Pulsars in binary systems

Pulsars are among the most precise instruments available to astrophysics. This the- sis reviews neutron star and pulsar physics, covering first how they work, then the binary systems that recycle them into millisecond pulsars, and the measurements they make possible. The study of four systems is undertaken to get a grasp of real examples which have shaped and are shaping the field. The first binary pulsar system B1913+16 is considered through its almost 50-year observational history, offering insights into many areas, from constraining neutron star masses to, most importantly, strong-field tests of general relativity, such as confirming geodetic spin precession and the indirect observation of gravitational waves through the identi- fication of post-Keplerian parameters. Then J0737−3039, the first double pulsar system, is explored, bettering the results found before thanks to the wealth of infor- mation available, in a relatively short window of time, due to the two visible beams. This system has offered the best tests of general relativity by determining seven in- dependent post-Keplerian parameters. Third, the newly discovered J0514−4002E, which may be a millisecond pulsar orbiting a black hole, is studied, making the system an exciting testing ground for strong-field gravity and helping to constrain neutron star and black hole masses. Finally, the first and only multi-messenger event GW170817 is considered, observed both by ground-based gravitational wave interferometers and electromagnetic detectors, which yields a vast number of results, from constraining the equation of state describing dense matter in neutron stars and several fundamental physics concepts, to explaining (some) short gamma-ray bursts and heavy-element synthesis.