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

¹ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
² Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010 6500 GL Nijmegen, The Netherlands
³ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
⁴ University of Oxford, Department of Physics, Astrophysics, Denys Wilkinson Building, Keble Road, OX1 3RH Oxford, United Kingdom
⁵ Department of Physics, University of Warwick, Coventry, CV4 7AL UK
⁶ Anton Pannekoek Institute for Astronomy, Universiteit van Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁷ International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987 Perth, WA, 6845 Australia
⁸ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester, M13 9PL United Kingdom
⁹ Department of Physics and Electronics, Rhodes University, P.O. Box 94 Makhanda, 6140 South Africa
¹⁰ South African Radio Astronomy Observatory, 2 Fir Street, Observatory, 7925 South Africa
¹¹ Department of Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa

^⋆ Corresponding authors: atri@astron.nl; sara.motta@inaf.it

Received: 31 October 2024
Accepted: 2 March 2025

Abstract

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 II 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 < L̇ < 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_jet^a < 1 × 10³⁵ 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 H_α 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.