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

TL;DR

This study uses JWST/MIRI spectroscopy to detect and characterize mid-infrared emission from GX 339–4 during a disc-dominated soft-state outburst, supplemented by a coordinated, quasi-simultaneous multiwavelength campaign. The MIR continuum is well described by synchrotron emission from a hot flow or corona overlaying the disc, with a derived magnetic field of $B \sim 3 \times 10^4$ G and an emitting region extending to $r_C \sim (10^2$–$10^3) R_G$, while alternative sources such as a circumbinary disc or winds are disfavored as the primary MIR power source. The observed MIR spectrum has a slope of $\alpha_{\rm MIRI}^{\rm obs} = 0.39 \pm 0.07$ and exhibits significant red-noise variability with a PSD slope of $\beta = -1.58 \pm 0.14$, and a tentative MIR lag behind optical by several hundred seconds that remains statistically marginal due to sampling. Archival MIR detections place GX 339–4 among the more MIR-luminous black hole X-ray binaries across states, revealing a complex MIR–X-ray plane with state-dependent flux ratios and suggesting a prominent inner synchrotron component even in quiescence. Overall, the work demonstrates JWST/MIRI’s power to probe the coronal/inner-disc physics in soft states and provides new constraints on accretion geometry and jet–disc coupling in black hole XRBs.

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)

Figures (10)

• Figure 1: ( Top) Long-term X-ray light curve of GX 339--4, with all known historical MIR observational epochs annotated and denoted by arrows. Recent X-ray data are from the MAXI camera, covering the 2--20 keV band. Pre-MAXI data from RXTE/ASM are plotted as gray dots before $\sim$ MJD 55,000, normalised to the MAXI photon flux by multiplying by a factor of 0.032, which converts between the respective full energy bands of the two cameras assuming a 1 keV blackbody model approximately appropriate for outburst peaks. X-ray accretion state is given in parentheses: soft (S), hard (H) and quiescent (Q). (Bottom) X-ray Hardness-Intensity diagrams using MAXI data, covering the 2023/24 outburst. The panels differ in the definition of the harder band: (4--10 keV) on the left and (10--20 keV) on the right, highlighting differences relevant to the dominance of the high-energy power-law tail. The red point denotes the observation date coinciding with JWST.
• Figure 2: Observed MIRI low-resolution spectrum of GX 339--4 during the 2024 March soft state. The points denote the median across all integrations, and error bars denote the standard deviation incorporating background and instrumental errors together with intrinsic source variability.
• Figure 3: Multi-wavelength time-series across all 1,400 integrations, split into annotated wavelength bins and normalised to the mean in each case. The bottom panel shows the background level with the same normalisation for the full (5--10 µ m) band.
• Figure 4: MIRI spectral-timing properties. (a) The excess fractional variability r.m.s. of the MIRI data as a function of wavelength (filled black circles), compared to that of the background (empty brown circles with dashed curve) and that of WISE hard state observations from 2010 (gray empty squares with dotted lines; Gandhi-2011). (b) The power spectral density (PSD) of the white-light time series, with a power-law (slope = --1.58 $\pm$ 0.14) and constant fit overplotted. (c) Cross-correlation functions (CCFs) between the respective annotated light curves and the shortest wavelength 5--6 µ m light curve (used as a baseline reference). The excellent match between the wavelengths is apparent.
• Figure 5: Observed broadband SED during the 2024 soft state in flux density units, compared to the hard state 2010 SED in gray. These are the observed values, not corrected for absorption. X-ray detector spectral responses have been folded out and instrumental cross-calibration removed for display. The instruments used are described in § \ref{['sec:obs']} and in Appendix \ref{['sec:coordination']}.