ATC, we have a wingman…

On a recent flight back from UK to the US, there was a sudden hubbub on the flight as people started looking out the widows and taking pictures. (After a few seconds of panic) when I looked out the window I found that we had picked up a wingman. A Swiss Air aircraft was flying very close to us off our port wing. Pictures are attached. As we flew on, the plane fell back.

I am not sure how far off the wing the aircraft was. It seemed much closer than any other aircraft I had seen fly close by at 36,000+ feet.