Galactic structure from binary pulsar accelerations: Beyond smooth models

Abstract

We measure the line-of-sight accelerations of 26 binary pulsars due to the Milky Way's gravitational potential, and produce a three-dimensional map of the acceleration field of the Galaxy. Acceleration measurements directly give us the change in the line-of-sight velocity at present day, without requiring any assumptions inherent to kinematic modeling. We measure the Oort limit (${\ensuremath{\rho}}_{0}=0.062\ifmmode\pm\else\textpm\fi{}0.017{M}_{\ensuremath{\bigodot}}/{\mathrm{pc}}^{3}$) and the dark matter density in the midplane (${\ensuremath{\rho}}_{0,\mathrm{DM}}=\ensuremath{-}0.010\ifmmode\pm\else\textpm\fi{}0.018{M}_{\ensuremath{\bigodot}}/{\mathrm{pc}}^{3}$); these values are similar to, but have smaller uncertainties than, previous pulsar timing measurements of these quantities. Here, we provide for the first time, values for the Oort constants and the slope of the rotation curve from direct acceleration measurements. We find that $A=15.4\ifmmode\pm\else\textpm\fi{}2.6\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{kpc}$ and $B=\ensuremath{-}13.1\ifmmode\pm\else\textpm\fi{}2.6\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{kpc}$ (consistent with results from Gaia), and the slope of the rotation curve near the Sun is $\ensuremath{-}2\ifmmode\pm\else\textpm\fi{}5\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{kpc}$. We show that the Galactic acceleration field is clearly asymmetric, but due to data limitations it is not yet clear which physical processes drive this asymmetry. We provide updated models of the Galactic potential that account for various sources of disequilibrium; these models are incompatible with commonly used kinematic potentials. This indicates that use of kinematically derived Galactic potentials in precision tests (e.g., in tests of general relativity with pulsar timing) may be subject to larger uncertainties than reported. The acceleration data indicates that the mass of the Galaxy within the Solar circle is $2.3\ifmmode\times\else\texttimes\fi{}{10}^{11}{M}_{\ensuremath{\bigodot}}$, roughly twice as large as currently accepted models. Additionally, the residuals of the acceleration data compared to existing Galactic models have a dependence on radial position; this trend can be explained if the Sun has an additional acceleration away from the Galactic Center.