Constraints on a Dark Matter Subhalo Near the Sun from Pulsar Timing

Using pulsar accelerations, we identify and constrain the properties of a dark matter subhalo in the Galaxy for the first time from analyzing the acceleration field of binary and solitary pulsars. The subhalo is characterized by analyzing a local deviation from a smooth potential. Our MCMC calculations show that this subhalo has a mass of 2.45_{-0.96}^{+1.07}×10^{7}M_{⊙} and is located at Galactocentric coordinates X=7.43_{-0.12}^{+0.2}  kpc, Y=0.38_{-0.16}^{+0.11}  kpc, Z=0.21_{-0.11}^{+0.06}  kpc, using flat, uninformative priors, where we have modeled the sub halo as a compact object. The Bayes factors for the models are in the range of ∼20-40, which indicates tentative evidence (though not yet decisive) for the subhalo. Modeling the subhalo with a NFW profile gives a subhalo mass of 6.19_{-2.03}^{+1.92}×10^{7}M_{⊙}, located at X=7.47_{-0.14}^{+0.21}, Y=0.38_{-0.16}^{+0.11}, Z=0.21_{-0.11}^{+0.06}; the mass within the scale radius (0.1 kpc) for the NFW profile is 0.48_{-0.16}^{0.15}×10^{7}M_{⊙}. We examine Gaia data and the atomic and molecular hydrogen data of our Galaxy and show that the measured deviation from a smooth potential cannot arise from the gas or the stars in our Galaxy. Additionally, by analyzing the full sample of binary pulsars with available acceleration measurements that span 3.4 kpc in Galactocentric radius from the Sun and 3.6 kpc in vertical height, we find that massive (with mass >10^{8}M_{⊙}) subhalos are disfavored for the Milky Way within several kiloparsec of the Sun. While smaller sub-halos may be present, they are beyond the reach of current direct acceleration measurements. The presence of a ∼10^{7}M_{⊙} subhalo within a few kpc of the Sun is potentially consistent with the expected number counts of sub-halos in the prevailing ΛCDM paradigm, for a substantial subhalo mass fraction. As the number and precision of direct acceleration measurements continues to grow, we will obtain tighter constraints on dark matter sub-structure in our Galaxy. This Letter now provides a proof of principle for probing nearby, low-mass subhalos, and has implications across many fields of astrophysics-from understanding the nature of dark matter to galaxy formation.