VLBI constraints on the "jet-line" of Cygnus X-1

TL;DR

This study uses VLBI observations to investigate the jet behavior of Cygnus X-1 during X-ray state changes, revealing persistent weak jets during soft states and structural jet changes without bright ejections.

Contribution

First high-resolution VLBI study showing continuous weak jets in Cygnus X-1 during soft states, challenging previous assumptions about jet quenching.

Findings

Weak, compact jets persist during soft X-ray states.

No evidence of bright, relativistic jet ejections during state transitions.

Structural changes in jets correlate with X-ray spectral states.

Abstract

Results are presented from recent VLBI observations of Cygnus X-1 during X-ray spectral state changes. Using the EVN in e-VLBI mode and the VLBA with disk recording, we observed the X-ray binary at very high angular resolution and studied changes in the compact jets as the source made transitions from hard X-ray states to softer states. The radio light curves show that these transitions were accompanied by radio flaring events followed by a quenching of the radio emission, as expected from the current paradigm for disc-jet coupling in X-ray binaries. While we see structural changes in the compact jets during these transitions, there was no evidence for the expected ejection of bright, relativistically-moving jet knots. However, we find strong evidence that the jet does not switch off completely in the soft X-ray state of Cygnus X-1, such that a weak, compact jet persists during this phase of radio quenching.