The wiki has a page about artifacts and their passive effects that seems to be up to date.
I’ll have to check to know where they are defined in the code, but this will take a little bit of time… I’ll report back with my findings, unless someone else already finds it before me.
Edit:
Alright, I’ve checked - and the effects are described in enums.h (enum art_effect_passive).
As for the messages that the player recieves, they can be found in game.cpp (function game::add_artifact_messages(…)).