We develop a method of an asymptotically exact treatment of threshold singularities in dynamic response functions of gapless integrable models. The method utilizes the integrability to recast the original problem in terms of the low-energy properties of a certain deformed Hamiltonian. The deformed Hamiltonian is local, hence it can be analysed using the conventional field theory methods. We apply the technique to spinless fermions on a lattice with nearest-neighbors repulsion, and evaluate the exponent characterizing the threshold singularity in the dynamic structure factor.PACS numbers: 02.30. Ik, 71.10.Pm, 75.10.Pq One-dimensional (1D) interacting systems [1, 2, 3] occupy a special place in quantum physics. Although interactions have much stronger effect in 1D than in higher dimensions, it is often possible to evaluate observable quantities exactly. Besides forming the basis of our understanding of strong correlations, 1D models have long served as a test bed for various approximate methods.Many properties of interacting 1D systems can be understood by considering integrable models [2]. The paradigmatic example is N spinless fermions on a lattice with L cites with periodic boundary conditions,Here n m = ψ † m ψ m and ∆ is the repulsion strength [4]. The experimentally relevant [5] response function for the model (1) is the dynamic structure factor|f is an eigenstate of Eq. (1) with energy ǫ f , and |0 is the ground state with ǫ 0 being the ground state energy.In any 1D system, conservation laws restrict the support of correlation functions in (ω, q) plane. For example, S(q, ω) > 0 only at ω > ω 0 (q), see Fig. 1. On general grounds, S(q, ω) is expected to exhibit a power-law singularity at the threshold [6],Although exact eigenstates of integrable models can be constructed using the Bethe ansatz [2], evaluation of dynamic correlation functions is very difficult. A considerable progress was achieved in understanding gapful models [7]. However, it is still largely an open problem in the gapless case, and, with few exceptions (see, e.g., [8,9,10,11]), the threshold exponents µ are not known. For the model (1), the most complete results so far were obtained by combining numerics with the algebraic Bethe ansatz [12,13]. The main limitation of this technique is very slow convergence towards the thermodynamic limit, which makes it very difficult to evaluate the exponent. In the alternative Luttinger liquid approach [1,3,14], a 1D system is described by two parameters, the sound velocity v = (dω 0 /dq) q→0 , and the Luttinger parameter κ characterizing the interaction strength. At q → 2k F , Luttinger liquid theory yields Eq. (3) with the exponentAt q → 0, however, one finds S ∝ δ(ω − vq). The discrepancy with Eq. (3) is due to the omission in the fixed-point Luttinger liquid Hamiltonian [3,14] of the irrelevant in the RG sense operators [13,14] representing the spectrum nonlinearity. Indeed, for small q, most of the spectral weight of S(q, ω) is confined to a narrow interval of ω of the width δω ∼ ω 1 − ω 0 about ...