Relations between threepoint configuration space shear and convergence statistics
^{1}
ArgelanderInstitut für Astronomie (AIfA), Universität Bonn,
Auf dem Hügel 71,
53121
Bonn,
Germany
email: xun@astro.unibonn.de
^{2} International Max Planck Research School (IMPRS) for
Astronomy and Astrophysics at the Universities of Bonn and Cologne, Germany
^{3}
Institute for Astronomy, University of Edinburgh, Royal
Observatory, Blackford
Hill, Edinburgh,
EH9 3HJ,
UK
Received:
11
May
2011
Accepted:
20
July
2011
With the growing interest in and ability of using weak lensing studies to probe the nonGaussian properties of the matter density field, there is an increasing need for the study of suitable statistical measures, e.g. shear threepoint statistics. In this paper we establish the relations between the threepoint configuration space shear and convergence statistics, which are an important missing link between different weak lensing threepoint statistics and provide an alternative way of relating observation and theory. The method we use also allows us to derive the relations between other two and threepoint correlation functions. We show the consistency of the relations obtained with already established results and demonstrate how they can be evaluated numerically. As a direct application, we use these relations to formulate the condition for E/Bmode decomposition of lensing threepoint statistics, which is the basis for constructing new threepoint statistics which allow for exact E/Bmode separation. Our work applies also to other twodimensional polarization fields such as that of the cosmic microwave background.
Key words: gravitational lensing: weak / largescale structure of Universe / methods: analytical / cosmology: theory
© ESO, 2011
1. Introduction
The statistical study of the weak gravitational lensing effect by the largescale structure, also called cosmic shear, is one of the established tools to investigate the matter distribution of the Universe and to constrain the parameters of cosmological models. The undergoing boosting of weak lensing survey size and quality gradually enables one to explore the small, nonlinear scales where a wealth of signal lies (Bernardeau et al. 2002; Pen et al. 2003; Jarvis et al. 2004; Semboloni et al. 2011). Along with this trend, more and more effort has been put into the development of statistical measures that can probe the nonGaussian signal arising from the nonlinear growth of structure on those scales. Among them, the threepoint (3pt) statistics may be the most widely used.
Two basic quantities considered in gravitational lens theory are the convergence κ and the shear γ. Defined as the dimensionless surface mass density, κ is a weighted projection of the threedimensional (3D) matter density contrast δ. The shear γ, on the other hand, is directly accessible from observations. Therefore, the theoretical framework of gravitational lensing should include the relation between configuration space κ and γ statistics as well as the one relating configuration space statistics to their Fourier space counterparts. At the level of twopoint (2pt) statistics, such relations have already been established. For 3pt statistics, the relation between the shear 3pt correlation functions (γ3PCFs) and the convergence bispectrum, which is the Fourier counterpart of the 3pt convergence correlation function (κ3PCF), has been derived by Schneider et al. (2005). The other nontrivial relation, the one between γ3PCFs and κ3PCFs, is still missing. One purpose of this work is to establish this missing link.
How to perform E/Bmode decomposition is also a major concern of the weak lensing community. For observational data an E/Bmode decomposition provides a necessary check on the possible systematics (e.g. Crittenden et al. 2002; Pen et al. 2002). In recent years there have been several efforts to construct better statistics which allow for an E/Bmode decomposition at the 2pt level (Schneider & Kilbinger 2007; Eifler et al. 2010; Fu & Kilbinger 2010; Schneider et al. 2010). They all use weight functions to filter the shear 2pt correlation functions (γ2PCFs), and the condition for E/Bmode decomposition transforms to a condition on the weight functions. Such a condition at the 3pt level is also missing so far. We will see that with the aid of the relation between the γ3PCFs and the κ3PCFs, one can easily formulate this condition.
The paper is organized as follows: In Sect. 2 we show how the relation between the γ3PCF and the κ3PCF is obtained. In Sect. 3 we investigate the correspondence between the derived relation and already established results. We then extend our results to other γ3PCFs in Sect. 4, and in Sect. 5 we present an application of the 3pt relations, deriving the condition for E/Bmode separation of 3pt shear statistics. How these relations can be numerically evaluated is demonstrated in Sect. 6, and we conclude in Sect. 7.
2. Relation between 3pt γ and κ correlation functions
2.1. The form of the relation
At the 2pt level, the relation between the configuration space shear and convergence statistics is the ξ_{+} − ξ_{−} relation (Crittenden et al. 2002; Schneider et al. 2002) (1)where H and are Heaviside function and 1D Dirac delta function, respectively. The functions ξ_{+} and ξ_{−} are defined as ξ_{+}(x): = ⟨ κκ ⟩ (x) and ξ_{−}(x): = ⟨ γγ ⟩ (x) e^{−4iφx}, with φ_{x} being the polar angle of the separation vector x, and ⟨ ⟩ indicating the ensemble average. Note that the shear γ is a spin2 quantity, i.e. it gets multiplied by a phase factor e^{−2iφx} when the coordinate x rotates by φ_{x}. Consequently, ⟨ γγ ⟩ (x) has a spin of 4. Being the product of ⟨ γγ ⟩ (x) and a phase factor of e^{−4iφx}, the quantity ξ_{−}(x) no longer depends on the polar angle of x.
The relation (1) has already taken both the statistical homogeneity and isotropy of the shear field into account and is therefore a onedimensional relation of quantities on the real domain. The derivation of the ξ_{+}−ξ_{−} relation originates from the relation between ξ_{+} and ξ_{−} and the convergence power spectrum P_{κ}, (2)Inverting one of the relations in (2) one can write ξ_{+} and ξ_{−} in terms of each other, e.g. (3)and the final form of the relation (1) can be reached by performing the 1D Bessel integral whose result can be obtained from Gradshteyn et al. (2000).
