how to store a reference to a object or a global object with angular?

I hope the title of this question makes sense. What i need is a way of storing a reference to a object in a controller and access it in another one.

For example, i have a AppCtrl where i initiate firebase:

.controller('AppCtrl', function ($scope, $ionicModal, $timeout, $firebase) {
        var ref = new Firebase("https://test.firebaseio.com/");
        var sync = $firebase(ref);
        $scope.db = sync.$asObject();

        var authClient = new FirebaseSimpleLogin(ref, function(error, user) {
            if (error) {
            } else if (user) {
                console.log(user);
            } else {
                // user is logged out
            }
        });

        ...

In another controller i want to do authClient.logout() but authClient is not available:

.controller('LogoutCtrl', function ($scope, $stateParams, $firebase) {
        authClient.logout();
})

Is there a way to access authClient in the second controller without the need to instantiating FirebaseSimpleLogin, like grabbing it from a global var?

I'm not sure what is the best approach, maybe is a good idea to instantiate it again..

I'm using firebase as an example here, but this could be any other class

any ideas?