In federated identity management systems, the Identity Provider (IdP) is responsible for authenticating the user's identity and creating security tokens known as assertions. These assertions, which include claims about the user's identity, are then sent to the Service Provider (SP), which validates the token and allows the user to access the resources without needing to create a local account. The resource server, directory service, and access gateway do not have the primary responsibility of authenticating users in a federated environment; they serve different roles in the identity management process.