Updated Proper Motion of the Neutron Star in the Supernova Remnant Cassiopeia A

Abstract

In this paper, we present updated estimates of the velocity of the neutron star (NS) in the supernova remnant (SNR) Cassiopeia A using over two decades of Chandra observations. We use two methods: 1.) recording NS positions from dozens of Chandra observations, including the astrometric uncertainty estimates on the data points but not correcting the astrometry of the observations, and 2.) correcting the astrometry of the 13 Chandra observations that have a sufficient number of point sources with identified Gaia counterparts. For method #1, we observe a heliocentric velocity of 275 $\pm$ 121 km/s, with an angle of 177 $\pm$ 22 degrees east of north. For method #2, we observe a heliocentric velocity of 436 $\pm$ 89 km s$^{-1}$ at an angle of 158 $\pm$ 12 degrees. Correcting for galactic rotation and the sun's peculiar motion decreases these estimates to 256 km s$^{-1}$ at 167$^\circ$ and 433 km s$^{-1}$ at 151$^\circ$, respectively. Both of our estimates match with the explosion-center-estimated velocity of $\sim$350 km s$^{-1}$ and the previous 10--year baseline proper motion measurement of 570 $\pm$ 260 km s$^{-1}$, but our use of additional data over a longer baseline has led to a smaller uncertainty by a factor of 2--3. Our estimates rule out velocities $\gtrsim$600 km s$^{-1}$ and better match with simulations of Cassiopeia~A that include NS kick mechanisms.