Abstract
The 0th order of affine perturbation series of the deflection angle of a ray near a photon sphere is more accurate than a deflection angle in a strong deflection limit, which is used often, because the later has hidden error terms. We investigate gravitational lensing by using 0th order affine perturbation series of the deflection angle in a general asymptotically-flat, static, and spherical symmetric spacetime with the photon sphere. We apply our formula to Schwarzschild black hole, Reissner–Nordström black hole, and Ellis–Bronnikov wormhole spacetimes as examples. By comparing observables by using the deflection angles, we show that we can ignore the effect of the hidden error terms in the the deflection angle in the strong deflection limit on the observables in a usual lens configuration with the photon sphere since the hidden error terms are tiny. On the other hand, in a retro lensing configuration, the deflection angle in the strong-deflection-limit analysis have error of several percent and the 0th order of affine perturbation series of the deflection angle has almost half of the error. Thus, in the retro lensing configuration, we should use the 0th order of affine perturbation series of the deflection angle rather than the deflection angle in the strong-deflection-limit analysis. The 0th order of affine perturbation series of the deflection angle can give a brighter magnification by a dozen percent than the one by using the deflection angle in the strong-deflection-limit analysis.
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1 Introduction
Gravitational lensing under a weak-field approximation is used to find massive and dark objects [1, 2]. From the leading term of the deflection angle of a ray reflected by a mass lens in the weak-field approximation, we can estimate the mass of the lensing object if a distance to the lensing object is known. We would reveal details of the lensing object if we detect the phenomena in a strong gravitational field by the lensing object.Footnote 1
Recently, gravitational waves from black holes have been reported by LIGO Scientific Collaboration and Virgo Collaboration [6] and the shadows of the candidates of supermassive black holes in the centers of a galaxy M87 and milky way have been reported by Event Horizon Telescope Collaboration [7, 8]. Investigation on phenomena in strong gravitational fields is important to understand compact objects.
In 1931, Hagihara pointed out that the image of a star at any position can be observed in a Schwarzschild spacetime [9] because the spacetime has a photon sphere [10,11,12,13,14,15,16,17,18,19,20] which is a sphere filled with unstable circular light orbits. The image due to the rays deflected by the photon sphere around a black hole and other compact objects has been revisited often [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]. In 1959, Darwin investigated the deflection angle of the ray deflected by the photon sphere in the Schwarzschild spacetime [21].
Bozza has investigated gravitational lensing in a strong deflection limit \(b\rightarrow b_\textrm{m}+0\), where b is the impact parameter of the ray and \(b_\textrm{m}\) is a critical impact parameter, in a general asymptotically-flat, spherical symmetric spacetime with the photon sphere [29]. Bozza has expressed the deflection angle \(\alpha \) of a ray reflected by the photon sphere as
where \(\bar{a}\) and \(\bar{b}\) can be calculated by using the metric of the spacetime.Footnote 2 In many spacetimes, \(\bar{a}\) is obtained as analytical forms while \(\bar{b}\) usually is calculated numerically. Analytic forms of \(\bar{a}\) and \(\bar{b}\) have been obtained only in simple spacetimes such as the Schwarzschild spacetime [28, 29], higher dimensional black hole spacetimes [42, 43], charged black hole spacetimes [39, 44, 45], rotating black hole spacetimes [46], and wormhole spacetimes [38, 47]. The analysis in the strong deflection limit has been extended and applied to various astrophysical situations [35, 38,39,40, 44, 46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70].
Iyer and Petters have investigated affine perturbation series of the deflection angle near the photon sphere in the Schwarzschild spacetime in the following form:
where \(b_\textrm{p}\) is defined by
and \(\lambda _0\), \(\sigma _0\), \(\sigma _1\), \(\sigma _2\), \(\sigma _3\), \(\rho _0\), \(\rho _1\), \(\rho _2\), and \(\rho _3\) are constant, and they have found the 0th order of the affine perturbation series
is more accurate than the deflection angle by Darwin [40]. Tsukamoto has investigated the affine perturbation series of the deflection angle in the Reissner–Nordström black hole spacetime and has confirmed the 0th order of affine perturbation series (1.4) is more accurate than the form of Eq. (1.1).
