Magnetic black hole in Einstein-Dilaton-Square root nonlinear electrodynamics

Abstract

Starting from the most general action in Einstein-Dilaton-Nonlinear Electrodynamics (NED) theory, we obtain the field equations. We apply the field equations for the specific NED known as the Liouville type plus a cosmological constant and solve the field equations. With a pure magnetic field it is shown that the square root model is the strong field limit of the Born-Infeld NED theory. In static spherically symmetric spacetime and a magnetic monopole sitting at the origin, the field equations are exactly solvable provided the intergation constants of the solution and the theory constants appearing in the action are linked through two constraints. As it is known such an exact solution in the absence of dilaton i.e., gravity coupled to square root NED doesn't exist. Therefore, the presence of the dilaton gives additional freedom to solve the field equations. The obtained spacetime is singular and non-asymptotically flat and depending on the free parameters it may be a black hole or a cosmological object. For the black hole spacetime, we study the thermal stability of the spacetime and show that the black hole is thermally stable provided its size is larger than a critical value.