737 Page 4 of 16 Eur. Phys. J. C (2019) 79 :737
It is straightforward to confirm that all the other components
of Eq. (4) are satisfied if (tt) component Eq. (21) holds, as
presented in the appendix. By integrating the equation, we
obtain the black hole solution as
f (r) =1+
r
2
4α
⎛
⎝
1−
1−
8α
l
2
+
8α
r
4
m +
q
2
3k
(e
−
k
r
2
−1)
⎞
⎠
,
(22)
where the ‘reduced’ mass m is the integration constant. The
ADM mass M of the geometry can be obtained by follow-
ing the expression given in Ref. [119] for the black holes in
Einstein–Gauss–Bonnet gravity coupled to nonlinear elec-
trodynamics and is given by
M =
S
3
16π
r
3
f
−12αrf
( f −1)−
3l
2
eff
4
1−
12α
l
2
eff
r
3
f
×
1
0
dt
1 − f
r
2
+
t
2
l
2
eff
r→∞
. (23)
Since the asymptotic expansion of the metric function f (r)
is given by
f (r) = 1 +
r
2
l
2
eff
−
1
1 −
4α
l
2
eff
m
r
2
+
q
2
3
1 −
4α
l
2
eff
1
r
4
+O(1/r
6
),
(24)
the ADM mass M of the black hole solution is related to the
‘reduced’ mass m as
M =
3S
3
16π
m. (25)
One may note that, if the reduced mass is in the region m <
q
2
3k
, the metric has branch point singularity at r = r
b
, which
is determined as
1 −
8α
l
2
+
8α
r
4
b
m +
q
2
3k
(e
−
k
r
2
b
− 1)
= 0. (26)
Therefore the reduced mass must satisfy the inequality, m ≥
q
2
3k
. As will be shown in the next section, this lower bound is
a special point where the curvature singularity disappears.
Before we describe the solution in detail, we present var-
ious limits of the solution, which correspond to the known
black hole solutions in the literatures.
• 5D Schwarzschild-AdS GB black holes In the limit q →
0, the metric function is given by
f (r) = 1 +
r
2
4α
1 −
1 −
8α
l
2
+
8αm
r
4
. (27)
which corresponds to the metric of 5D Schwarzschild-
AdS GB black hole [85].
• 5D charged AdS black holes with the GB term and
Maxwell electrodynamics In the limit k → 0, the
Lagrangian for the gauge field becomes the usual
Maxwell Lagrangian. In this limit, the solution becomes
of the form,
f (r) = 1 +
r
2
4α
⎛
⎝
1 −
1 −
8α
l
2
+
8αm
r
4
−
8αq
2
3r
6
⎞
⎠
,
(28)
which describes the charged AdS black hole solution
in Einstein–Gauss–Bonnet gravity. If we take additional
limit, l →∞, it becomes the Wiltshire black hole solu-
tion [84], in which the GB term plays the role of the effec-
tive cosmological constant. On the other hand, under the
limit, l →∞and q → 0, it reduces to the Boulware–
Deser black hole solution [83].
• 5D charged AdS black holes with higher order derivative
electrodynamics If we take the α → 0 limit, we obtain
another new charged black hole solution of Einstein grav-
ity with higher order derivative U(1) gauge field as
f (r) =
r
2
l
2
+ 1 −
m
r
2
−
q
2
3kr
2
e
−
k
r
2
− 1
. (29)
In this solution, the lower bound of the reduced mass is
also given by m = q
2
/3k, where it becomes the regular
black hole solution. Since all the thermodynamic prop-
erties of this solution can be trivially obtained by taking
α → 0 limit, we will present the thermodynamic prop-
erties for the general α case only.
3 Properties of black hole solutions
The behavior of the solutions depends on various parameters
in the theory. In this section we describe the basic properties
of the solutions, including the existence of curvature singu-
larity. Basically, for given coupling constants α and k and the
cosmological constant, there are two classes of the solutions.
The first one is when the mass is in the region , m >
q
2
3k
.In
this class, the behavior of the curvature scalar R near r = 0
is given by,
R =−
5
α
+
3
r
2
2
α
m −
q
2
3k
+ O
r
2
, (30)
which indicates the existence of a curvature singularity at
the center of the black hole. It describes the usual black hole
solutions with curvature singularity inside the event horizon.
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