2026/08/03 by Jie Liao, Wancheng Xu, Lang Cui +10
Physics and Astronomy · #astro-ph.HE
37 pages, 10 figures
arxiv created 2026/08/03 · arxiv updated 2026/08/04
PeVatrons are cosmic accelerators capable of driving particles to petaelectronvolt (PeV) energies. Recently, microquasar jets have emerged as compelling Galactic PeVatron candidates. This is especially the case for SS 433 as its >100 TeV gamma-ray emission is spatially coincident with an atomic cloud. However, the exact region where PeV protons are accelerated and injected within these jets remains unresolved. Here we report, using archival, multi-frequency VLBA observations, the magnetic field profile B(H) along the SS 433 inner jet on tens of AU scale, where H is the distance from the central compact object. We find that the field declines as B(H) ∝ H-0.50±0.12, demonstrating that the magnetic rigidity B(H)R\rm acc grows with H for a conical jet. This implies the Hillas limit (E\rm max ∝ BH) to lie well beyond a PeV at a few hundred-AU scale, which becomes a highly potential site for accelerating protons to energies E\rm cut ≃ 2.6 PeV inferred from the LHAASO gamma-ray spectrum. These results reveal a hidden PeVatron within the baryonic ejecta of microquasar SS 433, well upstream of the extended TeV-emitting lobes.