Active galactic nuclei (AGN), powered by accreting supermassive black holes, and X-ray binaries (XRBs), powered by accreting neutron stars or stellar-mass black holes, are two representative classes of compact accreting systems. Relativistic collimated jets are among the most spectacular phenomena in these systems. Understanding how these powerful outflows are launched, accelerated, and collimated remains a key puzzle in modern astrophysics. Jet collimation has been extensively studied in AGN, yet the jet collimation profiles in XRBs remain unknown.

Recently, a team of scientists carried out a detailed analysis of archival VLBI data of the famous X-ray binary SS 433. By measuring the jet width as a function of distance from the central black hole, they, for the first time, derived the jet collimation profile in an X-ray binary, marking an important step toward revealing the physical mechanisms governing jet collimation in stellar-mass accreting systems.

The research was led by YAN Xi, a postdoctoral researcher from the galaxy and cosmology research group/ VLBI research group at the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS), with his supervisor Prof. CUI Lang, and other international collaborators.

The findings have been published in The Astrophysical Journal Letters.

AI-generated schematic illustration of the jet width profiles of SS 433. (Image by XAO)

The 1995 and 1998 data show that both the approaching and receding jets of SS 433 exhibit quasi-parabolic collimation profiles. However, observations obtained in 2000 tentatively suggest a different jet collimation behavior.

They found that the widths of both jets exhibit a shift from a relatively flat to a steeper trend. Together with observations from 1999, these findings imply that the collimation behavior of SS 433 may itself be diverse, with the intrinsic collimation profile potentially differing substantially between different activity periods.

Significantly, such diversity in jet collimation is extremely challenging to observe within a single AGN. AGN evolve on timescales of millions of years or longer, so it is nearly impossible to monitor the evolution of an individual AGN jet collimation profile over a human lifetime. By contrast, XRBs evolve on timescales ranging from days to weeks, offering an excellent laboratory for monitoring the evolution of jet collimation.

According to the researchers, XRBs may thus offer unique opportunities to reveal a wide variety of jet morphologies and collimation behaviors.

Besides the collimation profile, the researchers also derived the first core-shift relation for an XRB using multi-frequency observations of SS 433. The derived relation is largely consistent with theoretical expectations and measurements from nearby AGN.

The core-shift measurements and jet collimation profiles of SS 433 now, for the first time, unify the studies of the nuclear environments and jet collimation physics of XRBs and AGN, providing an important foundation for future investigations into the formation, propagation, and evolution of jets in these two classes of accreting systems.

Notably, the research team has been actively studying the accretion-jet coupling and jet physics of XRBs through multi-wavelength observations, including a recent study on GRS 1915+105. The team has also long focused on high-resolution VLBI studies of AGN jets.

By systematically contrasting the similarities and differences in jet physics between these two classes of accreting systems, the team seeks to develop a more comprehensive understanding of the universal physical processes governing jet formation and evolution across vastly different cosmic scales.

This research was supported by the China Postdoctoral Science Foundation, the National Natural Science Foundation of China, and the National Key Research and Development Program of China.