Heavy Arbalest‘s ability grants to the equipped creature an activated ability that allows the controller of the creature to tap the creature to deal two points of damage to a targeted creature or player. Because the creature is the source of this ability, a creature with protection from artifacts can receive damage from this ability (as long as the equipped creature is not also an artifact.) Compare this to Blazing Torch. Blazing Torch’s ability specifically states that the Torch is the source of the damage.