Aging at criticality in models with absobing states

The long-time dynamics of the critical models with absobing states which are brought suddenly out of an uncorrelated initial situation undergoes ageing in close analogy with quenched magnetic systems. In particular, we show that time-translation invariance is broken and that dynamical scaling holds. We find that the autocorrelation and autoresponse exponents $\lambda_{\Gamma}$ and $\lambda_R$ are equal but, in contrast to systems relaxing to equilibrium, the aging exponents $a$ and $b$ are distinct. A recent proposal to define a non-equilibrium temperature through the short-time limit of the fluctuation-dissipation ratio is therefore not applicable.