I didn't realize you had aluminum arms. You have the late design.
The "bolt" you see poking through the arm is the end of the axle; I call it the stub axle. It is splined into a hub that rides in the bearing and that big nut holds everything together. The nut is a locking nut and it is supposed to be replaced any time it is removed because the locking feature wears when removed.
My guess is that the nut has backed off over time because it wasn't replaced when removed and likely wasn't properly torqued. Most people don't have a torque wrench capable of reaching the correct torque.
The bad news is that running with the nut loose may damage the bearing. I "fixed" my loose nut (after almost losing the wheel in a race) but when I went back and checked the bearing later, it was loose and didn't spin freely. There was no noise to indicate a problem.
Here is the replacement nut that I've been using instead of the factory lock nut:
www.disc-lock.com/safety-wheel-nut?item=93
I'm in Santa Clara and my impact wrench is pneumatic. You can try the penetrating oil but I don't think you'll have any luck.
If you have to replace the bearing, you'll also need a press. I use dry ice to shrink the bearings first before pressing in the new bearings.