Asymptotic front behavior in an A + B → 2A reaction under subdiffusion

We discuss the front propagation in the A + B → 2A reaction under subdiffusion, which is described by continuous-time random walks with a heavy-tailed power-law waiting-time probability density function. Using a crossover argument, we discuss the two scaling regimes of the front propagation: an intermediate asymptotic regime given by the front solution of the corresponding continuous equation, and the final asymptotics, which is fluctuation dominated and therefore lays out of reach of the continuous scheme. We moreover show that the continuous reaction subdiffusion equation indeed possesses a front solution that decelerates and becomes narrow in the course of time. This continuous description breaks down for larger times when the front gets atomically sharp. We show that the velocity of such fronts decays in time faster than in the continuous regime.