Electron-ion temperature equilibration at collisionless shocks in supernova remnants

The topic of this review is the current state of our knowledge about the degree of initial equilibration between electrons, protons and ions at supernova remnant (SNR) shocks. Specifically, the question has been raised as to whether there is an inverse relationship between the shock velocity and the equilibration. This review aims to compile every method that has been used to measure the equilibration and every SNR on which they have been tested. I review each method, its problems and uncertainties and how those would effect the degree of equilibration (or velocity) inferred. The final compilation of observed electron to proton temperature ratios as a function of shock velocity gives an accurate, conservative picture of the state of our knowledge and the avenues we need to pursue to make progress in our understanding of the relation between the velocity of a shock and the degree of equilibration.