String-scale black hole description in string theory

Determine the correct microscopic and Euclidean string-theoretic description of black holes when their horizon size approaches the string length, identifying the appropriate saddle (beyond Einstein gravity) that captures the physics at temperatures near the Hagedorn scale in asymptotically flat string theory.

Background

The paper studies the proposed correspondence between highly excited strings (string stars) and black holes, particularly as the black hole size shrinks toward the string scale due to Hawking radiation. In Euclidean string theory on asymptotically R^d × S^1_β, the usual Einstein gravity description breaks down for β ∼ l_s, raising the broader question of the correct string-theoretic description of such small black holes.

The authors explore this question with angular momentum turned on, arguing for a rotating string star saddle that interpolates between free rotating strings and rotating black holes. The explicit open question highlights the general unresolved status of the black hole description at the string scale in string theory.