The same procedure, however, fails to work for 3pt statistics since the corresponding Bessel integral actually consists of three integrals, and they have highly complicated dependencies on the arguments (see Schneider et al. 2005). A brute force numerical evaluation of these integrals is also extremely challenging due to the oscillatory behaviour of the Bessel functions.
Fig. 1 Definition of the geometry of a triangle (the leftmost sketch) and how it changes under permutations (the first three sketches from the left) and flip (the leftmost and the rightmost sketch) of the vertices. 

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Since the advantage of transforming to the Fourier plane and back no longer holds for 3pt statistics, we attempt to stay in configuration space, which at least avoids the problem of oscillatory integrals. One can see from (1) that the result of the Bessel integral in (3) is actually not oscillatory, as expected.
The configuration space 3pt shear correlator can be written as ⟨γ(X_{1})γ(X_{2})γ(X_{3})⟩, with X_{i} being the positions on the twodimensional (2D) plane where the shear signals are evaluated. Following the assumed statistical homogeneity of the shear field, the correlator depends only on the separations of these three positions. We choose x_{1} ≡ X_{1} − X_{3} and x_{2} ≡ X_{2} − X_{3} to be its arguments (see the leftmost sketch of Fig. 1) and write the correlator as ⟨ γγγ ⟩ (x_{1},x_{2}). After the same procedure is applied to the 3pt convergence correlator, the relation we are interested in will be shown to be of the form (4)where we have defined the convolution kernel G_{0} for which we need to find an explicit expression.
Writing the relation in the form of a convolution is motivated by the KaiserSquires (KS) relation between the convergence and the shear (Kaiser & Squires 1993), (5)which yields the result (4) and also allows us to express the kernel G_{0} as (6)Here, for simplicity, we have adopted the complex notation for the KS kernel, i.e. we have identified the 2D separation vectors with complex numbers. Throughout the text we will use the vector and complex notations interchangeably, and use x to indicate a complex quantity, x for its absolute value, and x^{∗} for its complex conjugate.
The integral in (6) is difficult to perform directly, so we first take a look at the more studied 2pt case. The relation between 2pt γ and κ correlation functions can be written in the same way as (7)with (8)Unlike the case of the ξ_{+} − ξ_{−} relation, we have not assumed a statistically isotropic field for (4) or (7). The ξ_{+} − ξ_{−} relation is actually what one should obtain after adding the assumption of isotropy to (7).
2.2. The form of the convolution kernels
Now we aim for obtaining the forms of the F and G_{0} kernels, which can be seen as the 2 and 3pt equivalence of the KS kernel (5). Introducing the symbols ∂ ≡ ∂_{1} + i∂_{2} and , the definitions of κ and γ read (9)i.e. both the convergence κ and the shear γ are secondorder derivatives of the deflection potential ψ. It is then convenient to use ψ as a link between κ and γ. Using the identities ∇lnx = x/x^{2} and which hold for a 2D x, one can easily verify the consistency of (9) with the relation between ψ and κ (e.g. Bartelmann & Schneider 2001), (10)Applying the operator ∂^{2} on both sides of (10) and taking (9) into account, one reaches the KS relation (5), since ∂^{2}lnz.
The same procedure can be generalized to secondorder statistics. The 2pt equivalence of (10) is (11)Using the statistical homogeneity of the κ field, and redefining the integration variables, (11) reduces to (12)where we have defined (13)Obviously, ℱ′ is infinite at every z, which is related to the fact that ψ is defined only up to an additive constant. However, we shall only need the derivatives of ℱ′. So we define (14)and will use ℱ and ℱ′ interchangeably. Let ϕ denote the angle between u and z, (14) can be rewritten as (15)The integral over ϕ yields zero if z < u, and 4πln(z/u) otherwise. Thus (16)We are now ready to apply differential operators to (12) to get the relations of 2pt shear and convergence statistics. As a consistency check, we first apply two ∇^{2} operators to (12), one acting on x_{1} and the other on x_{2}. According to (9), this turns the l.h.s. of (12) into 4⟨κ(x_{1})κ(x_{2})⟩. On the r.h.s. of (12) the operators act exclusively on ℱ, (17)with z = x_{1} − x_{2} − y here. Using (17), one easily sees that the r.h.s. of (12) after the operation gives 4⟨κκ⟩(x_{1} − x_{2}), which is equivalent to 4⟨κ(x_{1})κ(x_{2})⟩ under the assumption of statistical homogeneity of the κ field. Now we apply the operator on (12), which turns the l.h.s. of (12) into ⟨γ(x_{1})γ(x_{2})⟩. On the r.h.s. the operation again acts only on ℱ, (18)also with z = x_{1} − x_{2} − y. Remembering the definition of the kernel F (7), this leads to (19)For the 3pt kernel G_{0} we split the integral in (6) into (20)where we have assumed a ≠ b. From (19) as well as (21)we obtain (22)The forms of the kernels (19) and (22) hold rigorously outside their singularities (at z = 0 for F; at a = 0, b = 0, and a = b for G_{0}). One may wonder if additional delta functions exist at these singularities. We will show in Sect. 3 that this is not the case.
The method we used to derive the forms of the kernels (19) and (22) also allows one to derive the relations between other correlation functions of weak lensing quantities in a systematic way. We present explicit forms of some of the relations in Appendix B.
2.3. The relations
To summarize, we have obtained: (23)and (24)In these relations we have applied the statistical homogeneity of the convergence field, but not the statistical isotropy. Making use of the latter, one can derive the ξ_{+} − ξ_{−} relation from (23), as will be shown in Sect. 3.2.
