Quantifying jet–interstellar medium interactions in Cyg X-1: Insights from dual-frequency bow shock detection with MeerKAT

Context. Accretion and outflows are astrophysical phenomena observed across a wide range of objects, from white dwarfs to supermassive black holes. Developing a complete picture of these processes requires complementary studies across this full spectrum of jet-launching sources. Jet–interstellar medium (ISM) interaction sites near black hole X-ray binaries provide unique laboratories that provide insights into the energetics of the jets launched from stellar-mass black holes. Aims. This work aims to detect and characterise the bow shock near one black hole X-ray binary, Cyg X-1, and then use this bow shock structure to parametrise the properties of the jet launched by Cyg X-1 over its lifetime. Methods. We used the MeerKAT radio telescope to investigate the bow shock structure formed by the interaction between the jets of Cyg X-1 and the ISM. Using new L- and S-band detections of the bow shock, we constrained the density of the unshocked ISM and mapped the bow shock’s spectral index. These values were applied to self-similar models developed initially for FR <ns0:sc>II</ns0:sc> galaxies to estimate the energy transport rate and the age of Cyg X-1 jets. Results. We successfully detect the bow shock north of Cyg X-1 in the L and S bands and report its size and brightness. We present the spectral index distribution across the bow shock, which is in the range −0.9 ≤ α ≤ 0.4, with an error distribution (0.6 ≤ Δα ≤ 1.5) that peaks at unity. We determine that the unshocked ISM density is 6–7 cm−3 for a temperature range of 104–3 × 106 K. This temperature range suggests that the velocity of the bow shock is 21 km s$ ^{-1} < \dot{L} < 364 $ km s−1. The age of the Cyg X-1 jet responsible for the bow shock is 0.04–0.3 Myr, and the power of the jet is constrained to 2 × 1031 ergs s$ ^{-1} < Q_{\mathrm{jet}}^{\mathrm{a}} < 1\times10^{35} $ ergs s−1 for the case of opening angles of 0.3°–2.0°. We also detect new morphological features of the bow shock in the S-band image. The comparison of archival Hα maps with the new radio observations hints at different regions of emission, different temperature ranges, and different ISM densities. Conclusions. MeerKAT’s sensitivity and resolution effectively reveal low surface brightness features of the Cyg X-1 bow shock. The spectral index suggests a consistent emission origin across the structure. The ISM density around Cyg X-1 is on the higher end for Galactic environments, and our results indicate a lower jet energy transport rate than prior estimates. Further searches with MeerKAT will help build a statistically significant sample, advancing our understanding of black hole X-ray binary jets and their impact on their local environments.