A jet bent by a stellar wind in the black hole X-ray binary Cygnus X-1

TL;DR

This study provides the first empirical, instantaneous measurement of jet power in an accreting black hole by observing wind-induced bending of Cygnus X-1’s jets with 18 years of VLBI data. Through a physically grounded wind–jet interaction model, the authors extract $L_{ m jet}$, jet speed, and geometry, including a small jet–binary misalignment, and demonstrate that the instantaneous jet power is $ ext{log}_{10}(L_{ m jet}/{ m erg~s^{-1}})=37.3_{-0.2}^{+0.1}$, comparable to the accretion power inferred from X-rays. The results validate calorimetric jet-power estimates, constrain jet launching conditions, and have broad implications for jet feedback in galaxies and the origin of high-energy emission in Cygnus X-1. By showing stability of jet energetics over the system’s lifetime, this work directly informs the energy budgets used in large-scale cosmological simulations and black-hole accretion models.

Abstract

Jets provide an important channel for kinetic feedback from accreting black holes into their environment, without which models of the formation of large-scale structure in the universe fail to reproduce the observed properties of galaxies. Hence, an accurate measurement of jet power is critical for understanding black hole growth through accretion and also for quantifying the impact of kinetic feedback. However, the absence of instantaneous jet power measurements has precluded direct comparisons with the accretion luminosity, forcing kinetic feedback models to rely on ad hoc assumptions about how much jet power is released per accreted amount of mass. Here we report the detection of stellar wind-induced bending of the jets in the black hole X-ray binary Cygnus X-1, using 18 years of high-resolution radio imaging. By modeling jet-wind interactions, we determine the current kinetic instantaneous power of the jet to be log$_{10}(L_{\rm jet}/{\rm erg\,s}^{-1}) = 37.3_{-0.2}^{+0.1}$, comparable to the accretion energy determined from its bolometric X-ray luminosity. This result critically places prevailing assumptions about the energetics of black hole powered jets in both galaxy formation simulations, and in scaling models of black hole accretion, on a firm empirical footing.

Figures (12)

• Figure 1: High-resolution imaging of the jets in Cygnus X-1 over a full binary orbit in 2016. Panels (a-i) show the individual VLBI images (with the six 8.4-GHz VLBA observations in blue and the three 5-GHz EVN observations in purple), rotated counter-clockwise by $25^{\circ}$, and with an asymmetric axis ratio to help visualise the jet bending. The donor star's orbit (scaled up by a factor of 30 for the VLBA and 50 for the EVN) is shown in each image in red. The solid red circle indicates the star's position at the mid point of the observation. The orbital phase ($\phi$) of the observation is mentioned in the corresponding panels. (j) and (k) show the median position angle of the approaching and receding jets as measured from the core. The bending appears to lag by quarter of an orbit, due to the finite time taken by the bent jet to travel downstream. Panel l) shows the black hole orbit projected onto the plane of the sky, along with the phase coverage of each observation as shaded arcs in blue (VLBA) and purple (EVN), whose radii increase with time to avoid confusion.
• Figure 1: A montage of all 8.4-GHz VLBA observations, sorted by orbital phase. The contours are drawn at $\sigma \times \sqrt{2}^{n}$, where $n=3, 4, 5, ...$ and $\sigma$ is the image noise, as tabulated in Table \ref{['tab:fluxdensities']}. The red ellipse shows the orbit of the donor star as seen by the black hole, scaled up by a factor of 30 for visualisation purposes. The solid red circle shows the star's location at the mid-point of the observation. The grey ellipse in the bottom right of each panel shows the size of the restoring beam. The images have been rotated by $25^{\circ}$ counter clockwise.
• Figure 2: A model-independent demonstration of bent jets in Cygnus X-1. Panels (a-b) show VLBA images of Cygnus X-1 close to superior and inferior conjunction, when the jets showed significant deviations from the median position angle. The grey ellipse in each image indicates the synthesized beam size. The jet brightness profile was measured along lines of constant declination spaced by 1 mas, at the locations indicated by the horizontal lines for the core (dotted), approaching jet (solid), and receding jet (dashed). A linear fit to the jet ridge lines is shown in white, demonstrating that the approaching (solid) and receding (dashed) jets are not co-linear. Panels (c) and (d) show normalized cross-correlations of the downstream jet brightness profiles indicated in panels (a) and (b), respectively, with the brightness profile at the declination of the core. The negative of the lags are shown for the receding jets (dashed curves), to aid comparison with their approaching counterparts (solid curves). As the jets are not oriented north-south, the peak cross-correlation lag increases on moving downstream.
• Figure 2: A montage of the three 5-GHz EVN observations used in this work, sorted by orbital phase. The red ellipse and red circle denote the scaled-up donor star orbit and position, as described in Figure \ref{['FigVLBAMontage']}. Note that the image axis limits and the star's orbit have been scaled up by a factor of 1.7 relative to Extended Data Figure \ref{['FigVLBAMontage']}, to visualise the lower-resolution $5$-GHz EVN observations at the same aspect ratio as the $8.4$-GHz VLBA observations. The contour levels denote the same level of significance as Figure \ref{['FigVLBAMontage']}. The grey ellipse in the bottom right of each panel shows the size of the restoring beam. The images have been rotated by $25^{\circ}$ counter clockwise.
• Figure 3: Jet trajectories for each of the VLBA observations in 2016, determined from our physically-motivated jet model. Contour plots of each image are shown, with blue and red markers denoting the locations of the point source components required to model the approaching and receding jets, respectively, determined from fitting the visibility data in the uv-plane. The errors associated with these point source components are smaller than the size of the markers. The dashed line shows the median trajectory from the results of fitting the physical model to the point source locations, and the cyan shaded area shows a set of $200$ random draws from the posterior distribution. All images have been rotated counterclockwise by 25$^{\circ}$, to align the median jet axis vertically on the figure. An asymmetric axis ratio has been used to better visualise the jet bending. The date and orbital phase of each epoch is indicated in the bottom-left corner.