  The 21-cm signal from the cosmic reionization epoch can shed light on the
history of heating of the primordial intergalactic medium (IGM) at z~30-10. It
has been suggested that X-rays from the first accreting black holes could
significantly heat the Universe at these early epochs. Here we propose another
IGM heating mechanism associated with the first stars. As known from previous
work, the remnants of powerful supernovae (SNe) ending the lives of massive
Population III stars could readily expand out of their host dark matter
minihalos into the surrounding IGM, aided by the preceeding photoevaporation of
the halo's gas by the UV radiation from the progenitor star. We argue that
during the evolution of such a remnant a significant fraction of the SN kinetic
energy can be put into low-energy (E<30 MeV) cosmic rays that will eventually
escape into the IGM. These subrelativistic cosmic rays could propagate through
the Universe and heat the IGM by ~10-100 K by z~15, before more powerful
reionization/heating mechanisms associated with the first galaxies and quasars
came into play. Future 21-cm observations could thus constrain the energetics
of the first supernovae and provide information on the magnetic fields in the
primordial IGM.
