In last spec it seems to be impossible to register via Event interface an asynchronous observer.
It is possible for a method invocation, via

@Observes(notify=ASYNCHRONOUSLY)

in event param.

public interface Event<T> {
    public void fire(T event);
    public void addObserver(Observer<T> observer);
    public void removeObserver(Observer<T> observer);
    public Event<T> select(Annotation... bindings);
    public <U extends T> Event<U> select(Class<U> subtype, Annotation... bindings);
    public <U extends T> Event<U> select(TypeLiteral<U> subtype, Annotation... bindings);
}

maybe we need of a method overload like this

public void addObserver(Observer<T> observer, Notify type);