3. Consistency checks
3.1. The case of uniform κ
There is a physical condition which will directly serve as a test of the κ − γ relations (5), (23) and (24). At the 1pt level, for the KS relation, a uniform convergence field does not result in any shear. At the 2 and 3pt level, the physical condition could be that a uniform ⟨ κκ ⟩ (⟨ κκκ ⟩ ) field leads to a vanishing shear correlation ⟨ γγ ⟩ (⟨ γγγ ⟩ ).
One can easily see that both the F and G_{0} kernel we obtained satisfy this condition. If there are additional terms at the singularities of the kernels which contribute to the integral, a nonzero ⟨ γγ ⟩ (⟨ γγγ ⟩ ) term would be generated and the condition would not be satisfied anymore. Thus we argue that the expressions (23) and (24) are already complete.
3.2. Consistency with the ξ_{+} − ξ_{−} relation
Now we consider whether (23) is consistent with the ξ_{+} − ξ_{−} relation (1), which can be regarded as the isotropic form of (23). That the two relations are consistent is equivalent to (25)To verify that (25) indeed holds, we attempt to solve the φ_{y}integral on the l.h.s., (26)where φ = φ_{y} − φ_{x} has been defined. The φintegral can be carried out using the residual theorem, yielding 2π(x^{2} − 3y^{2})/x^{4} when x > y and zero when x < y. One can see that this result corresponds to the Heaviside function on the r.h.s. of (25).
At the singularity x = y the φintegral is not well defined, which means one cannot rule out the existence of additional delta function at x = y in the result of the φintegral. This ambiguity can again be eliminated by using the physical condition “a uniform ⟨ κκ ⟩ field leads to a null shear correlation ⟨ γγ ⟩ ”, which translates to “a constant ξ_{+} yields vanishing ξ_{−}” here. In this case, a delta function is indeed required to satisfy this condition, and the prefactor of the delta function can be determined to be π/2x, in consistency with (25).
3.2.1. KS relation and its isotropic form
A similar consistency exists between the KS relation and its isotropic form. As both forms are already wellknown, they can serve as a further support for our argument.
For an axisymmetric distribution of matter, i.e. κ(x) = κ(x), the following relation is established between the shear and the convergence (see e.g. Schneider et al. 1992) (27)with defined as (28)This is equivalent to (29)In the case of a uniform convergence field κ(x) = const., one can see that the integral of the Heaviside function and the delta function parts cancel each other.
The similarity between (29) and the ξ_{+} − ξ_{−} relation (1) is remarkable: they both have integrals of a Heaviside function part and a delta function part which cancel each other for constant κ and ⟨κκ⟩, respectively, and the 2D correspondences of both do not have an additional delta function at their singularities.
3.3. Fourier transformations
In the Fourier plane the relation between the shear and the convergence has a simple form. With denoting the Fourier transformation of the quantity g, it reads (30)with β being the polar angle of the wave vector ℓ. The identity (30) can be obtained by relating the Fourier transforms of the definitions (9) of γ and κ. It directly reflects the fact that γ is spin2 while κ is spin0, and leads to the wellknown result P_{γ} = P_{κ}. Comparing (30) to the configuration space relation (5) which means , using the convolution theorem, one can see that for ℓ ≠ 0.
The Fourier plane correspondences of (23) and (24) are also readily obtainable from the identity (30), as (31)and (32)with β_{i} denoting the polar angle of ℓ_{i}. These equations show that for ℓ ≠ 0, and that for ℓ_{1} ≠ 0 ≠ ℓ_{2} and ℓ_{3} = −ℓ_{1} − ℓ_{2} ≠ 0, since F/π^{2} and −G_{0}/π^{3} are the convolution kernels for the configuration space relations by their definitions.
In Appendix A we show explicitly that the Fourier transforms of F/π^{2} and −G_{0}/π^{3}, with F and G_{0} given in (19) and (22), are indeed e^{4iβ} and e^{2i(β1 + β2 + β3)}, respectively. However one cannot obtain the forms of F and G_{0} kernels simply through inverse Fourier transforming the phase factors e^{4iβ} and e^{2i(β1 + β2 + β3)}. This is due to the fact that the Fourier inversion theorem is valid strictly only for squareintegrable functions, which is not the case for the phase factors. The same situation occurs for the KS kernel .
4. The other γ3PCFs
Until now we have considered only the 3PCF of shear itself ⟨γ(X_{1})γ(X_{2})γ(X_{3})⟩, which is one of the four independent possible combinations considering that γ is a complex quantity. The other three are ⟨γ^{∗}(X_{1})γ(X_{2})γ(X_{3})⟩, ⟨γ(X_{1})γ^{∗}(X_{2})γ(X_{3})⟩, and ⟨γ(X_{1})γ(X_{2})γ^{∗}(X_{3})⟩, according to the choice made in Schneider & Lombardi (2003). Following Schneider et al. (2005), we denote these four γ3PCFs by (i = 0,1,2,3), with ‘cart’ emphasizing that the shear is measured in Cartesian coordinates, , and (i = 1,2,3) corresponding to the γ3PCF with γ^{∗} at position X_{i}. Since we have considered statistical homogeneity of the shear field, the Γ_{cart}’s depend only on the separation vectors of the position X_{1}, X_{2}, and X_{3}. The other γ3PCFs, i.e. those with two or three γ^{∗}’s, can be obtained by taking the complex conjugate of the Γ_{cart}’s.
