The Western Jet of SS 433/W50 Hard X-Ray Emission, Spectral Evolution, and Comparison to the Eastern Jet

The W50 nebula powered by the microquasar SS 433 is a unique laboratory for exploring several fundamental astrophysical phenomena. This study presents observations from NuSTAR and XMM-Newton, concentrating on the western lobe of W50. Detection of hard nonthermal X-ray emission is reported, extending up to ∼30 keV. This emission originates from a compact, knotty area referred to as the “Head,” located at ∼17' (equivalent to 26.5 pc at an assumed distance of 5.5 kpc) to the west of SS 433 and characterized by a power-law spectrum with a hard photon index of 1.55 ± 0.07 (0.5–30 keV). Moving westward from SS 433, the photon index gradually steepens, ultimately reaching a photon index of 2.10 ± 0.05 in the “w2” region centered at ∼35'  or ∼56 pc from SS 433. The distinct hard X-ray knots observed serve as clear markers for sites of particle acceleration within the western jet. The synchrotron radiation from the “Head” region implies an equipartition magnetic field strength of B ∼ 15 μG. Notably, these properties (western “Head” location, unusually hard spectral index, inferred magnetic field, and spectral evolution away from SS 433) are very similar to what has been observed in the eastern lobe, supporting a symmetric jet-driven origin. Finally, the broadband spectral energy distribution and X-ray morphology are modeled using semianalytic jet models, exploring different jet velocity and magnetic field configurations. The results favor a scenario in which in situ particle acceleration and synchrotron emission dominate, with implications for understanding particle transport, jet dynamics, and W50’s role as a Galactic PeVatron.