Advances in High Energy Physics

Volume 2018, Article ID 1953586, 7 pages

https://doi.org/10.1155/2018/1953586

## Heun Functions Describing Bosons and Fermions on Melvin’s Spacetime

Faculty of Physics, “Alexandru Ioan Cuza” University of Iaşi, Bd. Carol I, No. 11, 700506 Iaşi, Romania

Correspondence should be addressed to Marina-Aura Dariescu; or.ciau@aniram

Received 13 January 2018; Accepted 29 April 2018; Published 11 June 2018

Academic Editor: Saber Zarrinkamar

Copyright © 2018 Marina-Aura Dariescu and Ciprian Dariescu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP^{3}.

#### Abstract

Employing a pseudo-orthonormal coordinate-free approach, the solutions to the Klein–Gordon and Dirac equations for particles in Melvin spacetime are derived in terms of Heun’s biconfluent functions.

#### 1. Introduction

The study of relativistic particles in static magnetic fields has a long history and is still attracting considerable attention, especially for cases where someone deals with curved manifolds.

Even though on Minkowski spacetime the relativistic behavior of an electron in various magnetostatic configurations is well understood (see, for example, Johnson and Lippmann’s paper [1]), a weakness on curved spacetime regards the explicit gauge covariant formulation.

Recently, when dealing with slowly rotating neutron stars which have been termed as magnetars [2], it has been assumed that their huge magnetic induction in the core and crust, – (G), is affecting the spacetime geometry. A way out could be the search for general relativistic solutions with the magnetic field considered as a perturbation of the spherically symmetric background [3]. Another way is to assume that magnetized metrics, as the one belonging to the Melvin class [4, 5], may be reliable candidates for describing these highly compact astrophysical objects with a dominant axial magnetic field [6].

Within a coordinate-dependent formulation, switching between canonical and pseudo-orthonormal basis, the above-mentioned authors are integrating the system of four coupled first-order differential equations, in the first approximation, neglecting the terms in higher orders of the polar radial coordinate . Their solutions are expressed in terms of generalized Laguerre polynomials, similarly to the case of the Dirac equation in cylindrical coordinates on a flat manifold [7].

In the present work, we are applying a coordinates-free method to analyze the Klein–Gordon and Dirac equations describing particles evolving in Melvin’s spacetime. Employing Cartan’s formalism, we are computing all the essential geometrical objects for writing down the corresponding matter fields and Einstein’s equations.

It turns out that the -gauge covariant Klein–Gordon equation can be exactly solved, its solutions being given by the Heun biconfluent functions [8–10]. The same happens with the approximate expression of the second-order differential system derived from the Dirac equation.

The Heun functions, either general or confluent, are main targets of recent investigations and have been obtained for massless particles evolving in a Universe described by the metric function written as a nonlinear mixture of Schwarzschild, Melvine, and Bertotti-Robinson solutions [11].

#### 2. The Geometry

Recently, in [12], the procedure of transforming a known static symmetric solution to Einstein-hydrodynamic equations into a magnetized metric was presented, by (nonlinearly) adding the magnetic field. In spherical coordinates, this has the general form with the metric functions and depending only on and where, for the moment, is a parameter related to the magnetic field intensity.

In the pseudo-orthonormal Cartan frame corresponding to the metric (1),for the potential where is the strength of the magnetic field on the axis, and the Maxwell tensor components, corresponding to a poloidal magnetic field with and , are given by the relationspointing out a prolate (in shape) star.

Once we assume , we can switch to cylindrical coordinates , byso that the magnetized metric (1) turns into the simple Melvin expressionwith

Within an -gauge covariant formulation, we introduce the pseudo-orthonormal framewhose corresponding dual base isso that the metric (7) gets the Minkowskian form , with . The first Cartan equation, with and , leads to the following connection one-form:where is the derivative of with respect to .

Employing the second Cartan equation one derives the curvature two-forms , with , leading to the curvature componentspointing out the special radius value , for which only the components are surviving and the Weyl tensor vanishes.

Since the scalar curvature is zero, the Einstein tensor components are given by the Ricci tensor components, as

In the pseudo-orthonormal frame whose dual bases are (10), it turns out that the potential (4), generating the magnetic induction along , gets the familiar expression and the essential component of the Maxwell tensor reads where .

Using the energy-momentum tensor components in the Einstein equations , one gets the following relation between the parameters and : with .

#### 3. Exactly Solvable Klein–Gordon Equation

In this section, we are going to construct the wave function of the charged bosons, considered as test particles evolving in the crust of a relativistic magnetar. The complex scalar field of mass , minimally coupled to gravity, is described by the gauge covariant Klein–Gordon equation which, in the pseudo-orthonormal frame with the dual bases (10), reads The above form suggests the variables separation which leads to the following differential equation for the unknown function , with defined in (8).

This can be exactly integrated, its solution being expressed in terms of the Heun biconfluent function as [9, 10] where the variable and the parameters are, respectively, given by

Let us point out that the Heun biconfluent equation has one regular singularity at the origin and one irregular at and can be obtained, from the Heun general equation, by a process of successive confluences [10].

Regarding the asymptotic behavior of the function (24), solution to (23), that has a singularity in , due to the exponential term, this is vanishing for large -values. On the other hand, for a regular solution at the origin (where ), one has to choose the plus sign of in (26).

#### 4. The -Gauge Covariant Dirac Equation

For relativistic fermions of mass , coupled to the external magnetic field generated by (16), the Dirac equation has the -gauge covariant expression where “;” stands for the covariant derivative In view of the relations (12), the term expressing the Ricci spin-connection in (27) reads where we have introduced the function

With the explicit form of the Dirac equation (27) being one may use the variables separation to derive the differential equation satisfied by the part depending on ; i.e.,

With the following function substitution the above equation becomes where and we are going to use the Dirac representation for the matrices,withwhere denotes the usual Pauli matrices.

In the following, we are assuming that the particle is not moving along the magnetic field direction, i.e., , and the bispinor is of the form so that (35) decouples in two equations for the (two-component) spinors and ; i.e.,

Applying the usual procedure, one gets the following differential equations: which cannot be analytically solved. However, by imposing the condition and neglecting the powers of larger than 3, (41) get the simpler forms:The corresponding solutions, i.e.,are expressed in terms of Heun’s biconfluent functions [8–10]of variablesand parametersand therefore the components of the bispinor in (34) are given by

Using the expressions (47) in (32), one may compute the radial current density, meaning particles per unit time and per unit covariant 2-surface as and the corresponding (radial) current, represented in Figure 1, as a function of One may notice that, for , the current is suddenly increasing from zero to a maximum value, which depends on the ratio and on the magnetic field intensity.