For a thermalised system, at equilibrium, the usual quantum statistical ensembles are valid and we can define temperature at an equilibrium between system and its bath. But for integrable systems (non-thermalisable) we don't have those ensembles, so how do we define a temperature?
PS- I had this doubt specific to many body localized state. And my intution is that the ensemble for a many body localised state would be similar to any other integrable system. Please correct me if I am wrong.