Time evolution of the instantaneous and cumulative ionizing power of a stellar population of N = 10 ⁶ stars and a Kroupa IMF (upper and lower panels, respectively). Solid curves correspond to the contribution from MS stars only; curves of different line types correspond to the combined contribution of MS stars and HMXBs for five different values of the survival fraction f _sur with 0.01 ⩽ f _sur ⩽ 1.