Whittaker modules for the affine Lie algebra $A_1 ^{(1)}$

We prove the irreducibility of the universal non-degenerate Whittaker modules for the affine Lie algebra $\widehat{sl_2}$ of type $A_1^{(1)}$ with noncritical level which are also irreducible Whittaker modules over $\widetilde{sl_2} =\widehat{sl_2} + {\Bbb C} d$ with the same Whittaker function and central charge. We have to modulo a central character for ${sl_2}$ to obtain irreducible degenerate Whittaker $\widehat{sl_2}$-modules with noncritical level. In the case of critical level the universal Whittaker module is reducible. We prove that the quotient of universal Whittaker $\widehat{sl_2}$--module by a submodule generated by a scalar action of central elements of the vertex algebra $V_{-2}(sl_2)$ is irreducible as $\widehat{sl_2}$--module. We also explicitly describe the simple quotients of universal Whittaker modules at the critical level for $\widetilde{sl_2}$. Quite surprisingly, with the same Whittaker function and the same central character of $V_{-2}(sl_2)$, some irreducible $\widetilde{sl_2}$ Whittaker modules can have semisimple or free action of $d$. At last, by using vertex algebraic techniques we present a Wakimoto type construction of a family of generalized Whittaker irreducible modules for $\widehat{sl_2}$ at the critical level. This family includes all classical Whittaker modules at critical level. We also have Wakimoto type realization for irreducible degenrate Whittaker modules for $\widehat{sl_2}$ at noncritical level.

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