
457 Page 4 of 18 Eur. Phys. J. C (2015) 75 :457
where g(ρ) is an arbitrary function of radial coordinate, ρ,
and should be determined and l is a scale length factor which
is related to . The angular coordinate ϕ is dimensionless
and ranges in 0 ≤ ϕ<2π. The motivation for this curious
choice for the metric gauge [g
tt
∝ −ρ
2
and (g
ρρ
)
−1
∝
g
ϕϕ
] instead of the usual Schwarzschild like [(g
ρρ
)
−1
∝ g
tt
and g
ϕϕ
∝ ρ
2
] comes from the fact that we are looking for
magnetic solutions. It is easy to show that, using a suitable
transformation, the metric (12) can map to 3-dimensional
Schwarzschild like spacetime locally, but not globally.
It is well-known that the electric field is associated with
the time component of the vector potential, A
t
, while the
magnetic field is associated with the angular component A
ϕ
.
Since we want to investigate the magnetic solutions, so we
assume the vector potential as
A
μ
=
F
ϕρ
dρ
δ
ϕ
μ
. (13)
Now we continue our paper for obtaining the magnetic solu-
tions in the Einstein gravity and in presence of various models
of NED.
3.1 Static solutions
3.1.1 Class I: PMI NED model
In this case, we want to obtain the solutions in presence of
PMI NED and investigate the properties of the solutions.
Therefore, we consider the PMI Lagrangian with the follow-
ing form
L
PMI
(F) = (−κF)
s
, (14)
where κ and s are coupling and arbitrary constants, respec-
tively. It is straightforward to show that for s = 1, the PMI
Lagrangian (14) reduces to the standard Maxwell Lagrangian
(L
Maxwell
(F) =−κF). Since the Maxwell invariant is nega-
tive, hereafter we set κ = 1, without loss of generality. Using
the nonlinear Maxwell equation (3) and the Lagrangian of
PMI (14) with the metric (12), one can obtain
F
ϕρ
+ K (ρ) = 0, (15)
where
K (ρ) = (2s − 1)ρ F
ϕρ
, (16)
where the “prime” denotes differentiation with respect to ρ.
Eq. (15) has the following solution
F
ϕρ
=
q
ρ
1/(2s−1)
, (17)
where q is an integration constant. In order to have physical
asymptotical behavior, we restrict ourselves to s > 1/2. To
find the metric function g(ρ), one may insert Eqs. (17) and
(12) in the field equation (2). After some calculations, one
can show that
⎧
⎪
⎨
⎪
⎩
g
(ρ) + 2ρ − (2s − 1)
2q
2
l
2
ρ
1/s(2s−1)
s
= 0, ρρ (ϕϕ) component
g
(ρ) + 2 +
2q
2
l
2
ρ
2/(2s−1)
s
= 0, tt component
,
(18)
It is straightforward to show that these equations have the
following solutions
g(ρ) = m − ρ
2
+
⎧
⎨
⎩
2q
2
ln
(
ρ
l
)
l
2
, s = 1
2
s−1
(2s−1)
2
(
s−1
)
q
l
2s
ρ
2(s−1)/(2s−1)
, otherwise
,
(19)
where m is the integration constant which is related to the
mass parameter.
3.1.2 Class II: exponential form of NED (ENED)
Here, we consider ENED Lagrangian as
L
exp
(F) = β
2
exp
−
F
β
2
− 1
, (20)
where β is the ENED parameter and in the limit β −→ ∞ ,
L
exp
(F) reduces to the standard Maxwell form
L
Maxwell
(F) =−F. Inserting the Lagrangian of ENED (20)
in the nonlinear Maxwell equation (3) and using the metric
(12), one can obtain
1 −
2F
ϕρ
lβ
2
F
ϕρ
+
F
ϕρ
ρ
= 0. (21)
This equation has the following solution
F
ϕρ
=
lβ
2
−L
W
, (22)
where L
W
= LambertW (−4q
2
/l
2
β
2
ρ
2
) and the parameter
q is an integration constant. It is worthwhile to note that in
order to have a real electromagnetic field, we should consider
ρ with the following limitation
ρ>ρ
0
=
2q
lβ
exp
1
2
. (23)
Now, we want to obtain the function of f
(
ρ
)
. For this
purpose, we can take into account Eqs. (12) and (22)inthe
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