Distinctive rings in the 21 cm signal of the epoch of reionization
  It is predicted that sources emitting UV radiation in the Lyman band during
the epoch of reionization (EoR) showed a series of discontinuities in their
Ly-alpha flux radial profile as a consequence of the thickness of the Lyman
line series in the primeval intergalactic medium. Through unsaturated
Wouthuysen-Field coupling, these spherical discontinuities are also present in
the 21 cm emission of the neutral IGM. In this article, we study the effects
these discontinuities have on the differential brightness temperature of the 21
cm signal of neutral hydrogen in a realistic setting including all other
sources of fluctuations. We focus on the early phases of the EoR, and we
address the question of the detectability by the planned Square Kilometre
Array. Such a detection would be of great interest, because these structures
could provide an unambiguous diagnostic for the cosmological origin of the
signal remaining after the foreground cleaning procedure. Also, they could be
used as a new type of standard rulers. We determine the differential brightness
temperature of the 21 cm signal in the presence of inhomogeneous
Wouthuysen-Field effect using simulations which include (hydro)dynamics and
both ionizing and Lyman lines 3D radiative transfer with the code LICORICE. We
find that the Lyman horizons are clearly visible on the maps and radial
profiles around the first sources of our simulations, but for a limited time
interval, typically \Delta z \approx 2 at z \sim 13. Stacking the profiles of
the different sources of the simulation at a given redshift results in
extending this interval to \Delta z \approx 4. When we take into account the
implementation and design planned for the SKA (collecting area, sensitivity,
resolution), we find that detection will be challenging. It may be possible
with a 10 km diameter for the core, but will be difficult with the currently
favored design of a 5 km core.