How much does the difference of the deflection angles (1.1) and (1.4) affect observables in gravitational lensing? To answer this question, we investigate gravitational lensing in a general asymptotically-flat, static, and spherical symmetric spacetime with the photon sphere by using deflection angle in a form
which is the same as the 0th order of affine perturbation series (1.4) with the relations
and
in a usual lens configuration and a retro lensing configuration.
This paper is organized as follows. We investigate the 0th order of affine perturbation series of the deflection angle (1.5) in Sect. 2 and we consider the Schwarzschild black hole, Reissner-Nordström black hole, and the Ellis–Bronnikov wormhole spacetimes in Sect. 3. We investigate gravitational lensing by the photon sphere in a usual lens configuration in Sect. 4 and in a retro lens configuration in Sect. 5. We conclude and discuss Sect. 6. We review gravitational lensing under weak-field approximations in the usual lens configuration in Appendix A. We use the units in which the light speed and Newton’s constant are unity.
2 0th order of affine perturbation series of the deflection angle (1.5)
In this section, we investigate the 0th order of affine perturbation series of the deflection angle (1.5) in a general, asymptotically flat, static, and spherically symmetric spacetime with a metric
and with time translational and axial Killing vectors \(t^\mu \partial _\mu =\partial _t\) and \(\varphi ^\mu \partial _\mu =\partial _\varphi \), respectively.
We assume a photon sphere at \(r=r_\textrm{m}\) which is the largest positive solution of \(D(r)=0\), where D(r) is defined by
where the prime denotes a differentiation with respect to r. We also assume that A(r), B(r), and C(r) satisfy an asymptotically-flat condition
and that A(r), B(r), and C(r) are positive and finite for \(r>r_\textrm{m}\). We assume \(\vartheta =\pi /2\) without loss of generality because of spherical symmetry.
The trajectory of the ray is expressed by
where the dot denotes a differentiation with respect to an affine parameter along the trajectory. Conserved energy \(E\equiv -g_{\mu \nu }t^\mu \dot{x}^\nu =A(r)\dot{t}\) and angular momentum \(L\equiv g_{\mu \nu }\varphi ^\mu \dot{x}^\nu =C(r)\dot{\varphi }\) of the ray are constant along the trajectory and the impact parameter of the ray is defined by \(b\equiv L/E\). For simplicity, we assume that the impact parameter is positive in this section. The trajectory can be rewritten as
where V(r) is an effective potential defined by
where R(r) is defined by
We assume that the effective potential is negative \(V(r)<0\) for \(r_\textrm{m}<r<\infty \) so that the ray reaches to the photon sphere from spatial infinity.
We concentrate on a scatter case since we are interested in gravitational lensing. In this case, the ray is scattered at a closest distance \(r=r_0>r_\textrm{m}\). Equation (2.4) gives
at the closest distance \(r=r_0\). Here and hereafter, quantities with the subscript 0 denotes the quantities at \(r=r_0\). From Eq. (2.8), the positive impact parameter is expressed by
and R can be rewritten as
At the closest distance, we obtain
and
In a strong deflection limit \(r_0 \rightarrow r_\textrm{m}+0\) or \(b \rightarrow b_\textrm{m}+0\), where the critical impact parameter \(b_\textrm{m}\) is defined by
we obtain
We can rewrite Eq. (2.4) as
and we obtain the deflection angle \(\alpha (r_0)\) of the ray as
where \(I(r_0)\) is defined by
We change the radial coordinate r to a variable z defined by
and we obtain \(I(r_0)\) as
where \(f(z,r_0)\) is defined by
where \(G(z,r_0)\) is defined as
By using the expansions of a function F(r(z)) and its inverse 1/F(r(z)) in the power of z, which are expressed by
and
respectively, we obtain the expansion of \(R(r(z),r_0)\) in the power of z as
