A jet bent by a stellar wind in the black hole X-ray binary Cygnus X-1

Jets provide an important channel for kinetic feedback from accreting black holes into their environment, without which models of the formation of large-scale structure in the universe fail to reproduce the observed properties of galaxies. Hence, an accurate measurement of jet power is critical for understanding black hole growth through accretion and also for quantifying the impact of kinetic feedback. However, the absence of instantaneous jet power measurements has precluded direct comparisons with the accretion luminosity, forcing kinetic feedback models to rely on ad hoc assumptions about how much jet power is released per accreted amount of mass. Here we report the detection of stellar wind-induced bending of the jets in the black hole X-ray binary Cygnus X-1, using 18 years of high-resolution radio imaging. By modeling jet-wind interactions, we determine the current kinetic instantaneous power of the jet to be log${10}(L{\rm jet}/{\rm erg,s}^{-1}) = 37.3_{-0.2}^{+0.1}$, comparable to the accretion energy determined from its bolometric X-ray luminosity. This result critically places prevailing assumptions about the energetics of black hole powered jets in both galaxy formation simulations, and in scaling models of black hole accretion, on a firm empirical footing.

💡 Research Summary

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The authors present the first observational confirmation that the relativistic jets of a high‑mass black‑hole X‑ray binary (BHXRB) are bent by the stellar wind of the donor star, using Cygnus X‑1 as a test case. Over an 18‑year baseline they re‑analyze very‑long‑baseline interferometry (VLBI) data from the VLBA (1998, 2009, and a dense 2016 campaign) and the EVN, detecting both the approaching and receding jet components in every epoch. In the 2016 campaign the jet position angle varies systematically with orbital phase, and a model‑independent analysis shows the two jets bending in opposite directions, a pattern that cannot be reproduced by simple jet precession but is naturally explained by wind‑jet interaction.

The physical picture is that the black hole accretes from the O‑type supergiant’s powerful wind (mass‑loss rate ≈2.6 × 10⁻⁶ M⊙ yr⁻¹, wind speed ≈1500 km s⁻¹). The wind’s momentum flux pushes laterally on the jet, causing a deflection that is strongest within roughly one orbital separation of the jet nozzle (∼0.1 mas on the sky). The authors adopt an analytical model that balances wind momentum flux against the jet’s lateral momentum flux, includes the orbital motion, and predicts a helical jet trajectory. By fitting this model simultaneously to all six VLBA epochs from 2016 they derive:

• Instantaneous kinetic jet power L_jet = 10^{37.3 ± 0.2} erg s⁻¹.
• Jet launch speed β = 0.68^{+0.08}_{‑0.13} c (≈0.6 c).
• Jet half‑opening angle θ ≈ 0.8° ± 0.5°.
• Misalignment between jet axis and orbital angular momentum ≤ 8° (best‑fit ≈5°).

The derived jet power is essentially identical to the bolometric X‑ray luminosity of Cygnus X‑1 in the hard state (≈10^{37.3} erg s⁻¹), confirming the long‑assumed conversion efficiency of ~10 % of accretion power into kinetic jet power that underpins many galaxy‑formation simulations. Integrating the instantaneous power over the ∼10⁶ yr lifetime of the system yields a total kinetic feedback of ∼10^{50} erg, comparable to the energy released by a typical supernova, and thus demonstrates that microquasar jets can contribute significantly to the energy budget of the interstellar medium.

The paper also revisits earlier claims of a large (> 18°) spin‑orbit misalignment inferred from X‑ray polarization. The lack of any observable asymmetry in the wind‑jet bending, together with the stability of the mean jet position angle over 18 years, argues for a small misalignment. Consequently, the high X‑ray polarization must arise from other effects, such as a relativistic outflow in the corona, rather than a tilted jet.

Methodologically, the work establishes a new way to measure instantaneous jet power in accreting black holes: by exploiting the wind‑jet interaction in high‑mass X‑ray binaries. This approach bypasses the limitations of calorimetric estimates, which average jet power over Myr timescales and cannot capture rapid variability. The agreement between the instantaneous measurement and the time‑averaged calorimetric power validates the use of calorimetry for normalising black‑hole scaling relations (e.g., the fundamental plane of black‑hole activity) and for setting feedback parameters in cosmological simulations such as Illustris‑TNG.

In summary, the study provides (1) direct evidence of wind‑induced jet bending in Cygnus X‑1, (2) the first instantaneous measurement of jet kinetic power for any accreting black hole, (3) a robust estimate of the jet‑to‑accretion power conversion efficiency, and (4) a methodological framework that can be applied to other high‑mass X‑ray binaries to refine our understanding of black‑hole feedback on galactic scales.