An extremal black hole is a type of black hole that exists at the limit of certain physical parameters, particularly when considering its mass and charge or angular momentum. In the context of general relativity and modern theoretical physics, extremal black holes are characterized by having their mass equal to a specific value where they cannot radiate further or absorb additional charge or angular momentum without becoming non-extremal.
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An extremal black hole lies at the parameter bound where its inner and outer horizons coincide. Its surface gravity and hence its semiclassical Hawking temperature vanish.