Strings near BTZ black holes: a Carrollian chronicle

The BTZ black hole provides a tractable (2+1)-dimensional example for investigating string dynamics in curved spacetime. However, a systematic and robust analysis of the solution space of strings in the near-horizon region of BTZ black holes remains elusive in the literature. This work aims to fill this gap by employing the string-Carroll expansion. This formalism provides a natural setting for working with the near-horizon region, because near-horizon expansions for non-extremal black holes match string-Carroll expansions. Using this formalism, and expanding the string action and pullback fields in powers of an effective speed of light, we study the dynamics of closed bosonic strings in the near-horizon, nonextremal BTZ spacetime. Our approach classifies the general characteristics and further reveals some novel features of the families of string solutions.