We consider the signatures of a population of primordial black holes ( PBHs ) in future observations of 21cm radiation from neutral hydrogen at high redshift .
We focus on PBHs in the mass range 5 \times 10 ^ { 10 } \textrm { kg } \lesssim M _ { \textrm { PBH } } \lesssim 10 ^ { 14 } \textrm { kg } , which primarily influence the intergalactic medium ( IGM ) by heating from direct Hawking radiation .
Our computation takes into account the black hole graybody factors and the detailed energy dependence of photon and e ^ { \pm } absorption by the IGM .
We find that for black holes with initial masses between 5 \times 10 ^ { 11 } \textrm { kg } \lesssim M _ { \textrm { PBH } } \lesssim 10 ^ { 14 } \textrm { kg } , the signal mimics that of a decaying dark matter species .
For black holes in the range 5 \times 10 ^ { 10 } \textrm { kg } \lesssim M _ { \textrm { PBH } } \lesssim 5 \times 10 ^ { 11 } % \textrm { kg } , the late stages of evaporation produce a characteristic feature in the 21cm brightness temperature that provides a unique signature of the black hole population .
If no signal is observed , then 21cm observations will provide significantly better constraints on PBHs in the mass range 5 \times 10 ^ { 10 } \textrm { kg } \lesssim M _ { \textrm { PBH } } \lesssim 10 ^ { 12 } \textrm { kg } than are currently available from the diffuse \gamma -ray background .