The best competing explanation seems to be that it is an isolated Schwartzschild black hole or neutron star? Note that a perfect Schwartzchild metric will produce just two images (see, e.g., http://casa.colorado.edu/~ajsh/approach.html ). However, two arcseconds seems like a very large distance for a neutron star (which should give a splitting on the order of mas at 10 kpc.)