LOFAR as a Probe of the Sources of Cosmological Reionisation

We propose use of the thickness of the ionisation front as a discriminant between alternative modes of reionisation in the early universe, by stars or by miniquasars. Assuming a photoionisation-recombination balance, we find that for miniquasar sources the transition from neutral to ionised 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 megaparsecs, depending on the miniquasar power. The second feature is a long wing that represents a much slower transition from neutral fraction of $\approx 0.8$ 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 ionising photons from the stellar ones.