I know it would be awfully weak, but it's present even where all other power sources would be unavailable - could cosmic level radiation be used to power minimal set of systems of a probe, say, a clock to "wake it up" after reaching destination (in a couple hundred years) and activate more efficient energy sources "hibernated" for duration of the travel? What orders of magnitude of power input could be expected, per some unit (mass, surface?) of the "harvesting" apparatus?
Specifically, it appears the cosmic background - specifically, CMB - is not uniform.
The variance is only ±0.00057 K but the features (hot and cold spots) seem to remain mostly persistent over time, so the device, while needing to pivot to maintain orientation relative to CMB, doesn't have to reconfigure 'absorber' and 'radiator' layout on the fly.
The question remains, how can one use such tiny variance, especially with total input of 400–500 photons/cm$^3$ - and how much (...how little) energy could be gained that way?
As clarification what I'm asking for - any external energy source; it doesn't need to be CMB specifically - that is available during intergalactic travel, where you can't depend on solar wind, light of nearby stars or similar; you must depend on what's available there: thermal cosmic background radiation, cosmic microwave background, high-energy cosmic rays, whatever means are available in intergalactic space, other than what you brought with the craft.