🔭 astrophysics

The First Measurement of Jet Collimation Profiles in an X-ray Binary: the Case of SS 433

This paper reports the first direct measurement of jet collimation profiles in an X-ray binary using VLBI data from SS 433, revealing a quasi-parabolic profile and progressive collimation in the approaching jet while also deriving a core-shift relation and characterizing the receding jet despite absorption limitations.

Original authors: Xi Yan, Lang Cui, Zsolt Paragi, Sándor Frey

Published 2026-07-31

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Xi Yan, Lang Cui, Zsolt Paragi, Sándor Frey

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a cosmic construction site, where massive black holes and neutron stars act as the ultimate power plants. These engines don't just sit there; they spew out incredibly fast streams of particles called "jets," shooting them out into space at nearly the speed of light. For decades, astronomers have been trying to figure out how these jets stay together. Do they shoot out like a wide, messy firehose that spreads everywhere, or do they get squeezed into a tight, focused laser beam? We already know the answer for the super-massive black holes at the centers of giant galaxies, but for the smaller, "baby" black holes in our own neighborhood (called X-ray binaries), we've never been able to take a clear enough picture to see how they shape their jets. It's like trying to study the nozzle of a garden hose from a mile away—you know water is coming out, but you can't see if it's a spray or a stream.

This is where the star of our story, SS 433, comes in. It's a famous cosmic oddball located in our own galaxy, a system where a black hole is greedily eating a companion star and shooting out two powerful jets in opposite directions. Because SS 433 is so bright and active, it's the perfect candidate to finally get a good look at how these jets are formed. The big question is: as these jets travel away from their source, do they keep their shape, or do they wobble and spread out? Understanding this helps us figure out the rules of physics that govern how energy moves through the universe, whether it's in a tiny star system or a galaxy-sized monster.

In this new study, a team of astronomers decided to take a closer look at SS 433 using a super-powerful "virtual telescope" made by linking radio dishes across the globe. This technique, called Very Long Baseline Interferometry (VLBI), acts like a giant magnifying glass, allowing them to see details as small as a few thousandths of a second of arc. By looking at data collected in 1995, 1998, and 2000, they managed to measure the width of the jets as they traveled away from the central black hole.

The results are like watching a magic trick unfold. When they looked at the jet coming toward us (the "approaching" jet) using data from 1995 and 1998, they found it had a very specific shape: a "quasi-parabolic" profile. Imagine a water hose that starts wide and then gets squeezed tighter and tighter as the water shoots forward, rather than fanning out. The jet started with a wide opening angle of about 20 degrees but gradually narrowed down to just 3 degrees as it traveled further out. This suggests the jet is being "collimated," or squeezed into a tight beam, as it moves away from the source.

However, the story gets a bit wobbly when they looked at data from the year 2000. In those images, the jet width didn't follow a smooth line; instead, it had "local oscillations," or little bumps and humps, making it hard to draw a single perfect curve. It's as if the hose was vibrating or pulsing, making it difficult to say exactly how the shape was changing at that specific moment. Despite these bumps, the overall trend still showed the jet narrowing down, proving that the squeezing effect is real. The team also looked at the jet moving away from us (the "receding" jet), but it was harder to see clearly because it was being dimmed by a cloud of gas, kind of like trying to see a flashlight through a thick fog.

One of the coolest things they discovered was how the "core" of the jet—the bright spot right near the black hole—shifts depending on the radio frequency used to look at it. They found that the core moves closer to the center as the frequency gets higher, following a specific mathematical rule that matches what we see in giant galaxy jets. This is the first time such a relationship has been measured in a star system like SS 433.

So, what does this all mean? The authors suggest that SS 433 is the first X-ray binary where we can actually see the jet getting squeezed into a tight beam as it travels. While the 2000 data was a bit messy with its wiggles, the 1995 and 1998 data give us a clear picture of a jet that starts wide and gets narrower, proving that these cosmic streams are being actively shaped and confined by forces we are just beginning to understand. It's a major step forward in understanding how these cosmic engines work, showing that even the small black holes in our neighborhood follow some of the same rules as the giants at the centers of galaxies.

