A weak compact jet in a soft state of Cygnus X-1

We present evidence for the presence of a weak compact jet during a soft X-ray state of Cygnus X-1. Very-high-resolution radio observations were taken with the VLBA, EVN and MERLIN during a hard-to-soft spectral state change, showing the hard state jet to be suppressed by a factor of about 3-5 in radio flux and unresolved to direct imaging observations (i.e. < 1 mas at 4 cm). High time-resolution X-ray observations with the RXTE-PCA were also taken during the radio monitoring period, showing the source to make the transition from the hard state to a softer state (via an intermediate state), although the source may never have reached the canonical soft state. Using astrometric VLBI analysis and removing proper motion, parallax and orbital motion signatures, the residual positions show a scatter of ~0.2 mas (at 4 cm) and ~3 mas (at 13 cm) along the position angle of the known jet axis; these residuals suggest there is a weak unresolved outflow, with varying size or opacity, during intermediate and soft X-ray states. Furthermore, no evidence was found for extended knots or shocks forming within the jet during the state transition, suggesting the change in outflow rate may not be sufficiently high to produce superluminal knots.

💡 Research Summary

The authors present a comprehensive multi‑wavelength campaign that captured Cygnus X‑1 during a hard‑to‑soft X‑ray state transition in late June–early July 2010. High‑resolution radio imaging was performed with MERLIN (4 cm and 6 cm), the e‑EVN (6 cm), the VLBA (simultaneous 4 cm and 13 cm), and a single‑dish observation with the WSRT. Across all VLBI epochs the source remained unresolved, with an upper limit of ≲1 mas at 4 cm, indicating that the well‑known 15‑mas hard‑state jet collapsed to a size below the beam. The radio flux density dropped by a factor of three to five relative to the hard state, yet the spectrum remained flat or slightly inverted, suggesting the presence of a compact, partially self‑absorbed outflow rather than a complete quenching.

Simultaneous X‑ray monitoring with RXTE‑PCA, RXTE‑ASM, and Swift‑BAT traced the spectral evolution. The fractional RMS variability fell from ≈8 % (intermediate state) to <5 % as the source entered a soft state, while the power‑density spectrum changed from broadband noise to a narrow peak around 3 Hz, consistent with canonical soft‑state timing properties. These X‑ray diagnostics confirm that the radio observations were indeed taken while Cygnus X‑1 was in a soft (or at least intermediate‑soft) state.

Crucially, the authors performed precise astrometric analysis. After removing the contributions of proper motion, parallax, and orbital motion (using the ephemeris of Brocksopp et al. 1999), the residual positions at 4 cm displayed a scatter of ~0.2 mas, and at 13 cm a scatter of ~3 mas. Both scatter patterns are aligned with the known jet position angle (≈22° west of north). This alignment is interpreted as the motion of the τ ≈ 1 surface of a partially self‑absorbed jet whose optical depth changes due to modest variations in jet power, electron density, or magnetic field. The larger scatter at 13 cm is partly attributed to frequency‑dependent calibration shifts of the phase‑reference calibrator, but the overall directional consistency remains robust.

No discrete ejecta or shock‑related knots were detected during or after the transition, contrary to earlier reports of a transient knot ≈50 mas from the core (Fender et al. 2006). The absence of such features implies that the reduction in jet power was insufficient to cross the “jet‑line” (hardness ratio HR ≈ 0.4) with enough abruptness to generate super‑luminal ejecta. This challenges the universality of the jet‑line concept and suggests that jet suppression in soft states can be gradual rather than instantaneous.

In summary, the paper provides strong evidence that Cygnus X‑1 retains a weak, compact jet even in a soft X‑ray state. The jet is significantly dimmer and more compact than in the hard state, but its presence is inferred from both the residual astrometric scatter aligned with the jet axis and the flat/inverted radio spectrum. These findings refine our understanding of the disk‑jet coupling in black‑hole X‑ray binaries, indicating that jet quenching in soft states is not absolute and that subtle changes in jet opacity can be probed with high‑precision VLBI astrometry. The work also underscores the importance of coordinated, multi‑band monitoring to capture the nuanced physics of state transitions.