From my understanding, the "light" didn't appear in the same spot in the two photographs taken at roughly the same time of day. So two cosmic ray strikes are possible, since they show up in the pictures on a weekly basis. It's unclear how many pictures they took in that direction at that time of day. If they took more than two (sets), that seems to make the cosmic ray explanation even more likely. The shape of the "light" also seems to be consistent with a cosmic ray strike.