Probing Nonequilibrium Fluctuations through Linear Response

Linear response analysis in the nonequilibrium steady state (Gaussian regime) provides two independent fluctuation-response relations. One, in the form of the symmetric matrix, manifests the departure from the equilibrium formula through the quantity so-called {\it irreversible circulation}. The other, in the anti-symmetric form, connects the asymmetries in the fluctuation and the response function. These formulas represent characteristic features of fluctuations far from equilibrium, which have no counterparts in thermal equilibrium.