The Asymmetric Bowshock/PWN of PSR J2124$-$3358

We describe new measurements of the remarkable H$\alpha$/UV/X-ray bowshock and pulsar wind nebula of the isolated millisecond pulsar PSR J2124$-$3358. {\it CXO} imaging shows a one-sided jet structure with a softer equatorial outflow. KOALA IFU spectroscopy shows that non-radiative emission dominates the bow shock and that the H$\alpha$ nebula is asymmetric about the pulsar velocity with an elongation into the plane of the sky. We extend analytic models of the contact discontinuity to accommodate such shapes and compare these to the data. Using {\it HST} UV detections of the pulsar and bow shock, radio timing distance and proper motion measurements and the {\it CXO}-detected projected spin axis we model the 3-D PWN momentum flux distribution. The integrated momentum flux depends on the ionization of the ambient ISM, but for an expected ambient WNM we infer $I = 2.4 \times 10^{45} {\rm g\,cm^2}$. This implies $M_{NS} = 1.6-2.1 M_\odot$, depending on the equation of state, which in turn suggests that the MSP gained significant mass during recycling and then lost its companion. However, this conclusion is at present tentative, since lower ionization allows $\sim 30\%$ lower masses and uncertainty in the parallax allows up to 50% error.