Due to gravitation? Do they include gravitation in their “vacuum” models?
Yes, I'm guessing, indirectly due to gravity. I haven't even begun trying to read the paper, and don't think I will (my math just isn't good enough), but the article says their method involves a "high-energy electron beam combined with an intense laser pulse." Pulsars and neutron stars produce something like that in the form of intense jets of material and radiation that erupt from opposite poles of the star.
Now, whether or not they include gravitation in their models in a more substantive way, I can't say. It'd be fantastic if they do. I'd love to see that even if 99% of it went right over my head, as I'm sure it would! :-)