Note that , , and are different functions, since x_{1} (x_{2}) is defined to be the difference of the positions of the first (second) and the third γ in the bracket. Due to the same reason, they can be transformed into each other through permutations and flips of the vertices of the triangle formed by their arguments (see Fig. 1), and thus are not independent if argument permutations and flips are allowed. As an example, one has (33)where different lines correspond to different ways of labeling the same triangle with side lengths x_{1}, x_{2}, and x_{1} − x_{2}. The same permutations and flips also reveal the inherent symmetry of , (34)In the case that the shear is measured relative to a center of the triangle, transforms to Γ^{(i)}, the natural components of the γ3PCF as defined in Schneider & Lombardi (2003). For a general triangle configuration, all four Γ_{cart}’s are expected to be nonzero, thus all of them should be used to exploit the full 3pt information of cosmic shear.
Before relating the other Γ_{cart}’s to the κ3PCFs, we extend κ to a complex quantity κ = κ^{E} + iκ^{B}. Although the physical convergence is a real quantity, the convergence field corresponding to the measured shear signals can have an imaginary part due to e.g. systematical errors and noise. The shear component which corresponds to this unphysical imaginary part of the convergence field is identified as the Bmode, on which we will elaborate more in Sect. 6. When taking the Bmode into consideration, the 3pt correlation functions of the convergence field can be written as (35)Apart from the Emode ⟨ κ^{E}κ^{E}κ^{E} ⟩ term, there are still additional Bmode contributions to the real parts of the K’s, namely ⟨ κ^{E}κ^{B}κ^{B} ⟩ , ⟨ κ^{B}κ^{E}κ^{B} ⟩ , and ⟨ κ^{B}κ^{B}κ^{E} ⟩ . The imaginary part of the K’s are composed of the parity violating terms which are expected to vanish due to parity symmetry (Schneider 2003). The property of K^{(i)} under permutations and flips of the vertices of the triangle formed by their arguments is the same as that of .
Similar to (4), the relations between and K^{(i)} for i = 1,2,3 can be written as (36)(37)and (38)where the convolution kernels G_{1}, G_{2}, and G_{3} have been defined. Again with the aid of the KS relation, we can write these convolution kernels as (39)(40)and (41)When a ≠ b, the product of the three terms in the integrand of (41) can be split into products of two, as (42)These terms are also obtainable from doing derivatives to the kernel ℱ, (43)(44)This way we obtain the form of the convolution kernel G_{3}. The forms for the kernel G_{1} and G_{2} can be obtained likewise. The results are (45)(46)(47)The symmetries in the Γ_{cart}’s and K’s are also reflected in the G kernels. One can verify that G_{2}(a,b) = G_{1}(b − a, −a), G_{3}(a,b) = G_{1}(−b,a − b) as results of the symmetry under permutations, and G_{2}(a,b) = G_{1}(b,a), G_{3}(a,b) = G_{3}(b,a) as results of the symmetry under flips, in consistency with (33). Similarly, one has G_{0}(a, b) = G_{0}(b − a, −a) = G_{0}(−b,a − b) = G_{0}(b, a), in consistency with (34).
5. Inverse relations
So far we have obtained the expressions of the four γ3PCFs as functions of the κ3PCFs. Written in a short form, they are (48)where i runs from 0 to 3. The forms of the G_{i} kernels are given by (22), (45), (46), and (47).
These relations can be inverted. We define the kernels of the inverse relations to be , i.e. (49)Using the convolution theorem, it is apparent from (48) and (49) that (50)From the corresponding Fourier plane relations of (48), we also know (51)which implies (52)Comparing (50) and (52) one has (53)and further, (54)i.e. the convolution kernel for the inverse relation is the complex conjugate of the original kernel.
This property of the convolution kernel has its root in the fact that and differ only by a phase factor. This fact also endows the convolution kernels in the 1pt and 2pt relations between γ and κ with the same property. As is well known for the 1pt relation, the inverse relation of (5) is (55)where the kernel is the complex conjugate of the KS kernel . The inverse relation of the 2pt relation (7) can also be shown to be (56)
6. Condition of 3pt E/B decomposition
Being mathematically a polarization field, the shear field can be decomposed into a curlfree component and a divergencefree component, usually called the Emode and the Bmode, respectively. Performing such a decomposition when treating cosmic shear data has long been recognized as a necessity, since it provides a valuable check on the possible systematics (e.g. Crittenden et al. 2002; Pen et al. 2002).
The E/Bmode decomposition can be done either on the shear field itself (e.g. Bunn et al. 2003; Bunn 2011), or at the level of correlation functions (e.g. Schneider 2006). The complex survey geometry after masking, which is especially characteristic for a lensing survey (e.g. Erben et al. 2009), renders the first option barely feasible, and singles out the correlation function as the basic statistic to be applied directly to the data. Thus the natural way to perform the E/Bmode decomposition on cosmic shear data is to derive statistics based on the shear correlation functions.
A commonly used statistic for this purpose is the aperture mass statistic which can be expressed as a linear combination of ξ_{+} and ξ_{−} (Schneider et al. 2002), (57)where the forms of the weight functions and are given explicitly in Schneider et al. (2002). The chosen forms of the weight functions guarantee that responds only to the Emode and only to the Bmode.
The aperture mass statistics has been generalized to 3pt level by Jarvis et al. (2004) and Kilbinger & Schneider (2005), and is the only statistics available up to now which allows an E/Bmode decomposition at the 3pt level. However, as found by Kilbinger et al. (2006), it cannot ensure a clean E/Bmode decomposition when applied to real data. The lack of shearcorrelation measurements on small and large scales, which arises from the inability of shape measurement for close projected galaxy pairs and the finite field size, prohibits one from performing the integral in (57) from zero to infinity, and thus introduces a mixing of the E and Bmodes.
