Čerenkov radiation
From Halopedia, the Halo wiki
Cherenkov radiation is a type of radiation emitted by plutonium, such as in HORNET mines, when transitioning from Slipspace to realspace. It is extremely powerful, reaching across light-minutes. This is what gives Slipspace entry and exit the unique blue coloration, and usually alerts in-system enemy ships to their arrival. Covenant ships apparently also give off Cherenkov radiation, though whether it is emitted by plutonium or some other material on their ships is unknown.
