We construct several new integrable systems corresponding to nonlocal versions of the Hirota equation, which is a particular example of higher order nonlinear Schrödinger equations. The integrability of the new models is established by providing their explicit forms of Lax pairs or zero curvature conditions. The two compatibility equations arising in this construction are found to be related to each other either by a parity transformation P, by a time reversal T or a PT -transformation possibly combined with a conjugation. We construct explicit multi-soliton solutions for these models by employing Hirota's direct method as well as Darboux-Crum transformations. The nonlocal nature of these models allows for a modification of these solution procedures as the new systems also possess new types of solutions with different parameter dependence and different qualitative behaviour. The multi-soliton solutions are of varied type, being for instance nonlocal in space, nonlocal in time of time crystal type, regular with local structures either in time/space or of rogues wave type.
Integrable nonlocal Hirota equationswith constants ε, α, β, γ ∈ R. Besides the NLSE for ε = 0, four cases are known to be integrable. When the ratio of the constants are taken to be α : β : γ = 0 : 1 : 1 or α : β : γ = 0 : 1 : 0 one obtains the derivative NLSE of type I [8] and II [9], respectively, which are in fact related to each other by a dependent variable transformation [10]. For α : β : γ = 1 : 6 : 3 one obtains the Sasa-Satsuma equation [11] and for α : β : γ = 1 : 6 : 0 the Hirota equation [12]. Variations of the latter are the subject of this manuscript.We notice that the additional term in the HNLSE when compared to the NLSE, i.e. (1.1) for ε = 0, shares the same PT -symmetry with the NLSE, as it is invariant with respect to PT :Hence HNLSEs may also be viewed as PT -symmetric extensions of the NLSE. Similarly as for many other PT -symmetric nonlinear integrable systems [13], various other PT -symmetric generalizations have been proposed and investigated by adding terms to the original equation, e.g. [14,15,16]. A further option, that will be important here, was explored by Ablowitz and Musslimani [17,18] who identified a new class of nonlinear integrable systems closely related to the NLSE by exploiting a hitherto unexplored PTsymmetry present in the zero curvature condition. Exploring this option below for the Hirota equation will lead us to new integrable systems with nonlocal properties.Our manuscript is organized as follows: In section 2 we discuss the zero curvature condition or AKNS-equation for the new class of integrable systems. The solutions to these systems involve fields at different points in space or time and reduce in certain limits to the standard Hirota equation, so that we refer to them as nonlocal Hirota equations. The equations possess two types of solutions of qualitatively different behaviour and parameter dependence. We identify the origin for this novel feature within the context of Hirota's direct method as w...