In recent years, there have been several efforts to construct better statistics which allow E/Bmode decomposition (Schneider & Kilbinger 2007; Eifler et al. 2010; Fu & Kilbinger 2010; Schneider et al. 2010), all of them focusing on the cosmic shear 2pt statistics. These new statistics are based on the idea that the weight functions and used in the aperture mass statistics are just one example out of the many possibilities. In general one can define secondorder statistics in the form (Schneider & Kilbinger 2007) (58)for which the condition that EE responds only to Emode and BB only to Bmode is found to be (59)Note that instead of being functions of the separation length as the aperture mass statistics, EE and BB are just numbers. At first sight one seems to have reduced the information quantity by integrating over the scale dependence in (58). In fact, the information can be easily regained by constructing a set of weight functions satisfying (59). As one example, and for any θ value can be reconstructed in the framework of (58) by specifying and .
Since the condition for E/Bmode decomposition (59) still leaves large freedom for the choice of the weight functions, one can construct statistics which fulfill additional constraints, e.g. a finite support over the separation length. If one requires that EE and BB respond only to ξ_{+}(ϑ) and ξ_{−}(ϑ) with ϑ_{min} < ϑ < ϑ_{max}, where ϑ_{min} and ϑ_{max} are the chosen small and largescale cutoff, T_{+}(ϑ) and T_{−}(ϑ) must vanish outside the same range. Since T_{+} and T_{−} are interrelated by (59), one can specify only one of them to satisfy this constraint. The requirement that the other weight function also vanishes outside the specified range needs to be put as additional integral constraints. As shown by Schneider & Kilbinger (2007), if one chooses T_{−} to vanish for ϑ < ϑ_{min} and ϑ > ϑ_{max}, then to allow an E/Bmode decomposition on a finite interval ϑ_{min} < ϑ < ϑ_{max}, T_{−} has to satisfy additionally, (60)which would guarantee that T_{+} vanishes for ϑ < ϑ_{min} and ϑ > ϑ_{max}.
Similar statistics are needed at the 3pt level as well. The first step required is to formulate the conditions for 3pt weight functions to allow E/Bmode decomposition, in analogy to (59). As we will show in this section, the relations between the γ3PCFs and κ3PCFs that we derived provide a natural way of formulating such conditions.
A pure Emode shear 3pt statistics is related only to the Emode κ3PCF ⟨ κ^{E}κ^{E}κ^{E} ⟩ but not to other 3PCFs with κ^{B} contribution. Therefore we first write the 3PCFs of κ^{E} and κ^{B} as linear combinations of the real and imaginary parts of the K^{(i)}’s, using (35), and then relate them with the Γ_{cart}’s through (49), as the Γ_{cart}’s are the directly measurable statistics from a lensing survey. The results read (61)and (62)which shows how the E and Bmode κ3PCFs can be computed when the full information of the Γ_{cart}’s is available. In the ideal case that there exists no noise or systematical effects, only the Emode term κ3PCF ⟨ κ^{E}κ^{E}κ^{E} ⟩ is expected to be nonzero, since it corresponds to the correlation in the physical density field which leads to the correlation in the shear signal.
Following the ideas of Schneider & Kilbinger (2007), we construct a new statistic (63)which by definition responds only to Emode. With the help of (61) we can link EEE to the observable Γ_{cart}’s, as (64)where in the first equation we have specified U to be a real function, and in the second equation we have used the fact that G_{i}(−a, −b) = G_{i}(a,b).
Denoting (65)the expression of EEE (64) has a similar form as (58). We can see in this form that EEE responds only to the Emode if the weight function T’s satisfy (66)with U being a real function. One can easily verify that these conditions are also necessary conditions. Noticing that −T^{(i)} ∗ G_{i}/(4π^{3}) is the corresponding weight on K^{(i)}, the condition that T^{(i)} ∗ G_{i} being purely real is required to separate the parityviolating and nonviolating terms in (35). In addition, (66) is required to cancel the parity nonviolating Bmode terms ⟨ κ^{E}κ^{B}κ^{B} ⟩ , ⟨ κ^{B}κ^{E}κ^{B} ⟩ and ⟨ κ^{B}κ^{B}κ^{E} ⟩ .
The statistics containing the contribution from only one of the Bmode terms can be constructed in the same way. Omitting the arguments for notational simplicity, they can be expressed as (67)and (68)For all of them the condition that T^{(0)} ∗ G_{0} is real has been imposed.
The four parity violating statistics (68) can be used as a check on parity violating systematical errors, while the other Bmode statistics (67) allows for a further examination of the Bmodes.
With (52) one can easily invert the conditions on the weight functions to express the weight function T’s directly in terms of each other. Take the conditions for EEE (66) for example. One can write the Fourier transforms of T^{(1)}, T^{(2)} and T^{(3)} as functions of the Fourier transform of T^{(0)} as (69)In order to simplify these relations, we now attempt to give simple expressions for for i = 1,2,3. With (51) one has , which does not depend on ℓ_{2}. Noticing that (see Sect. 3.3), we actually have , which yields in configuration space (70)Similarly one can derive that (71)Inserting (70) and (71) into (69), one obtains (72)(73)(74)where we have used the expression of F (19).
To summarize, if one chooses an arbitrary form of T^{(0)} which makes T^{(0)} ∗ G_{0} real, constructs T^{(1)}, T^{(2)} and T^{(3)} according to (72), (73) and (74), and uses these weight functions to weight the measured ’s, then the resulting statistic EEE as defined in (64) receives contributions only from ⟨ κ^{E}κ^{E}κ^{E} ⟩ but not the terms affected by the Bmode. The Bmode statistics can be obtained from the measured ’s through a similar procedure. Equations (72)–(74) are the analogue of (59) for 3pt functions.
