Conventionally, Iranian languages are grouped in "western" and "eastern" branches. These terms have little meaning with respect to Old Avestan as that stage of the language may predate the settling of the Iranian peoples into western and eastern groups. The geographic terms also have little meaning when applied to Younger Avestan since it isn't known where that dialect (or dialects) was spoken either. Certain is only that Avestan (all forms) and Old Persian are distinct, and since Old Persian is "western", and Avestan was not Old Persian, Avestan acquired a default assignment to "eastern". Confusing the issue is the introduction of a western Iranian substrate in later Avestan compositions and redactions undertaken at the centers of imperial power in western Iran (either in the south-west in Persia, or in the north-west in Nisa/Parthia and Ecbatana/Media).

Answer this question, if possible (if impossible, reply "unanswerable"): What were the centers of imperial power in Persia?
unanswerable