From Eqs. (2.25)–(2.28), \(G(z,r_0)\) can be expanded in the power of z as
where \(c_1(r_0)\) and \(c_2(r_0)\) are obtained as
and
respectively. In the strong deflection limit \(r_0 \rightarrow r_\textrm{m}+0\), we obtain
and
where we define
and then we get
We assume that \(D_\textrm{m}^\prime \) does not vanish. Under the assumption, the term \(I(r_0)\) diverges logarithmically in the strong deflection limit \(r_0 \rightarrow r_\textrm{m}+0\).Footnote 3 We define the divergent part \(I_\textrm{D}(r_0)\) of the term \(I(r_0)\) as
where \(f_\textrm{D}(z,r_0)\) is defined as
We expand \(c_1(r_0)\) and \(b(r_0)\) in powers of \(r_0-r_\textrm{m}\) as
and
respectively. Therefore, we obtain the relation, in the strong deflection limit \(r_0 \rightarrow r_\textrm{m}+0\) or \(b \rightarrow b_\textrm{m}+0\),
By using the relation, we express the divergent part \(I_\textrm{D}(b)\) in the strong deflection limit \(b \rightarrow b_\textrm{m}+0\) as
We define the regular part \(I_\textrm{R}(r_0)\) of the term \(I(r_0)\) as
where \(f_\textrm{R} (z,r_0)\) is defined as
and expand it in power of \(r_0 -r_\textrm{m}\) and we only consider the first term in which we are interested. Then, we get
or
From \(I=I_\textrm{D}+I_\textrm{R}\), we obtain the deflection angle in the strong deflection limit \(b\rightarrow b_\textrm{m}+0\) as
where \(\bar{a}\) and \(\bar{b}\) are given by
and
respectively.
3 Examples of deflection angles
In this section, we apply the formulas in the previous section to the Schwarzschild black hole, Reissner–Nordström black hole and Ellis–Bronnikov wormhole spacetimes. We obtain \(\bar{a}\) and \(\bar{b}\) in the deflection angles (1.1) and (1.5) and we show the percent errors of the deflection angles (1.1) and (1.5) defined by
and
respectively.
3.1 Schwarzschild black hole
In the Schwarzschild spacetime with a mass M, the functions A(r), B(r), and C(r) are given by
and
respectively.
The critical impact parameter is given by \(b_\textrm{m}= 3\sqrt{3}M\) and the photon sphere is at \(r=r_\textrm{m}=3M\). As obtained in Refs. [28, 29], coefficients \(\bar{a}\) and \(\bar{b}\) of the deflection angles in the strong deflection limit are obtained as
and
respectively. The percent errors of the deflection angles (1.1) and (1.5) are shown in Fig. 1.
3.2 Reissner–Nordström black hole
A Reissner–Nordström black hole is often considered as the simplest extension of the Schwarzschild black hole. Gravitational lensing [29, 39, 44, 53, 56, 72, 73], shadow [7, 74,75,76,77,78], and time delay [79] by the Reissner-Nordström black hole have been investigated.
Eiroa has considered gravitational lensing by the Reissner–Nordström black hole in the strong deflection limit \(b \rightarrow b_\textrm{m}+0\) in numerical [53]. Coefficients \(\bar{a}\) in an analytical form and \(\bar{b}\) in numerical have been obtained by Bozza [29]. The analytical forms of \(\bar{a}\) and \(\bar{b}\) have been obtained in Refs. [39, 44].
In the Reissner-Nordström black hole spacetime for \(0\le Q^2/M^2\le 1\), where Q is an electrical charge,Footnote 4 the functions A(r), B(r), and C(r) are given by
and
respectively.
We obtain \(r_\textrm{m}\) as
and \(b_\textrm{m}\) as
The coefficients \(\bar{a}\) and \(\bar{b}\) of the deflection angles in the strong deflection limit are given by
and
respectively. We show the percent errors of the deflection angles (1.1) and (1.5) in Fig. 1.
3.3 An Ellis–Bronnikov wormhole
An Ellis–Bronnikov wormhole is the solution of Einstein equations with a phantom scalar field [82, 83]. The deflection angle in the Ellis–Bronnikov wormhole spacetime has investigated by Chetouani and Clément [84] and it has been revisited by several authors [63, 85,86,87,88,89,90,91]. The visual appearance of the wormhole [92] and images due to the photon sphere [31, 39, 47, 48, 63, 85, 90, 93,94,95,96,97] have been investigated.
