Neutron stars and pulsar timing arrays as axion giant gyroscopes

We consider the three-dimensional rotating motions of neutron stars blown by ''axion wind.'' Neutron star precession and spin can change from the magnetic moment coupling to the oscillating axion background field, which is analogous to gyroscope motions with a driving force and the laboratory nuclear magnetic resonance detections of the axion. This effect modulates the pulse arrival time of pulsar timing arrays (PTAs) through changes in the pulsars' periods. It has appeared as a signal in the timing residual and two-point correlation function of recent Nanograv and PPTA data. Thus, the current measurement of PTAs can cast constraints on the axion-nucleon, for example, coupling as gaNN ∼ 10⁻¹² GeV⁻¹ with an axion mass of approximately 10⁻²³ eV.