X-ray polarization hints at turbulence around SS 433’s eastern jet

SS 433 is a compact binary system inside the nebula W50. It launches fast jets, including an eastern jet that shines in X-rays. Earlier observations have been interpreted as evidence that particles in the extended jets reach very high energies. To understand the conditions that may support this acceleration, researchers can study X-ray polarization, which carries clues about magnetic fields.

Polarization describes the preferred orientation of a light wave’s electric field. The measured electric-vector position angle, or EVPA, is not itself the magnetic-field direction: translating one into the other depends on the emission model. In this study, the authors interpret the X-ray polarization as synchrotron emission, for which the inferred mean magnetic field is perpendicular to the EVPA. Polarization strength can also indicate how ordered a field is, but it is not a unique or direct turbulence meter. Tangled fields and unresolved changes in field direction can both reduce the net signal.

Ningyue Fan, Jack T. Dinsmore and Roger W. Romani reanalyzed IXPE observations of the eastern extended X-ray jet, taken in 2023. The jet is faint against X-ray and particle backgrounds. The researchers used the public LeakageLib fitting package, combining IXPE event data with spatial and spectral information from Chandra observations. Their event-by-event maximum-likelihood method weights how likely each event is to come from the source or background. They tested two ways of representing the source: three separate regions, and a model in which the spine and sheath overlap.

Those regions are the head, a bright area at the jet’s onset that may correspond to a recollimation shock; the spine, the bright central ridge; and the sheath, fainter emission around the spine. In both model setups, a three-region description improved the fit over treating the jet as one polarization region, by more than three standard deviations. This supports allowing the regions to have different polarization properties. It does not mean that polarization was individually detected with high significance in each region.

The head and spine have higher fitted polarization than the sheath. The measured EVPAs are consistent across regions, but the sheath’s angle is especially poorly constrained: the individual-region polarization detections are below two standard deviations. The study reports EVPA values around 0 degrees, while the jet axis is about 90 degrees in the paper’s sky-coordinate convention. Under the assumed synchrotron interpretation, the magnetic field is perpendicular to the EVPA, so the inferred mean field is aligned with the jet. The authors’ shared-field interpretation is consistent with the data, but the sheath does not have a well-established angle of its own. The result concerns the IXPE-observed base, not the entire extended jet.

To estimate magnetic turbulence, the authors converted polarization measurements into a ratio of turbulent to ordered magnetic field, assuming isotropic turbulence and a model of the emission. The resulting sheath estimate is roughly twice that of the head and spine. This is a model-derived comparison, not a direct measurement of the field ratio. The inferred turbulence excess is weak: depending on the assumed spatial distribution of sheath emission, it is supported at about 1 to 1.8 standard deviations. The sheath’s polarization is itself poorly constrained. The stronger statistical result is that a multi-region fit improves on a single-region fit—not that a turbulence difference has been firmly established.

One possible explanation is the Kelvin–Helmholtz instability, which can arise where flows move at different speeds. Shear at the boundary between the jet and surrounding material could stir and mix the flow, potentially increasing magnetic disorder. The measurements are consistent with this idea but do not directly detect the instability or establish it as the cause. Unresolved changes in field direction could also affect polarization.

The analysis illustrates how detailed event weighting can extract information from faint, background-contaminated X-ray data. Its interpretation nonetheless depends on the regional definitions and magnetic-field assumptions; in the overlapping model, the spine and sheath estimates are correlated. The authors note that higher sensitivity and spatial resolution will be needed to explore the polarization structure more fully and connect it to particle acceleration. This study analyzes the eastern jet in one 2023 observation; it does not establish the same pattern in the western jet or in jets generally.

The supplied manuscript is identified as a November 2026 arXiv draft. Although its acknowledgments mention an anonymous referee, the evidence available here does not establish the manuscript’s publication or review status. The findings are best read as a model-dependent hint of greater turbulence near the eastern jet boundary, rather than a settled measurement or explanation.