Evolution of primordial black hole spin due to Hawking radiation

Near extremal Kerr black holes (BHs) are subject to the Thorne limit |$a\lt a^*_{\rm lim}=0.998$| in the case of thin disc accretion, or some generalized version of this in other disc geometries. However, any limit that differs from the thermodynamics limit a* < 1 can in principle be evaded in other astrophysical configurations, and in particular if the near extremal BHs are primordial and subject to evaporation by Hawking radiation only. We derive the lower mass limit above which Hawking radiation is slow enough so that a primordial black hole with a spin initially above some generalized Thorne limit can still be above this limit today. Thus, we point out that the observation of Kerr BHs with extremely high spin should be a hint of either exotic astrophysical mechanisms or primordial origin.