We cannot apply directly Bozza’s method [29] to an ultrastatic spacetime with a time translational Killing vector with a constant norm such as the Ellis–Bronnikov wormhole spacetime.Footnote 5 An extended method for the ultrastatic spacetime has been investigated and the deflection angle in the strong deflection limit in the Ellis–Bronnikov wormhole spacetime has been calculated in Refs. [38, 39, 47].
A line element in the Ellis–Bronnikov wormhole spacetime is given by
where a is a positive constant. We cannot apply formulas in Sect. 2 in a radial coordinate l since the photon sphere is at \(l=0\). We use a radial coordinate r defined by \(r\equiv l+p\), where p is a positive constant, so that the photon sphere is at \(r=r_\textrm{m}=p>0\). Under the radial coordinate r, we get
and
The critical impact parameter is given by \(b_\textrm{m}=a\) and coefficients of the deflection angles in the strong deflection limit are obtained as [38]
and
The percent errors of the deflection angles (1.1) and (1.5) are shown in Fig. 2.
4 Gravitational lensing in usual lens configuration
We consider that a ray with an impact parameter b, which is emitted by a source S with a source angle \(\phi \), is deflected with a deflection angle \(\alpha \) by a lens object L and its image I with an image angle \(\theta \) is observed by an observer O as shown in Fig. 3. The distances between O and S, between L and S, and between O and L are denoted by \(D_{\textrm{os}}\), \(D_{\textrm{ls}}\), and \(D_{\textrm{ol}}=D_{\textrm{os}}-D_{\textrm{ls}}\), respectively.
By using an effective deflection angle \(\bar{\alpha }\) defined by
a small-angle lens equation [99] is expressed by
where we have assumed \(\left| \bar{\alpha } \right| \ll 1\), \(\left| \theta \right| = \left| b \right| /D_{\textrm{ol}} \ll 1\), and \(\left| \phi \right| \ll 1\). The deflection angle \(\alpha \) can be expressed by
where N is a winding number of the ray. We define an angle \(\theta ^0_N\) by
and we expand the deflection angle \(\alpha (\theta )\) around \(\theta =\theta ^0_N\) as
4.1 By using the deflection angle (1.5)
We express the 0th order of affine perturbation series of the deflection angle (1.5) as
where \(\theta _\infty \equiv b_\textrm{m}/D_{\textrm{ol}}\) is the image angle of the photon sphere. From Eqs. (4.4) and (4.6), we get
From
and Eqs. (4.3)–(4.5) and (4.7), the effective deflection angle \(\bar{\alpha }(\theta _N)\), where \(\theta =\theta _N\) is the positive solution of the lens equation for a positive winding number N, is obtained as
By substituting the effective deflection angle (4.9) to the lens Eq. (4.2), we obtain the image angle as
and the image angle of an Einstein ring with the winding number N as
The difference of image angles between the outermost image and the photon sphere is given by
The magnification of the image is given by
The ratio of the magnifications of the outermost image to the sum of the other images is obtained by
Notice that we can get a negative solution \(\theta =\theta _{-N}(\phi )\sim -\theta _N(\phi )\) of the lens equation for each winding number N while we have concentrated on the positive solution \(\theta =\theta _N(\phi )\). The separation of the positive and negative image angles for each N is given by \(\theta _N(\phi )-\theta _{-N}(\phi )\sim 2\theta _N(\phi )\). The magnification of the negative image angle \(\theta _{-N}(\phi )\) is given by \(\mu _{-N}(\phi )\sim -\mu _{N}(\phi )\). The total magnification \(\mu _{N\textrm{tot}}(\phi )\) of the positive and negative image angles for each N is given by
Table 1 shows the observables with the deflection angle (1.5).
4.2 By using the deflection angle (1.1)
As a reference, we consider the deflection angle (1.1) in the strong deflection limit. It is rewritten in
By using Eqs. (4.4) and (4.16), we obtain
From
and Eqs. (4.3)–(4.5) and (4.17), we obtain the effective deflection angle \(\bar{\alpha }(\theta _N)\) as
From Eqs. (4.2) and (4.19), we get the image angle
the image angle of an Einstein ring for each N
and the difference of image angles between the outermost image and the photon sphere
The magnification of the image is obtained as
and the ratio of the magnifications of the outermost image to the sum of the other images is given by
The separation of two images are obtained as \(2\theta _N(\phi )\) and their total magnification for each N is given by
The observables with the deflection angle (1.1) are shown in Table 2.
