Microquasar remnants as hidden PeVatrons

TL;DR

The paper proposes microquasar remnants (MQRs) as long-lived reservoirs for ultra-relativistic cosmic rays, which, after the central engine shuts down, remain confined in a jet-inflated cocoon and later irradiate nearby molecular clouds to produce gamma rays via $pp$ interactions. CR production during the active MQ phase is modeled with a hadron-dominated spectrum accelerated at the reverse shock, yielding a maximum energy $E_{ m max}\approx 25\,\mathrm{PeV}$ and confinement within the cocoon under Bohm-like diffusion with $D(E)=\zeta D_B(E)$ and $\zeta=5$. The resulting gamma-ray emission from illuminated clouds can extend to very-high-energy (VHE) and ultra-high-energy (UHE) regimes, potentially detectable by LHAASO and companion observatories, and may explain several unidentified LHAASO Galactic sources. The study also explores robustness to turbulence regimes (Kolmogorov and Kraichnan) and discusses observational signatures, including faint extended radio remnants, offering a path to identify these hidden PeVatrons and clarify the origin of a subset of UHE Galactic gamma-ray sources.

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 transfer has 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 $pp$ collisions. Remnants of former super-Eddington systems can act as dark PeVatrons, releasing particles up to $\sim$10 PeV that illuminate surrounding clouds producing gamma rays reaching hundreds of TeV. This scenario provides a natural explanation for several unidentified Galactic LHAASO sources.

Figures (5)

• Figure 1: Conceptual scheme, not to scale. Left panel: MQ ($t<t_0$). The stellar-mass BH and the star are in the core. The backward shock at the end of the jets accelerates particles to relativistic energies. An overpressured cocoon evolves with the jets and envelops the entire system. Right panel: MQR ($t>t_0$). The central engine shuts down, and an MQR forms. CRs distributed within the survivor cocoon interact with a fragment of molecular cloud that enters the MQR, illuminating it.
• Figure 2: Evolution of proton distributions. Top: CRs within the cocoon. The color bar distinguishes the MQ ($t<t_0$, orange) and MQR ($t>t_0$, blue) phases. Bottom: CR propagation to a distance of 100 pc from the MQR over time, for a source with a continuous injection of particles.
• Figure 3: SEDs of the illuminated clouds at different times (see color bar). Leptonic emission is produced by secondary pairs created in the cloud via $pp$ interactions. Thick dashed lines show the sensitivity curves of Fermi (10 yr, grey), SWGO (5 yr, light blue), and LHAASO (1 yr, green) for a Galactic MQR at 5 kpc. Top: Cloud in the MQR. Bottom: Cloud at 100 pc from the MQR (continuous scenario).
• Figure 4: Same as in Fig.\ref{['fig: proton_distribution']} but assuming Kolmogorov diffusion inside the cocoon.
• Figure 5: Same as in Fig. \ref{['fig: seds']} but assuming Kolmogorov diffusion inside the cocoon.