# Extensions of the generalized hedgehog ansatz for the Einstein-nonlinear $\sigma$-model system: black holes with NUT, black strings and time-dependent solutions

## TL;DR

This paper develops extended ansätze for the Einstein-nonlinear sigma-model system, enabling the construction of new exact solutions like black holes with NUT charge, black strings, and dynamic spacetimes, enhancing understanding of these complex gravitational configurations.

## Contribution

It introduces generalized ansätze that simplify the field equations, leading to new and rederived exact solutions in various spacetime geometries within the Einstein-nonlinear sigma-model framework.

## Key findings

- Constructed stationary black holes with NUT parameter
- Derived uniform black string solutions
- Obtained time-dependent solutions including Robinson-Trautman and Kundt spacetimes

## Abstract

We consider a class of ans\"atze for the construction of exact solutions of the Einstein-nonlinear $\sigma$-model system with an arbitrary cosmological constant in (3+1) dimensions. Exploiting a geometric interplay between the $SU(2)$ field and Killing vectors of the spacetime reduces the matter field equations to a single scalar equation (identically satisfied in some cases) and simultaneously simplifies Einstein's equations. This is then exemplified over various classes of spacetimes, which allows us to construct stationary black holes with a NUT parameter and uniform black strings, as well as time-dependent solutions such as Robinson-Trautman and Kundt spacetimes, Vaidya-type radiating black holes and certain Bianchi~IX cosmologies. In addition to new solutions, some previously known ones are rederived in a more systematic way.