We note again that , and are not independent under transformation of their arguments. Thus the statistics EEE (64) as well as the Bmode statistics (67) and (68) can all be written in terms of linear combinations of and alone. However we shall keep the current redundancy since it allows for simple analytical expressions of the relations between the weight functions.
So far one still has much freedom in choosing the form of T^{(0)}(x_{1},x_{2}). This freedom can be exploited to construct statistics which do not respond to the γ3PCF at smaller or larger angular separations than can be probed by the survey. We leave this to future work.
7. Numerical evaluation
7.1. Design of the sampling grid
We have written configuration space relations between weak lensing statistics in the form of convolutions, e.g. (23) and (24), where the convolution kernels are complex, have nontrivial spin numbers, and feature singularities. The convolutions can be performed numerically, but special care must be taken of these properties of the integration kernel.
One can take the KS kernel −1/z^{ ∗ 2} as an example of this kind of convolution kernel. The KS kernel has an integer spin of 2, so an azimuthal integration of the kernel around its singularity at z = 0 should give zero, i.e. the values of the kernel along the circle cancel themselves due to their opposite phases. This property renders its singularity harmless, but entails the condition that the sampling grid should guarantee the cancellation. Such a requirement of a special grid design has already been realized in early lensing mass reconstruction works (e.g. Seitz & Schneider 1996). For a spin2 kernel like the KS kernel, a common square grid already suffices if the singularity is placed at a center of rotational symmetry, i.e. either onto a grid point or at the center of four grid points. In the case of the former, the grid point at the singularity has to be discarded. The latter, as shown by the left panel in Fig. 2, is a better choice considering the sampling homogeneity.
Fig. 2 Examples of sampling grids applicable for 2D integration over singular kernels of different spin values. The cross in the center indicates the position of the singularity in the integration kernel. 

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In general we need to deal with convolution kernels with different spin numbers. For example, the kernel F between γ2PCF and κ2PCF (19), which is proportional to z/z^{ ∗ 3}, has a spin of 4. In this case the square grid cannot guarantee the phase cancellation around the singularity any more (see Fig. 3). With a similar analysis as shown in Fig. 3, one can see that a triangular grid (middle panel, Fig. 2) can actually guarantee the phase cancellation around the singularity of a spin4 kernel. When using a triangular grid, the singularity can also be put either on a grid point or at the center of three grid points. The former choice loses the grid point at the singularity, but is applicable to spin3 kernels where the latter fails.
Fig. 3 A square grid guarantees phase cancellation around the singularity of spin2 kernels (left panel), but not that of spin4 kernels (right panel). The crosses indicate the position of the singularity of the integration kernel, while the dots are the grid points closest to the singularity. The polar angles of the kernel at the grid points are indicated by the directions of the arrows. For a spin2 kernel they cancel each other on a square grid already. For a spin4 kernel they cancel each other if the square grid is duplicated, rotated 45 degrees and put on top of the original grid. 

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To achieve a high numerical accuracy, it is required that the circle integral around the singularity is wellsampled. If one uses a square (triangular) grid, the innermost circle is only sampled by four (six) grid points, which is not enough for many κmodels. To remedy this, one can duplicate the sampling grid, rotate it around the singularity, and put it on top of the original grid. We show the result with the square grid and 45 degrees of rotation in the right panel of Fig. 2. The resulting grid is also applicable for spin4 kernels, as shown in the right panel of Fig. 3. This nonstandard way of constructing sampling grids, although not creating the best grids in terms of sampling efficiency, deals very well with the singularity of the integration kernel, and can easily generate sampling grids applicable for kernels of any spin number. We use this kind of grid in our numerical sampling.
Additional complications arise when performing the integration for 3pt statistics. There are two 2D integrals in (4), (36), (37) and (38). The corresponding integration kernels (22), (45), (46) and (47) all have three singularities. Luckily we can split each integration kernel into four additive terms and perform the integrals over each of them separately. Moreover, one can apply translational shifts to the integrands so that in each 2D integral there is only one singularity in the integration kernel. The singularity can also be shifted to the origin of the grids for numerical simplicity. After all these procedures, the four relations can be written as (75)(76)(77)(78)We can see that the integration kernels are either spin2, spin4, or spin3. All sampling grids shown in Fig. 2 are applicable to spin3 kernels.
7.2. Numerical results for 2PCF
We now construct several toy models for 2pt and 3pt convergence and shear correlations, and use them to test the relations derived as well as the numerical sampling method.
In the 2pt case, we build two models for the convergence correlation function: ⟨κκ⟩(r) = 1/r, and ⟨κκ⟩(r) = e^{−r2}. Using the wellestablished ξ_{+} − ξ_{−} relation (1) we can obtain the corresponding models for the shear correlation function: ⟨γγ⟩(r) = e^{4iφr}/r, and ⟨γγ⟩(r) = e^{4iφr}^{ [}(r^{4} + 4r^{2} + 6) e^{−r2} + 2r^{2} − 6^{] }/r^{4}.
Fig. 4 Left panels: ⟨ γγ ⟩ (r) with fixed φ_{r} = π/6 as a function of r. Right panels: polar angle of ⟨ γγ ⟩ (r) with fixed r = 20 as a function of φ_{r}. The black curves are the expected values while the dots are the results from numerical evaluation of 2D integral in (23). The ⟨ κκ ⟩ used in the upper panels is ⟨ κκ ⟩ (r) = 1/r and in the lower panels ⟨ κκ ⟩ (r) = exp(−r^{2}). 

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Figure 4 shows the comparison between these shear correlation function models and the shear correlation functions sampled using (23), with the corresponding convergence correlation function models as input. The numerically sampled values match the analytical models very well.
