They would probably just leave the ship in storage for study. Having a warship like this is a logistical nightmare, even to the UEF (huge ammunition expenditure if actually used in combat, and more fighter squadrons means more supplies need to be diverted to the ship). I got the impression that the Solaris followed the philosophy of the rest of the UEF: Costly, high performance for a short period. It excels at defending single system with secured supply chains, but I can't imagine it handling an offensive campaign without having a disproportionate logistical drain. Granted, the GTVA might use it as a core system defender, but having to take a ship with completely different system architecture and attempting to integrate it into the fleet would probably be more trouble than its worth. Not to mention that the ship's main armament is antimatter based, which would be very difficult for the GTVA to constantly replace (at least until they gain total infrastructural control of the UEF).
E: Typos.