Angular and Rails are frameworks that leave the decision of authentication up to you.

You can do token authentication, basic authentication, OAuth, OAuth2, or even roll your own. For a rails api <-> angular combo, I usually stick to token or OAuth 1/2 authentication.

I have open sourced an angular application (technically ionic but it uses angular under the hood) and the rails api that it uses as a backend which uses token authentication. https://github.com/johnkelly/mojave https://github.com/johnkelly/kalahari