Microquasar remnants as hidden PeVatrons

Abstract

The Large High Altitude Air Shower Observatory (LHAASO) has revealed numerous ultrahigh-energy gamma-ray sources of unknown origin. We propose that a fraction of them can be explained by microquasar remnants, i.e., binary systems where mass transferhas ceased and the central engine is quenched. Cosmic rays injected during the active phase of a microquasar may remain confined within its cocoon and subsequently interact with nearby molecular clouds, producing bright gamma-ray emission through ppcollisions. Remnants of former super-Eddington systems can act as dark PeVatrons, releasing particles up to ∼10 PeV that illuminate surrounding clouds producing gamma rays reaching hundreds of TeV. This scenario provides a natural explanation for severalunidentified Galactic LHAASO sources.