Yes. All activated capabilities have reached immediate speed unless they especially indicate otherwise*. Because the capability is created with a colon, it is an activated capability. It generally does not must be in the battlefield to be activated, plus opponent doesn't have to understand it is present to stimulate it (although you must reveal it, demonstrably). Within specific cast, you're discarding the card included in the price. That's the only reason you need to discard the card. If it don't say discard, you could activate the power as much times while you desired.
It is spelled call at 112.3b, focus my own.
112.3b Activated abilities have a cost and a result. They've been written as "[Cost]: [Effect.] [Activation guidelines (if any).]" A new player may trigger such an ability whenever he or she has priority.
Compare this on principles text for instants,
304.1. A person who may have concern may throw an instant card from his / her hand.
So the timing for activated abilities matches the time for instants.
As for it being within hand, this is spelled out here:
602.2a The gamer declares that he or she is activating the capability. If an activated capability has been triggered from a concealed area, the card that features that capability is uncovered. That ability is established on the stack as an object that is not a card.
Note: I said especially suggest otherwise. This specification does not need becoming on card. As an example, the Comprehensive Rules suggest that planeswalker turn on capabilities is only able to be activated on your change at sorcery rate, but this is not reported on the card.