Validity of the Onsager relations in relativistic binary mixtures

In this work we study the properties of a relativistic mixture of two nonreacting dilute species in thermal local equilibrium. Following the conventional ideas in kinetic theory, we use the concept of chaotic velocity. In particular, we address the nature of the density, or pressure gradient term that arises in the solution of the linearized Boltzmann equation in this context. Such an effect, also present for the single component problem, has, so far, not been analyzed from the point of view of the Onsager resciprocity relations. To address this matter, we propose two alternatives for the onsagerian matrix which comply with the corresponding reciprocity relations. The implications of both representations are briefly analyzed.