Particle acceleration and multi-messenger radiation from Ultra-Luminous X-ray Sources: A new class of Galactic PeVatrons

TL;DR

This work demonstrates that ULX wind bubbles, powered by super-Eddington accretion, can accelerate protons to PeV energies at wind termination shocks through diffusive shock acceleration. By solving a space- and energy-dependent transport equation and considering realistic bubble structure and magnetic diffusion, the authors predict strong hadronic gamma-ray emission and accompanying neutrino and radio signatures, with SS433 serving as a concrete Galactic example that can explain the >100 TeV gamma rays observed by LHAASO. The study also forecasts neutrino detectability by KM3NeT and a potentially measurable radio component linked to the bubble, while evaluating the ULX contribution to Galactic cosmic rays near the knee. Overall, ULX wind bubbles emerge as a plausible new class of Galactic PeVatrons with detectable multi-messenger signals, though their overall contribution to the knee CR flux remains uncertain due to source numbers and diffusion effects.

Abstract

Super-Eddington accretion onto stellar-mass compact objects powers fast outflows in ultra-luminous X-ray sources (ULXs). Such outflows, which can reach mildly relativistic velocities, are often observed forming bubble structures. Wind bubbles are expected to develop strong wind termination shocks, which are sites of great interest for diffusive shock acceleration. We developed a model of diffusive shock acceleration in the wind bubbles powered by ULXs. We find that the maximum energy in these objects can easily reach the PeV range, promoting winds from ULXs as a new class of PeVatrons. We specialized our model in the context of the Galactic source SS433 and show that high-energy protons in the bubble might explain the highest energy photons (>100 TeV) and their morphology recently observed by LHAASO. In this paper, we discuss the detectability of such a source in neutrinos, and we analyze the possible radio counterpart of ULXs focusing on the case of W50, the nebula surrounding SS433. Finally, we discuss the possible contribution of Galactic ULXs to the cosmic-ray flux at the knee, concluding that their role could be significant only if one of these sources, currently undetected, were sufficiently close.

Figures (5)

• Figure 1: ULX sketch showing various components (not to scale) that are relevant to our model. Two scenarios for the density distribution in the shocked wind that are investigated are displayed.
• Figure 2: Typical timescales regulating behavior of non-thermal particles in ULX wind bubble. The acceleration timescale (blue dashed) is compared with the timescales of the main escape processes: advection (yellow dot-dashed) and diffusion (green dotted). The age of the system is also shown as a solid black line.
• Figure 3: Proton spectra at different radii. The spectrum at the TS (solid black) is compared with the spectra at progressively larger radii approaching the FS (from the red dotted to the blue dashed).
• Figure 4: $\Dot{M}-V_{w}$ parameter space of typical Galactic wind bubbles and achievable maximum energy computed according to Eq. \ref{['Eq: E_max']}.
• Figure 5: Spectral energy distribution of gamma-ray emission from SS 433. Colored markers indicate flux measurements obtained with H.E.S.S. and HAWC. More recent data obtained by LHAASO are shown with black markers. The upper limits imposed by Fermi-LAT on the GeV emission from the eastern (E) and western (W) jets are overplotted with colored downward-pointing arrows. The predicted (hadronic) gamma-ray spectrum from the bubble is overplotted for the two scenarios presented in Fig. \ref{['Fig: ULX sketch']}. The shaded region shows the expected range of gamma-ray emission when varying the terminal wind speed by a factor of two.