LOFAR as a probe of the sources of cosmological reionization

We propose use of the thickness of the ionization front as a discriminant between alternative modes of reionization in the early Universe, by stars or by miniquasars. Assuming a photoionization-recombination balance, we find that for miniquasar sources the transition from neutral to ionized intergalactic medium is extended and has two features. The first is a sudden steep increase in the neutral fraction with a typical width of 5-10 comoving mega-parsecs, depending on the miniquasar power. The second feature is a long wing that represents a much slower transition from a neutral fraction of ≈ 0.8 to 1. The angular resolution of LOFAR is expected to resolve these scales and will, therefore, play an important role in discriminating the hard sources of ionizing photons from the stellar ones.