Broadband emission of microquasar remnants

Abstract

Microquasar remnants (MQRs), the long-lived cocoons inflated by extinct microquasar jets, have recently been proposed as hidden Galactic PeVatrons capable of producing ultra-high-energy gamma rays without an active central engine. While hadronic interactions can account for bright gamma-ray emission from nearby clouds, the direct detection of MQRs remains challenging because their intrinsic emission is expected to be extended and of low surface brightness. In this work, we explore the broadband emission of MQRs by focusing on the leptonic component confined within the cocoon and on particle interactions in the shocked shell surrounding it. We model the injection and time-dependent transport of relativistic particles, including stochastic re-acceleration driven by internal turbulence, treated as a second-order Fermi process. We consider sub-Eddington and super-Eddington microquasar systems and compute the resulting non-thermal emission from radio to gamma-ray energies, together with the thermal soft X-ray emission produced in the shocked shell. In the super-Eddington case, the intrinsic emission reaches peak values of $νL_ν\sim 10^{35}-10^{36}\,{\rm erg\,s^{-1}}$, whereas sub-Eddington remnants are typically several orders of magnitude fainter. At 1.3 GHz, the modeled cocoon surface brightness is of order $Σ_ν\sim 10^{-19}\,{\rm W\,m^{-2}\,Hz^{-1}\,sr^{-1}}$ for young powerful remnants and decreases rapidly as the remnant evolves. We find that the direct detectability of MQRs is therefore controlled mainly by surface brightness rather than by integrated luminosity. Powerful remnants may be detectable as extended synchrotron radio cocoons and shell-dominated soft X-ray structures, whereas sub-Eddington remnants are expected to be much harder to identify directly. Our results suggest that MQRs may constitute a hidden population of extended Galactic non-thermal sources.