Source: Exotic engrams; extremely rare world drops.
Lore
"I dunno. Hunter just came in one day with this odd Arc-reacting circuit, asking what it might be good for. All she could tell me was she scored it off some derelict satellite that crashed in the EDZ. So I ran diagnostics on them and it ebbed and flowed with massive Arc fluctuations. No pattern, no match to known solar or extra-solar events. Just random-like. So I hooked one into an old Malina vest we had lying around, and wished her luck. Word's got out, I guess." —Overheard in the Tower