Impact of cosmic rays on the global 21-cm signal during cosmic dawn
  It is extremely important to understand the processes through which the
thermal state of the inter-galactic medium (IGM) evolved in the early universe
in order to study the evolution of HI 21-cm signal during cosmic dawn. Here, we
consider the heating of the IGM due to cosmic ray protons generated by the
supernovae from both early Pop III and Pop II stars. The low energy cosmic ray
protons from Pop III supernovae can escape from minihalos and heat the IGM via
collision and ionization of hydrogen. Furthermore, high energy protons
generated in Pop II supernovae can escape the hosting halos and heat the IGM
via magnetosonic Alfv\'en waves. We show that the heating due to these cosmic
ray particles can significantly impact the IGM temperature and hence the global
21-cm signal at $z\sim 14-18$. The depth, location, and duration of the 21-cm
absorption profile are highly dependent on the efficiencies of cosmic ray
heating. In particular, the EDGES signal can be well fitted by the cosmic ray
heating along with the Lyman-$\alpha$ coupling, and the dark matter-baryon
interaction that we consider to achieve a `colder IGM background'. Further, we
argue that the properties of cosmic rays and the nature of first generation of
stars could be constrained by accurately measuring the global 21-cm absorption
signal during the cosmic dawn.
