawar
M. E. Ressler
S. K. Rout
P. Saikia
T. Shahbaz
G. R. Sivakoff
Abstract
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)
