21-cm constraints on spinning primordial black holes
  Hawking radiation from primordial black holes (PBH) can ionize and heat up
neutral gas during the cosmic dark ages, leaving imprints on the global 21-cm
signal of neutral hydrogen. We use the global 21-cm signal to constrain the
abundance of spinning PBHs in mass range of $[2 \times 10^{13}, 10^{18}]$
grams. We consider several extended PBH distribution models. Our results show
that 21-cm can set the most stringent PBH bounds in our mass window. Compared
with constraints set by {\it Planck} cosmic microwave background (CMB) data,
21-cm limits are more stringent by about two orders of magnitudes. PBHs with
higher spin are typically more strongly constrained. Our 21-cm constraints for
the monochromatic mass distribution rule out spinless PBHs with initial mass
below $1.5 \times 10^{17}\ \r{g}$, whereas extreme Kerr PBHs with reduced
initial spin of $a_0=0.999$ are excluded as the dominant dark matter component
for masses below $6 \times 10^{17}\ \r{g}$. We also derived limits for the
log-normal, power-law and critical collapse PBH mass distributions.
