Countdown and the novelization. This is also implied by the film due to the apparent relative proximity to Romulus, in conjunction with the fact that no Federation worlds were said to be destroyed in the same timeframe as Romulus. For example, we know from Unification that Vulcan is also a "short hop" from the RNZ. Star Trek Online, in addition to the above sources. They can indicate writer intent. For example: Trespassing vessel: you have entered the jurisdiction of the Klingon Empire. This is another reference which indicates that the border the writers had in mind for the Kelvin incident was the Klingon-Federation border. Statements by Orci & Kurtzman appear to say otherwise.