An optical bow shock around the nearby millisecond pulsar J2124-3358

We report the discovery of an Hα-emitting bow shock nebula powered by the nearby millisecond pulsar J2124-3358. The bow shock is very broad and is highly asymmetric about the pulsar's velocity vector. This shape is not consistent with that expected for the case of an isotropic wind interacting with a homogeneous ambient medium. Models that invoke an anisotropy in the pulsar wind, a bulk flow of the surrounding gas, or a density gradient in the ambient medium either perpendicular or parallel to the pulsar's direction of motion also fail to reproduce the observed morphology. However, we find an ensemble of good fits to the nebular morphology when we consider a combination of these effects. In all such cases, we find that the pulsar is propagating through an ambient medium of mean density 0.8-1.3 cm^-3 and bulk flow velocity ∼15-25 km s^-1 and that the star has recently encountered an increase in density by 1-10 cm^-3 over a scale ≲0.02 pc. The wide variety of models that fit the data demonstrate that in general there is no unique set of parameters that can be inferred from the morphology of a bow shock nebula.