Problem
While significant progress has been made in characterizing jet collimation profiles in Active Galactic Nuclei (AGN), similar measurements in X-ray binaries (XRBs) have remained elusive. This gap exists because steady jets in the hard state of XRBs are typically too faint and compact for sufficient detection, while transient jets often manifest as discrete ejecta rather than continuous flows. Furthermore, the lack of precise core-shift measurements has hindered efforts to trace jet width as a function of distance from the central accretor. Consequently, it remains an open question how relativistic jets in accreting compact objects are collimated, particularly in Galactic systems accreting at high Eddington ratios.

Methodology
The authors utilized archival Very Long Baseline Interferometry (VLBI) data from the hyper-Eddington system SS 433, obtained via the Very Long Baseline Array (VLBA) combined with VLA antennas (1995, 1998) and European VLBI Network (EVN) stations (2000). The analysis involved:

Image Processing: Standard calibration in AIPS and iterative phase/amplitude self-calibration imaging in DIFMAP.
Core Shift Determination: Measuring frequency-dependent core positions using optically thin jet components (E1, E2, E3) at 1.7, 5, 8.4, and 15 GHz to locate the central binary system relative to the jet base.
Jet Width Measurement: Following AGN collimation study protocols, the authors restored images with circular beams, aligned the jet axis, and extracted transverse intensity profiles. These were fitted with Gaussian functions to derive the Full Width at Half Maximum (FWHM). Deconvolved jet widths ( $W$ ) were calculated by subtracting the synthesized beam size.
Geometric Correction: Using the well-established kinematic model of SS 433 (intrinsic speed $\beta \approx 0.26$ , precession period $\sim 162$ days), the authors calculated the jet viewing angle ( $\theta_{view}$ ) for each epoch to convert projected distances and widths into intrinsic values.
Profile Fitting: Jet widths were fitted to power-law relations ( $W \propto r^a$ ) to distinguish between cylindrical ( $a=0$ ), parabolic ( $a=0.5$ ), and conical ( $a=1$ ) geometries.

Key Contributions and Results
This study presents the first direct measurement of jet collimation profiles in an XRB.

Jet Collimation Profiles:
- 1995 and 1998 Data: The approaching jet exhibits a well-constrained quasi-parabolic profile ( $W \propto r^{0.74 \pm 0.02}$ ) over deprojected distances of $\sim 3 \times 10^{14}$ to $8 \times 10^{15}$ cm. The receding jet also follows a parabolic trend ( $W \propto r^{0.46 \pm 0.23}$ ), though measurements are scattered due to free-free absorption (FFA).
- 2000 Data: While the 2000 observations offer improved resolution, the jet width measurements display prominent local oscillations ("humps"), preventing a robust characterization via a single power law. However, these data still confirm progressive collimation.
Jet Opening Angle Evolution: The intrinsic opening angle of the approaching jet decreases gradually from $\sim 20^\circ$ at $8 \times 10^{14}$ cm to $\sim 3^\circ$ at $9 \times 10^{15}$ cm. This indicates progressive collimation of the bipolar outflow on these scales.
Core-Shift Relation: The authors measured core positions at four frequency bands and derived a core-shift relation of $\Delta r_{core, R.A.} \propto \nu^{-1.1 \pm 0.3}$ . This index is consistent with theoretical expectations for synchrotron self-absorbed jets ( $\nu^{-1}$ ) and values reported for nearby AGN.
Jet Geometry Variability: The study notes a potential diversity in collimation behavior, as the 2000 data show oscillatory features not present in the smoother 1995/1998 profiles, and a 1999 observation suggested a roughly cylindrical geometry on larger scales.

Significance
The paper establishes SS 433 as the first X-ray binary in which jet collimation profiles, opening-angle evolution, and core-shift relations have been constrained. These results provide direct evidence that jets in Galactic XRBs can undergo progressive collimation similar to AGN, challenging the notion that XRB jets are purely conical or unconfined. The findings suggest that mechanisms such as magnetic hoop stress (supported by polarimetric data showing perpendicular field orientations) and the presence of cold protons (baryonic jets) play crucial roles in confining the jet. By linking the inner X-ray jet confinement to the radio jet collimation, the study highlights a complex interplay between the jet and its surrounding environment over several orders of magnitude in scale. The derived core-shift relation further validates the applicability of AGN jet physics to Galactic XRBs.

More like this