A magnetar is a neutron star with a very strong magnetic field. As far as I know, they’re not electrically charged (though of course they have electric fields if they rotate), so the Kerr-Newman metric wouldn’t properly describe them in general relativity.
So, which metric does describe them? What’s the metric called that describes an electrovacuum solution based on a magnetic field without a net electrical charge?
Most metrics have never been solved analytically nor given names, and it wouldn’t surprise me at all if the electrically neutral magnetic dipole were one of that vast set. Even worse, they rotate, about an axis in general different from the magnetic axis.
And you probably already know this, but any electrovacuum solution could only be at best the exterior metric. The interior metric, where all of the baryonic matter is, is even more of a mess.
It looks like the metric has been found in this paper:
Kerr–Newman metric endowed with magnetic dipole moment
Of course, it costs $28 to purchase the article, so I might have to look this one up in the actual :eek: paper journal instead. 
Oh, wow, that is impressive. It still doesn’t address rotation of the dipole, and might in fact only apply to the dipole being on the rotation axis, both of which are significant. Still impressive, though.