Requirements for Cosmological 21cm Masers

We explore the prospects for a population inversion in the hyperfine levels of atomic hydrogen at high redshifts when the Universe was still predominantly neutral. The existence of 21-cm masers during the epoch of reionisation (EoR) could greatly boost the expected EoR signals. We perform Monte-Carlo calculations of the radiative transfer of Lyα photons emitted by a source embedded in a neutral collapsing gas cloud and compute the Lyα spectrum as function of radius and time. In such scenarios, the Lyα color temperature may be negative in large volumes surrounding the source. Classically, for sources with Lyα luminosities exceeding ∼ 10 − 10 erg s, a population inversion in the hyperfine transition via the Wouthuysen-Field effect is established, thus resulting in a 21-cm maser. However, quantum interference can strongly reduce the strength of the WF-effect in the Lyα line wings, which implies that the required Lyα luminosities should be larger by orders of magnitude. Furthermore, we show that the astrophysical conditions that are required to establish a population inversion are very difficult to meet: A low flux of Lyα photons produced via x-ray and/or cosmic ray heating, or through cascades of atoms which are photo-excited by higher Lyman-series photons, can cause Tα to become positive. If, despite these obstacles, 21-cm masers do exist then we find that the background Cosmic Microwave background could be amplified by a maximum factor of ∼ 100, and maser saturation is not important.