7.3. Numerical results for 3PCF
We build models for 3pt shear and convergence correlation functions via the 3pt correlation function of the deflection potential ψ. Suppose we evaluate the fields at positions X, Y and Z. By definition we have and (81)Now we assume a model for ⟨ ψ(X)ψ(Y)ψ(Z) ⟩ as (82)with x = X − Z and y = Y − Z. This model is special in the sense that it does not depend on the angle between x and y, but is nevertheless simple and rather local. The statistical homogeneity of the field enables us to write the 3pt correlation function as a function of two 2D spatial coordinates, which we have chosen here to be x and y.
Performing the derivatives, we find the corresponding 3pt shear and convergence correlation functions also depend only on X − Z = x and Y − Z = y, and read and (85)Using (83) as the input model for ⟨ κκκ ⟩ , we numerically evaluate ⟨ γγγ ⟩ (x,y) and ⟨ γγγ^{∗} ⟩ (x,y) using (75) and (78). The results are then compared to the analytical models for the 3pt shear correlation functions (84) and (85). As shown in Fig. 5, the numerical evaluations closely match the analytical models.
Hence, we have proven numerically that the relations between γ3PCFs and κ3PCFs (75) and (78) are correct, no additional delta functions are needed. At the same time we have shown that these relations can be numerically evaluated with a high accuracy. Therefore these relations provide a better way of relating the observable shear signal to the underlying matter density field than the original way, i.e. using the relations between the γ3PCFs and the κ bispectrum, since the latter does not allow for an easy accurate numerical evaluation.
Fig. 5 Comparison of numerically evaluated ⟨ γγγ ⟩ (x,y) and ⟨ γγγ^{∗} ⟩ (x,y) with their analytical toy models described in this section adopting α = 0.05. “Re” and “Im” indicate the real and imaginary parts of the shear correlation functions. Left panels: the functions are evaluated at x = 0.25k e^{−iπ/3} and y = 0.17k e^{iπ/8} for different values of k; right panels: the functions are evaluated at x = 0.75 e^{−iπ/3} and y = 0.45 e^{iφ} for 50 equally spaced φ values. 

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8. Conclusion
We derived the relations between the 3pt shear and convergence correlation functions which had been an important missing link between weak lensing threepoint statistics. As an intermediate step, we found the 2pt analogue of these relations and proved that it is the nonsymmetrized form of the existing ξ_{+} − ξ_{−} relation. By drawing analogy to the corresponding 1pt relations, namely the KaiserSquires relation and its isotropic form, we have further revealed that the newly derived relations and already established results fit into the same theoretical framework. The consistency of the configuration space relations with the known Fourier space relations have also been shown.
The 3pt relations derived are simple both analytically and numerically. They can be used as an alternative way of relating the measurable 3pt weak lensing statistics with the statistics of the underlying matter density field. Up to now one has to use the relations between the γ3PCFs and the convergence bispectrum to link theory to the observable 3pt shear signal. Since the γ3PCFs are very oscillatory and complicated functions of the convergence bispectrum (Schneider et al. 2005), it is hard to study the behavior of the 3pt shear signal for a given convergence bispectrum model. With the relations we derived, one can instead study the properties of the 3pt shear signal by constructing models for the κ3PCFs.
The method we used to derive these relations is based on the relation between the 2pt correlation functions of the lensing deflection potential and the convergence. The same method also allows one to systematically derive the relations between correlations functions of other weak lensing quantities, including the deflection potential ψ, the shear γ, the convergence κ, and the deflection angle α. We present the forms of some 2 and 3pt relations in Appendix B. Some of them are potentially of interest for studies of galaxygalaxy(galaxy) lensing and lensing of the cosmic microwave background.
Since the relations we obtained have complex kernels with nontrivial spin number and singularities, special care is needed when they are used numerically. We demonstrated how the numerical evaluation can be done, in particular the design of the sampling grid. Examples of numerical evaluation were shown for both 2 and 3pt relations using toy models for the convergence correlation function. Their results match very well with the analytical expectations.
Separating E and Bmodes from measurements of the γ3PCFs is particularly important since the systematic effects at the 3pt level are less understood. So far the only 3pt statistics allowing for an E/Bmode decomposition is the aperture mass statistics (Jarvis et al. 2004; Schneider et al. 2005) which is plagued with the same problem as the 2pt aperture statistics pointed out by Kilbinger et al. (2006). To amend this problem, one needs to construct the 3pt correspondence of the newly developed 2pt statistics (Schneider & Kilbinger 2007; Eifler et al. 2010; Fu & Kilbinger 2010; Schneider et al. 2010) which allows for an E/Bmode decomposition on a finite interval. As a direct theoretical application of the 3pt relations derived in this paper, we used them to formulate the conditions for E/Bmode decomposition of lensing 3pt statistics, in analogy to the 2pt condition given by Schneider & Kilbinger (2007). These conditions are the basis of formulating additional constraints which lead to E/Bmode decomposition over a finite region, therefore they provide a starting point for future works on constructing better 3pt shear statistics.
Our work was done for the case of weak lensing, but since it has only used the mathematical structure of the shear and the convergence, it applies also to other 2D polarization fields such as the polarization fluctuations of the cosmic microwave background.
Acknowledgments
X.S. would like to thank Fujun Du, Guangxing Li, and Andy Taylor for helpful discussions. B.J. acknowledges support by a UK Space Agency Euclid grant. This work was supported by the Deutsche Forschungsgemeinschaft under the Collaborative Research Center TR33 “The Dark Universe” and the EC within the RTN Network “DUEL”.
