On the foundation of equipartition in supernova remnants

Abstract

A widely accepted paradigm is that equipartition (eqp) between the energy density of cosmic rays (CRs) and the energy density of the magnetic field cannot be sustained in supernova remnants (SNRs). However, our 3D hydrodynamic supercomputer simulations, coupled with a non-linear diffusive shock acceleration model (NLDSA), provide evidence that eqp may be established at the end of the Sedov phase of evolution in which most SNRs spend the longest portions of their lives. We introduce term "constant partition" for any constant ratio between the CR energy density and the energy density of magnetic field in an SNR, while term "equipartition" should be reserved for the case of approximately the same values of the energy density (also, it is {constant partition} in the order of magnitude) of ultrarelativistic electrons only (or CRs in total) and the energy density of magnetic field. Our simulations suggest that this approximate constant partition does exist in all but the youngest SNRs. We speculate that since evolved SNRs at the end of the Sedov phase of evolution can reach eqp between CRs and magnetic fields, they may be responsible for initializing this type of eqp in the interstellar medium. Additionally, we show that eqp between the electron component of CRs and magnetic field may be used for calculating the magnetic field strength directly from observations of synchrotron emission from SNRs. The values of magnetic field strengths in SNRs given here are approximately 2.5 times lower than values calculated by Arbutina et al. (2012, 2013).