2026/08/04 by Chao-Nan Tong, Hong-Bin Tan, Ruo-Yu Liu
Physics and Astronomy · #astro-ph.HE
Recently, the Large High Altitude Air Shower Observatory (LHAASO) detected ultrahigh-energy (UHE; photon energy E>100TeV) γ-ray emission toward SS 433, the microquasar embedded in the W50 nebula, making it a promising Galactic PeVatron candidate. We reanalyze the archival XMM-Newton observations covering the bipolar jets and the thermal X-ray shell north of SS 433, and derive spatially resolved profiles of the nonthermal X-ray intensity and photon index along both jets. The jet emission softens with distance from the source, implying a correspondingly evolving electron population. In particular, a hard electron component appears close to the jet bases, which can account for the UHE emission from SS 433 via inverse Compton radiation if the magnetic field remains approximately uniform along the jets. The result, however, is highly sensitive to the magnetic field profile. For flux-conserving configurations in which the field decreases as the jet expands, the stronger field required in the inner regions may reduce the number of X-ray-emitting electrons and suppress their inverse Compton emission. Furthermore, electron transport calculations show that injection only at the jet bases cannot reproduce the observed intensity and spectral evolution, particularly the downstream re-brightening features, indicating additional particle injection and/or re-acceleration within the jets.