Probing gravity and accretion using neutron stars

The overarching theme of this thesis is the use of neutron stars to understand basic physical processes like accretion and the nature of gravity. We investigated accretion primarily through joint radio and X-ray observations of low-mass X-ray binaries containing a neutron star. Astrophysical accretion occurs in a variety of contexts, and thus understanding the underlying processes can have broad application. Combining (quasi-)simultaneous radio and X-ray measurements is a powerful way to probe accretion because it traces both the inflow of material (the inner accretion disk and potential matter transfer onto the neutron star) and the outflow (a collimated jet or other type of outflow). In this thesis we explore such disk-jet connections in three neutron star low-mass X-ray binary systems, probing new or not-well-established phenomena over larger, previously unexplored luminosity ranges. Regarding the use of neutron stars to investigate the nature of gravity, we performed a fundamental test of gravitational theory using a radio millisecond pulsar that is in a hierarchical triple system with two white dwarf companions. This test exploits the precise, clock-like nature of millisecond pulsars and their high gravitational binding energy. In particular, we use the triple-system millisecond pulsar to test the strong equivalence principle, which is the central tenet of Einstein's theory of general relativity, and states that the gravitational acceleration experienced by bodies does not depend on their mass or composition.