  An active bath, made of self-propelling units, is a nonequilibrium medium in
which the Einstein relation $D=\mu k_B T$ between the mobility $\mu$ and the
diffusivity $D$ of a tracer particle cannot be expected to hold a priori. We
consider here heavy tracers for which these coefficients can be related to
correlation functions which we estimate. We show that, to a good approximation,
an Einstein relation does hold in an active bath upon using a different
temperature which is defined mechanically, through the pressure exerted on the
tracer.
