AdS_3 x S^3 (Un)twisted and Squashed, and an O(2,2;Z) Multiplet of Dyonic Strings

TL;DR

The paper investigates AdS$_3\times S^3$ solutions in type II string theory carrying NS-NS and RR 3-form charges and develops dual descriptions via Hopf T-duality along the S$^3$ and AdS$_3$ fibers. It reveals how S$^3$ can untwist to S$^2\times S^1$, become lens spaces S$^3/\mathbb{Z}_p$, or become squashed, with the squashing tied to the charges; similar structures arise for AdS$_3$, and the two dualizations lead to isentropic, entropy-preserving mappings between black holes. An explicit $O(2,2;\mathbb{Z})$ multiplet of dyonic strings is constructed within consistent six-dimensional truncations that capture NS-NS/R-R and electric/magnetic dualities. The work also analyzes Killing spinors to determine how supersymmetry behaves under Hopf dualities, showing orientation-dependent preservation or breaking, and demonstrates isentropic connections between five- and four-dimensional black holes via Hopf reductions. Overall, the paper provides a unified duality framework linking near-horizon AdS$_3\times S^3$ geometries across dimensional reductions and charge configurations, with implications for holography and BPS spectra.

Abstract

We consider type IIB configurations carrying both NS-NS and R-R electric and magnetic 3-form charges, and whose near horizon geometry contains AdS_3 x S^3. Noting that S^3 is a U(1) bundle over CP^1 \sim S^2, we construct the dual type IIA configurations by a Hopf T-duality along the U(1) fibre. In the case where there are only R-R charges, the S^3 is untwisted to S^2 x S^1 (in analogy with a previous treatment of AdS_5 x S^5.) However, in the case where there are only NS-NS charges, the S^3 becomes the cyclic lens space S^3/Z_p with its round metric (and is hence invariant when p=1), where p is the magnetic NS-NS charge. In the generic case with NS-NS and R-R charges, the S^3 not only becomes S^3/Z_p but is also squashed, with a squashing parameter that is related to the values of the charges. Similar results apply if we regard AdS_3 as a bundle over AdS_2 and T-dualise along the fibre. We show that Hopf T-dualities relate different black holes, and that they preserve the entropy. The AdS_3 x S^3 solutions arise as the near-horizon limits of dyonic strings. We construct an O(2,2;Z) multiplet of such dyonic strings, where O(2,2;Z) is a subgroup of the O(5,5) or O(5,21) six-dimensional duality groups, which captures the essence of the NS-NS/R-R and electric/magnetic dualities.