A Survey for H$\alpha$ Pulsar Bow Shocks

We report on a survey for H$\alpha$ bow shock emission around nearby $\gamma$-detected energetic pulsars. This survey adds three Balmer-dominated neutron star bow shocks to the six previously confirmed examples. In addition to the shock around {\it Fermi} pulsar PSR J1741$-$2054, we now report H$\alpha$ structures around two additional $\gamma$-ray pulsars, PSR J2030+4415 and PSR J1509$-$5850. These are the first known examples of H$\alpha$ nebulae with pre-ionization halos. With new measurements, we show that a simple analytic model can account for the angular size and flux of the bow shocks' apices. The latter, in particular, provides a new pulsar probe and indicates large moments of inertia and smaller distances than previously assumed, in several cases. In particular we show that the re-measured PSR J0437$-$4715 shock flux implies $I = (1.7\pm 0.2) \times 10^{45}/(f_{HI} {\rm sin}i) {\rm g\,cm^2}$. We also derive a distance $d\approx 0.72$kpc for the $\gamma$-ray only pulsar PSR J2030+4415 and revised distances for PSRs J1959+2048 (1.4kpc) and J2555+6535 ($\sim 1$kpc), smaller than the conventional DM-estimated values. Finally we report upper limits for 94 additional LAT pulsars. An estimate of the survey sensitivity indicates that for a warm neutral medium filling factor $\phi_{WNM}\sim 0.3$ there should be a total of $\sim$ nine H$\alpha$ bow shocks in our LAT-targeted survey; given that seven such objects are now known, a much larger $\phi_{WNM}$ seems problematic.