5 Retro lensing
Gravitational lensing with the deflection angle \(\alpha \sim \pi \) is called retro lensing. Retro lensing in black hole spacetimes [44, 56, 57, 100,101,102,103], wormhole spacetimes [47, 63], naked singularity spacetimes [81, 104], and black bounce spacetimes [105, 106] were investigated. In this section, we investigate retro lensing with a configuration that a lens object L with a photon sphere, an observer O, and a source S are almost aligned in the order as shown in Fig. 4.
A light ray emitted by the source is reflected by the photon sphere of the lens object and it is observed by the observer as an image I. The Ohanian lens equation [24, 99] is expressed as
where \(\beta \sim 0\) is a source angle defined by \(\angle \)OLS and \(\bar{\theta }\) is an angle between a line LS and the light ray at S. We also assume that the terms \(\theta =b/D_{\textrm{ol}}\) and \(\bar{\theta }=b/D_{\textrm{ls}}\) are small and that they can be neglected in the lens equation. We obtain a positive solution \(\theta =\theta _N\) for every winding number N and its magnification is obtained as [44, 56, 57]
where \(s(\beta )\) for a point source is given by
and for an uniform-luminous disk with a size \(\beta _\textrm{s} \equiv R_\textrm{s}/D_{\textrm{ls}}\), where \(R_\textrm{s}\) is the radius of the source, on a source plane,
where \(\beta ^\prime \) is a radial coordinate divided by \(D_{\textrm{ls}}\) on the source plane, \(\Phi \) is an azimuthal coordinate around the origin of the coordinates on the source plane. By fixing the point of an intersection between an axis \(\beta =0\) and the source plane as the origin of the coordinates, \(s(\beta )\) is given by
for \(\beta \le \beta _\textrm{s}\) and
for \(\beta >\beta _\textrm{s}\). A perfectly-aligned case \(\beta =0\) for the uniform-luminous disk with the finite size, we obtain
5.1 By using the deflection angle (1.5)
We investigate retro lensing by using the deflection angle (1.5) or (4.6). From Eqs. (4.3), (4.6), and (5.1), we obtain the image angle \(\theta _{N}\) with the winding number N as
and its magnification as
We also get a negative image angle as \(\theta _{-N} (\beta )\sim -\theta _N (\beta )\) and their total magnification as
Table 3 shows observables by retro lensing with the deflection angle (1.5).
5.2 By using the deflection angle (1.1)
As a reference, we consider retro lensing with the deflection angle (1.1). From Eqs. (4.3), (4.16), and (5.1), we obtain the image angle \(\theta _{N}\) with the winding number N as
and its magnification as
The total magnification of the positive and negative images for the winding number N is obtained as
The observables by retro lensing with the deflection angle (1.1) are shown in Table 4.
6 Conclusion and discussion
We have shown the 0th order of affine perturbation series of the deflection angle (1.5) is more accurate than the deflection angle (1.1), which is often used in the strong deflection limit \(b\rightarrow b_\textrm{m}+0\), not only in the black hole spacetimes but also in the wormhole spacetime. We have investigated gravitational lensing by using the 0th order of affine perturbation series of the deflection angle (1.5). As shown Tables 1 and 2, under the usual lens configuration with the photon sphere, the observables obtained by using the 0th order of the affine perturbation series of the deflection angle (1.5) and by using the deflection angle (1.1) in the strong deflection limit are almost the same. Thus, we can ignore the effect of the hidden errors in the deflection angle (1.1) in the strong deflection limit on the observables in the usual lens configuration.
On the other hand, in a retro lensing configuration, the error of the deflection angle (1.1) in the strong-deflection-limit analysis increases to be several percents and the error of the 0th order of affine perturbation series of the deflection angle (1.5) reduces to be the almost half of the error of the deflection angle (1.1). We have shown that the hidden errors in the deflection angle (1.1) can affect on the magnification by a dozen percent. Thus, we conclude that we should use the 0th order of affine perturbation series of the deflection angle (1.5) rather than the deflection angle (1.1) when we consider retro lensing. The 0th order of affine perturbation series of the deflection angle can give a brighter magnification by a dozen percent than the one by using the deflection angle (1.1) in the strong-deflection-limit analysis.