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Appendix A: Fourier transform of the F and G_{0} kernels
The F and G_{0} kernels, as defined in (7) and (4) relate the 2 and 3pt shear and convergence correlation functions, respectively. In Sect.2 we have derived their explicit forms, see (19) and (22). According to the Fourier space relations of the shear and convergence statistics (31) and (32), one expects that F/π^{2} and e^{4iβ} are Fourier pairs, as well as −G_{0}/π^{3} and e^{2i(β1 + β2 + β3)}. Here we perform the Fourier transforms of F/π^{2} and −G_{0}/π^{3}, with F and G_{0} given in (19) and (22).
The Fourier transformation of the kernel F is a 2D integral of the form (A.1)The Fourier transformation of the kernel G_{0} can be greatly simplified by performing translational shifts to the integration variables. It turns out that the full transformation is composed of 2D integrals similar to that in (A.1), (A.2)Performing the 2D integrals in polar coordinates results in (A.3)
(A.5)and (A.6)Combining (A.1) and (A.3) yields (A.7)which demonstrates that the Fourier transformation of F/π^{2} is indeed the phase factor e^{4iβ} in (31).
For the G_{0} kernel we still need to take account that ℓ_{3} = ℓ_{3} e^{iβ3} ≡ −ℓ_{1} − ℓ_{2}, so that (A.8)Thus the Fourier transformation of −G_{0}/π^{3} equals the phase factor e^{2i(β1 + β2 + β3)} in (32), as expected.
Appendix B: Relations between other correlation functions
In Sect. 2.2 we have derived the relations between the shear and the convergence 2 and 3pt correlation functions. The method we used is based on the relation between the 2pt correlation functions of the convergence κ and the deflection potential ψ (12). Thus the method can easily be generated to derive relations between the correlation function of κ and that of any weak lensing quantity g which can be expressed as derivatives of ψ. We denote g = D_{g}ψ, and write these 2pt relations in a general form (B.1)
Listed below are some candidates for g and the corresponding operator D_{g}, where α is the deflection angle defined as α = ∂ψ.
Let D_{g}D_{g′} act on both sides of the relation between ⟨ψψ⟩ and ⟨κκ⟩ (11), and use the statistical homogeneity of the κ field, one can obtain an integration form of the kernel ℋ, in analogy to (8). Let the same operator act on both sides of (12), one can express ℋ as derivatives of the convolution kernel ℱ in (12), as ℋ(x_{1} − x_{2} − y) = D_{g}D_{g′}ℱ(x_{1} − x_{2} − y). Further inserting the explicit form of ℱ (19) allows one to obtain the explicit form of ℋ as a function of z = x_{1} − x_{2} − y. We summarize some of the 2pt relations in Table B.1. Note that the form of ℋ(ℱ) for ⟨αα⟩ has a minus sign, which is due to the fact that ∂_{x1}∂_{x2}ℱ(x_{1} − x_{2} − y) = −∂^{2}ℱ(z) with z = x_{1} − x_{2} − y.
We write the relations between ⟨κκκ⟩ and the 3pt correlation functions of the g’s also in a uniform convolutional form, (B.2)To find the explicit form of the convolution kernel I, we first write it into an integral form, in analogy to (6), and then split it into terms which can be expressed also as derivatives of the kernel ℱ, like (20). Then with the explicit form of ℱ one can reach the explicit form of I. We list the forms of the convolution kernel I for some 3pt statistics in Table B.2.
Some of these relations, e.g. those for ⟨κγ⟩, ⟨γγκ⟩, and ⟨γκκ⟩, can find their application in galaxygalaxy(galaxy) lensing. Some other relations, e.g. those for ⟨αα⟩, ⟨ακ⟩, and ⟨αακ⟩, are potentially of interest for studies of the lensing effects on the cosmic microwave background and its crosscorrelation with galaxy weaklensing maps (Hu 2000).
All Tables
All Figures
Fig. 1 Definition of the geometry of a triangle (the leftmost sketch) and how it changes under permutations (the first three sketches from the left) and flip (the leftmost and the rightmost sketch) of the vertices. 

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In the text 
Fig. 2 Examples of sampling grids applicable for 2D integration over singular kernels of different spin values. The cross in the center indicates the position of the singularity in the integration kernel. 

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In the text 
Fig. 3 A square grid guarantees phase cancellation around the singularity of spin2 kernels (left panel), but not that of spin4 kernels (right panel). The crosses indicate the position of the singularity of the integration kernel, while the dots are the grid points closest to the singularity. The polar angles of the kernel at the grid points are indicated by the directions of the arrows. For a spin2 kernel they cancel each other on a square grid already. For a spin4 kernel they cancel each other if the square grid is duplicated, rotated 45 degrees and put on top of the original grid. 

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In the text 
Fig. 4 Left panels: ⟨ γγ ⟩ (r) with fixed φ_{r} = π/6 as a function of r. Right panels: polar angle of ⟨ γγ ⟩ (r) with fixed r = 20 as a function of φ_{r}. The black curves are the expected values while the dots are the results from numerical evaluation of 2D integral in (23). The ⟨ κκ ⟩ used in the upper panels is ⟨ κκ ⟩ (r) = 1/r and in the lower panels ⟨ κκ ⟩ (r) = exp(−r^{2}). 

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In the text 
Fig. 5 Comparison of numerically evaluated ⟨ γγγ ⟩ (x,y) and ⟨ γγγ^{∗} ⟩ (x,y) with their analytical toy models described in this section adopting α = 0.05. “Re” and “Im” indicate the real and imaginary parts of the shear correlation functions. Left panels: the functions are evaluated at x = 0.25k e^{−iπ/3} and y = 0.17k e^{iπ/8} for different values of k; right panels: the functions are evaluated at x = 0.75 e^{−iπ/3} and y = 0.45 e^{iφ} for 50 equally spaced φ values. 

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In the text 