The C*-algebra of a Hilbert Bimodule

Abstract

We regard a right Hilbert C*-module X over a C*-algebra A endowed with an isometric *-homomorphism ϕ: A\to L_A(X) as an object X_A of the C*-category of right Hilbert A-modules. Following a construction by the first author and Roberts, we associate to it a C*-algebra O_{X_A} containing X as a ``Hilbert A-bimodule in O_{X_A}''. If X is full and finite projective O_{X_A} is the C*-algebra C*(X), the generalization of the Cuntz-Krieger algebras introduced by Pimsner. More generally, C*(X) is canonically embedded in O_{X_A} as the C*-subalgebra generated by X. Conversely, if X is full, O_{X_A} is canonically embedded in the bidual of C*(X). Moreover, regarding X as an object A_X_A of the C*-category of Hilbert A-bimodules, we associate to it a C*-subalgebra O_{A_X_A} of O_{X_A} commuting with A, on which X induces a canonical endomorphism ρ. We discuss conditions under which A and O_{A_X_A} are the relative commutant of each other and X is precisely the subspace of intertwiners in O_{X_A} between the identity and ρon O_{A_X_A}. We also discuss conditions which imply the simplicity of C*(X) or of O_{X_A}; in particular, if X is finite projective and full, C*(X) will be simple if A is X-simple and the ``Connes spectrum'' of X is the circle.