Cosmic Ray Production Rates in Supernova Remnants

Abstract

Supernova Remnants (SNRs) are the most likely sources of the galactic cosmic rays up to energies of about 10¹⁵ eV/nuc. The large scale shock waves of SNRs are almost ideal sites to accelerate particles up to these highly non-thermal energies by a first order Fermi mechanism which operates through scattering of the particles at magnetic irregularities. In order to get an estimate on the total amount of the explosion energy E _SNconverted into high energy particles the evolution of a SNR has to be followed up to the final merging with the interstellar medium. This can only be done by numerical simulations since the non-linear modifications of the shock wave due to particle acceleration as well as radiative cooling processes at later SNR stages have to be considered in such investigations. Based on a large sample of numerical evolution calculations performed for different ambient densities n _ext, SN explosion energies, magnetic fields etc. we discuss the final ‘yields’ of cosmic rays at the final SNR stage where the Mach number of the shock waves drops below 2. At these times the cosmic rays start to diffuse out of the remnant. In the range of external densities of10^-2 ≤ n _ext/[cm^-3] ≤ 30 we find a the total acceleration efficiency of about 0.15 E _SN with an increase up to 0.24 E _SN at maximum for an external density of n _ext = 10 cm^-3. Since for the larger ambient densities radiative cooling can reduce significantly the total thermal energy content of the remnant dissipation of Alfvén waves can provide an important heating mechanism for the gas at these later stages. From the collisions of the cosmic rays with the thermal plasma neutral pions are generated which decay subsequently into observable γ-rays above 100 MeV. Hence, we calculate these γ-ray luminosities of SNRs and compare them with current upper limits of ground based γ-raytelescopes. The development of dense shells due to cooling of the thermal plasma increases the γ-ray luminosities and e.g. an external density of n _ext = 10 cm^-3 with E _SN = 10⁵¹ erg can lead to a γ-ray flux above 10^-6 ph cm^-2 s^-1 for a remnant located at a distance of 1 kpc.