Effect of relativistic equation of state and diffusion coefficient on diffusive shock acceleration

Abstract: Diffusive Shock Acceleration, resulting from first-order Fermi acceleration occurring near Magnetohydrodynamic shock waves, is essential in explaining the power law spectrum in various astrophysical radiation and cosmic rays. We perform Monte Carlo simulations to model the stochastic scattering process in Fermi acceleration, capturing the confinement of particles around the shock within the ambient fluid. The model is tested and validated by comparing it with the spectral index obtained with analytical calculation. Assuming a relativistic EoS, we calculate the power-law spectral index for different diffusion coefficients. With constant diffusion co-efficient and stiffer EoS, the observed range of the spectral index is very narrow; however, as the EoS becomes softer, the range increases. With varying diffusion co-efficient stiffer EoS fails to give a well-defined spectral index (no linear spectrum); however, as the EoS becomes softer, the spectral index lies between $2-4$. For ultra-relativistic shocks, we consistently obtained a linear spectrum; however, the spectral index range varied considerably with the diffusion coefficient.