Empirical Modeling of Magnetic Braking in Millisecond Pulsars to Measure the Local Dark Matter Density and Effects of Orbiting Satellite Galaxies

Abstract

We present a novel method that enables us to estimate the acceleration of individual millisecond pulsars (MSPs) using only their spin period and its time derivative. For our binary MSP sample, we show that one can obtain an empirical calibration of the magnetic braking term that relies only on observed quantities. We find that such a model for magnetic braking is only valid for MSPs with small surface magnetic field strengths ($<3\times10^8$ G) and large characteristic ages ($>$ 5 Gyr). With this method we are able to effectively double the number of pulsars with line-of-sight acceleration measurements, from 27 to 53 sources. This expanded dataset leads to an updated measurement of the total density in the midplane, which we find to be $ρ_0$ = 0.108 $\pm$ 0.008 \textit{stat}. $\pm$ 0.011 \textit{sys} M$_\odot$/pc$^3$, and the first $>3σ$ measurement of the local dark matter density from direct acceleration measurements, which we calculate to be $ρ_{0,\mathrm{DM}}$ = 0.0098 $\pm$ 0.0025 \textit{stat.} $\pm$ 0.0003 \textit{sys}. M$_\odot$/pc$^3$ (0.37 $\pm$ 0.10 GeV/cm$^3$). This updated value for $ρ_{0,\mathrm{DM}}$ is in good agreement with literature values derived from kinematic estimates. The pulsar accelerations are very asymmetric above and below the disk; we show that the shape and size of this asymmetry can be largely explained by the north-south asymmetry of disk star counts and the offset in the Milky Way disk and halo centers of mass due to the Large Magellanic Cloud.