Proper Motion of the Neutron Star in the Supernova Remnant G18.9-1.1

Abstract

In this paper, we present the first direct measurement of the proper motion of the neutron star (NS) in the supernova remnant (SNR) G18.9-1.1 using a 15-year Chandra baseline. After correcting the observations' astrometric solutions using reference Gaia stars' positions, we measure a total proper motion of 24.7 $\pm$ 6.8 mas yr$^{-1}$ at an angle of $336^\circ \pm$ 16$^\circ$ east of north. Using the distance estimates from literature of 2.1 kpc and 3.8 kpc, this proper motion corresponds to Galactic rotation-corrected transverse velocities of 264d$_{2.1}$ $\pm$ 79 km s$^{-1}$ and 474d$_{3.8}$ $\pm$ 129 km s$^{-1}$, respectively. Our power ratio method analysis of SNR ejecta slightly favors the higher velocity, as multipole moments calculated from the back-evolved center using the farther distance are more consistent with values from other CCSNRs. The NS's motion is directly opposite the motion of bulk ejecta in G18.9$-$1.1, providing yet more evidence that NS kicks are generated via a conservation of momentum-like process between the NS and the ejecta, as has been observed in other SNRs.