Microscopic entropy of AdS3 black holes revisited

We revisit the microscopic description of AdS(3) black holes in light of recent progress on their higher dimensional analogues. The grand canonical partition function that follows from the AdS(3)/CFT2 correspondence describes BPS and nearBPS black hole thermodynamics. We formulate an entropy extremization principle that accounts for both the black hole entropy and a constraint on its charges, in close analogy with asymptotically AdS black holes in higher dimensions. We are led to interpret supersymmetric black holes as ensembles of BPS microstates satisfying a charge constraint that is not respected by individual states. This interpretation provides a microscopic understanding of the hitherto mysterious charge constraints satisfied by all BPS black holes in AdS. We also develop thermodynamics and a nAttractor mechanism of AdS(3) black holes in the nearBPS regime.