Gone with the Wind: JWST-MIRI Unveils a Strong Outflow from the Quiescent Stellar-mass Black Hole A0620-00

Journal article

We present new observations of the black hole X-ray binary A0620-00 using the Mid-Infrared (MIR) Instrument on the James Webb Space Telescope, during a state where the X-ray luminosity is 9 orders of magnitude below Eddington, and coordinated with radio, near-infrared, and optical observations. The goal is to understand the nature of the excess MIR emission originally detected by Spitzer redward of 8 μm. The stellar-subtracted MIR spectrum is well modeled by a power law with a spectral index of α = 0.72 ± 0.01, where the flux density scales with frequency as Fν ∝ να. The spectral characteristics, along with rapid variability—a 40% flux flare at 15 μm and 25% achromatic variability in the 5–12 μm range—rule out a circumbinary disk as the source of the MIR excess. The Low Resolution Spectrometer reveals a prominent emission feature at 7.5 μm, resulting from the blend of three hydrogen recombination lines. While the contribution from partially self-absorbed synchrotron radiation cannot be ruled out, we argue that thermal bremsstrahlung from a warm (a few tens of thousands of Kelvin) wind accounts for the MIR excess; the same outflow is responsible for the emission lines. The inferred mass outflow rate indicates that the system’s low luminosity is due to a substantial fraction of the mass supplied by the donor star being expelled through a wind rather than accreted onto the black hole.

Authors: Zuo, Z; Cugno, G; Michail, J; Gallo, E; Russell, DM

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal
• Volume: 991
• Issue: 2
• Article number: 157
• Publication date: 2025-09-24
• Acceptance date: 2025-07-30
• DOI: 10.3847/1538-4357/adf6b9
• EISSN: 1538-4357
• ISSN: 0004637X, 0004-637X
• Language: English
• Keywords: Low-mass x-ray binary stars; Stellar mass black holes; Infrared excess; High energy astrophysics