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This theoretical study examines how cosmic rays escape from wind-driven bubbles around massive stars using time-dependent transport models. The researchers found that the energy spectrum of escaping cosmic rays can be harder (steeper at high energies) than the standard E^-2 expectation from conventional shock acceleration theory. Additionally, the model predicts suppression of low-energy particles depending on turbulence conditions, which could lead to enhanced particle confinement within these bubbles.
Why it matters
These findings could help explain observed cosmic ray spectra that deviate from standard predictions and improve our understanding of how massive stellar winds contribute to the overall cosmic ray population in galaxies. The predicted low-energy suppression may have observable consequences for multi-messenger astronomy, linking cosmic ray observations with gamma-ray and neutrino detections from these stellar environments.
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⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
Abstract: Overview: Wind-driven bubbles are dynamic systems that can accelerate cosmic rays, depending on their physical properties, up to very high energies. We investigate how a time-dependent description of the particle transport may impact the escaping cosmic-ray flux. Model: The wind bubble system is modeled as spherically symmetric. Cosmic rays are continuously injected at the position of the termination shock and propagate through advection and diffusion until the escape at the time-dependent position of the forward shock, which is treated as a free escape boundary. Methods: The one-dimensional spherical time-dependent transport equation is solved by transforming it into the corresponding set of stochastic differential equations, and integrated with a modified version of the open source cosmic-ray propagation framework CRPropa. Results: We find that, during the wind driven phase, the downstream escaping spectra from wind bubbles can be harder than $sim E^{-2}$, the conventional expectation from diffusive shock acceleration. Depending on the turbulence model the initial energy spectrum can be significantly suppressed at lowest energies, which could be an observable feature to distinguish between different turbulence realizations. This effect could lead to an efficient confinement of low energy particles, potentially leading to observable implication in terms of multi-messenger radiation and cosmic-ray accumulated grammage within the bubble.
Source: Time-dependent cosmic-ray escape from wind bubbles: hard spectra formation