
Eur. Phys. J. C (2019) 79 :922 Page 3 of 12 922
G
μν
= R
μν
−
R
2
g
μν
, (4)
H
μν
= 2
RR
μν
− 2R
μω
R
ω
ν
− 2R
ωβ
R
μωνβ
+R
ωβ γ
μ
R
νωβγ
−
1
2
g
μν
L
GB
. (5)
The energy-momentum tensor T
μν
corresponding to the mat-
ter fields is obtained from S
matter
.
So, by taking n = 5, the five-dimensional line element
for a static spherically symmetric spacetime has the standard
form
ds
2
=−e
2ν(r)
dt
2
+ e
2λ(r)
dr
2
+r
2
(dθ
2
+ sin
2
θ dφ
2
+sin
2
θ sin
2
φdψ
2
), (6)
in coordinates (x
i
= t, r,θ,φ,ψ). For our model the energy-
momentum tensor for the stellar fluid is taken to be
T
μν
= diag
(
−ρ, p
r
, p
t
, p
t
, p
t
)
, (7)
where ρ, p
r
, and p
t
are the proper energy density, the radial
pressure, and the tangential pressure, respectively. By con-
sidering the comoving fluid velocity as u
a
= e
−ν
δ
a
0
,theEGB
field equation (3) leads to the following set of independent
equations:
κρ =
3
e
4λ
r
3
4αλ
+re
2λ
−re
4λ
−r
2
e
2λ
λ
− 4αe
2λ
λ
,
(8)
κp
r
=
3
e
4λ
r
3
(r
2
ν
+r + 4αν
)e
2λ
−re
4λ
− 4αν
, (9)
κp
t
=
1
e
4λ
r
2
12αν
λ
− e
4λ
− 4αν
− 4αν
2
+
1
e
2λ
r
2
1 − r
2
ν
λ
+ 2rν
− 2rλ
+r
2
ν
2
+
1
e
2λ
r
2
r
2
ν
− 4αν
λ
+ 4αν
2
+ 4αν
. (10)
Besides, we have considered units such that the speed of light
c and the constant G
5
are set to unity. Then κ = 8π.Here
denotes differentiation with respect to the radial coordinate
r.
3 Solution of the field equations
To solve the above field equations (8)–(10) we choose λ(r) =
ln(1 + ar
2
+ br
4
) and ν = Br
2
+ 2lnC with a, b, B and
C as constants. These metric potentials conform to the well-
known Tolman–Kuchowicz [43,44] spacetime. This choice
on e
λ
and e
ν
is well motivated because both metric potentials
are free from physical and mathematical singularities at every
point inside the compact star. Moreover, at the center of the
structure they have the appropriate behavior, i.e., e
λ(r)
|
r=0
=
Fig. 1 Variation of metric potentials for 4U 1538-52 using the param-
eter provided in Table 1
1 and e
ν(r)
|
r=0
= C
2
, which implies (e
λ
)
|
r=0
= (e
ν
)
|
r=0
=
0, as is required for a well behaved model. The trend of the
inner geometry is displayed in the upper panel of Fig. 1.
A completely regular behavior is observed, also as α grows
e
λ
and e
ν
take higher values, in distinction with GR whose
values are dominated by those of EGB theory for all r. So,
inserting e
λ
and e
ν
into Eqs. (8)–(10)wearriveat
κρ =
3
r
3
4
8αr (a + 2br
2
) −
8αr (a + 2br
2
)
+2r
3
(a + 2br
2
) −r
2
+r
4
)
, (11)
κp
r
=
3
r
3
4
(r + 8αBr + 2Br
3
)
2
− 8α Br − r
4
,
(12)
κp
t
=
1
5
48α B(a + 2br
2
) + 8α B(a − 2B + br
2
)
−4
a − 2B + 2aα B + B
1
r
2
2
− (a + br
2
)
4
−
3
a + 2B + br
2
− 4B
2
(4α + r
2
)
. (13)
The anisotropic factor defined by ≡ p
t
− p
r
is obtained:
κ =
2
5
24α B(a + 2br
2
) − 8α B(a + B + br
2
)
−2(a − 2B + 8aα B + B
2
r
2
)
2
+ (a + br
2
)
4
+
3
a + br
2
+ 2B(−2 + 4α B + Br
2
)
(14)
where
B
1
=−aB + b(2 + 6α B),
= (1 + ar
2
+ br
4
),
B
2
=−aB + 2b(1 + 6α B).
The behavior of the metric function, density, pressure,
anisotropy and equation of state parameter are given in Figs.
1, 2, 3, 4 and 5. The interior red-shift can be found to be
z(r) = e
−ν/2
− 1 (15)
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