NSbranes

NSbranes, or Neveu-Schwarz branes, are branes in string theory that couple to the Neveu-Schwarz (NS-NS) sector of the theory. The most familiar examples are the fundamental string, often called the NS1 or F1, and the NS5-brane. These objects are distinguished from D-branes, which couple to Ramond-Ramond (RR) fields. In ten-dimensional Type II string theories, NS-branes appear as classical solitonic solutions of the low-energy supergravity and include both perturbative (F1) and nonperturbative (NS5) configurations.

The fundamental string is an electric source for the NS-NS two-form B2 and has tension T_F1 = 1/(2π α′), which is independent of the string coupling g_s. Its magnetic dual is not a standard elementary object, while the NS5-brane is a magnetic source for the three-form H = dB and has tension T_NS5 = 1/[(2π)^5 α′^3 g_s^2]. The NS5-brane carries NS-NS flux through surrounding spheres and induces a nontrivial dilaton profile in its vicinity. NS-branes thus alter both the metric and the dilaton in the corresponding supergravity solutions.

NS-branes are related by string dualities to other objects. In Type IIB string theory, S-duality maps NS-branes to D-branes (the F1 to D1 and the NS5 to the D5). T-duality can transform NS-branes into other configurations such as Kaluza-Klein monopoles, reflecting their role in the nonperturbative duality web of string theory. The near-horizon limit of multiple NS5-branes yields a throat with a linear dilaton background, and the low-energy dynamics on the brane is described by little string theory, a six-dimensional nonlocal theory decoupled from gravity.

In summary, NSbranes form a fundamental part of the NS-NS sector, providing a counterpoint to RR-charged D-branes and illuminating the nonperturbative structure and dualities of string theory.