There are 2 Dead Ghosts to be found in Archer's Line. They can be collected on any mission or during a patrol. Each Ghost can only be collected once per account.
Climb up onto the crates on the right-hand side of the accelerator. Look toward the accelerator itself and you will see the faint light of a Dead Ghost:
This Dead Ghost is a little tricky to get back from. Head over to the ruined dome that is just beyond the accelerator, to the right of the Dead Ghost. You just want to head to the right of the pole and look into the ravine that is there:
Below you you can see the Dead Ghost. Just drop down to reach it.
You should be able to Double Jump from there back to the ledge above.