On this paper, we have investigated gravitational lensing by a photon sphere in a general, asymptotically flat, static, and spherically symmetric spacetime with a photon sphere. One might think that some numerical treatments of gravitational lensing in given spacetimes are more practical than analytical treatments and one does not need analytical studies. However, we can reveal universal property of the gravitational lensing in the general spacetime with the photon sphere by using the analytical method while we have to rely on concrete examples by numerical methods. Therefore, it is still meaningful to consider the analytical treatment even if some examples were calculated numerically.
Gravitational lensing by rotating black holes in strong deflection limit has been investigated in Refs. [46, 54, 57, 58, 58, 60, 70]. On this paper, we have concentrated on asymptotically flat, static, and spherically symmetric spacetimes and our results can be extended to the rotating black holes and alternatives. The extension to the rotating case is left as future work.
Data Availability
This manuscript has no associated data or the data will not be deposited. [Authors’ comment: This is a theoretical study and no experimental data.]
Notes
In Ref. [29], the order of the error of Eq. (1.1) is estimated as \(O\left( \frac{b}{b_\textrm{m}}-1 \right) \). In Refs. [38, 39], Tsukamoto claims that the order of the error should read as \(O\left( \left( \frac{b}{b_\textrm{m}}-1 \right) \log \left( \frac{b}{b_\textrm{m}}-1 \right) \right) \). Iyer and Petters [40] and Tsukamoto [41] discuss hidden error terms in the deflection angle (1.1).
We can apply indirectly Bozza’s method to the ultrastatic Ellis–Bronnikov wormhole [98].
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Appendix A: Weak-field approximations
Appendix A: Weak-field approximations
In this appendix, we review gravitational lensing under weak-field approximations in the usual lens configuration.
1.1 A.1 Schwarzschild and Reissner–Nordström spacetimes
Under the weak-field approximation \(\left| b \right| \gg M\) in the Schwarzschild and Reissner–Nordström spacetimes, the deflection angle (2.20) can be expressed by
By using the deflection angle and Eqs. (4.2) and (4.3), \(\theta =b/D_{\textrm{ol}}\), and \(N=0\), we get the reduced lens equation
and its solutions as
where \(\hat{\theta }\equiv \theta /\theta _{\textrm{E}0}\) and \(\hat{\phi }\equiv \phi /\theta _{\textrm{E}0}\) are a reduced image angle and a reduced source angle, respectively, and \(\theta _{\textrm{E}0}\) is the image angle of an Einstein ring given by
The magnifications of the images and its total magnification are obtained by
and
respectively.
1.2 A.2 Ellis–Bronnikov wormhole spacetime
Under the weak-field approximation \(\left| b \right| \gg a\) in the Ellis–Bronnikov wormhole spacetime [107,108,109,110,111,112,113,114,115,116,117,118,119], the deflection angle (2.20) is given by
From Eqs. (4.2), (4.3), and (A7), \(\theta =b/D_{\textrm{ol}}\), and \(N=0\), we get the reduced lens equation
and the image angle of the Einstein ring is obtained as
The lens equation always has a positive solution \(\hat{\theta }=\hat{\theta }_{+0}(\hat{\phi })\) and a negative one \(\hat{\theta }=\hat{\theta }_{-0}(\hat{\phi })\) and their magnifications are expressed by
In Tables 1, 2, 3, 4, we set parameter a as
so that \(\theta _{\textrm{E}0}\) in the Ellis–Bronnikov wormhole spacetime is the same as \(\theta _{\textrm{E}0}\) in the Schwarzschild and Reissner–Nordström spacetimes.
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Tsukamoto, N. Gravitational lensing by using the 0th order of affine perturbation series of the deflection angle of a ray near a photon sphere. Eur. Phys. J. C 83, 284 (2023). https://doi.org/10.1140/epjc/s10052-023-11419-9
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DOI: https://doi.org/10.1140/epjc/s10052-023-11419-9