{#sig_map}
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Let us assume that there is an isolated source radiating photons in the neutral and cold IGM. Our first aim is to study the detectability of the 21-cm pattern around such a source. We later study a more complex and realistic model where multiple sources form within a cosmological volume. The main steps to simulate the [H [i ]{}]{} signal maps around an isolated source are as follows:

-   For a given source model, we generate one-dimensional ionization profiles of the hydrogen and helium species and the kinetic temperature around each source by solving the radiative transfer equations [@1994MNRAS.269..563F; @thomas08]. The main features of the method are described in details in our earlier works @ghara15a [@ghara15c], which is based on @thomas08. We assume that the IGM consists of hydrogen and helium of uniform density contrast $\delta$. We assume that the IGM is completely neutral when the source starts to radiate.

-   We calculate the ${\rm {Ly{\alpha}}}$ photon flux by considering the ${\rm {Ly{\alpha}}}$ contribution from the continuum spectrum, recombination in the interstellar medium and the secondary ionization due to the X-rays. The ${\delta T_{\rm b}}$ profile, as well as the detectability of the source, critically depends on the 