Breaking the quantum limit: the magnetic field of neutron stars in extra-galactic Be X-ray binaries

Neutron stars are some of the most magnetic objects that have ever been observed, and so they provide physicists with unique environments where fundamental laws of physics can be tested. Neutron stars are typically thought to have magnetic fields between 108 and 1014 G. The effects of the quantum electrodynamics are important above the quantum critical field (BQED) of 4.4×1013 G. In this thesis, I provide evidence that there may be many more neutron stars with B > BQED than previously thought, and that all neutron stars in binary systems that are close to spin equilibrium follow the same relationship between spin period (P) and magnetic field. In Chapter 2, I determine the long-term average X-ray luminosity, spin period, and rate of change of spin period for 42 Be X-ray binaries (BeXB) in the Small Magellanic Cloud (SMC). I use this information, combined with orbital data, to show that the neutron stars in all of these systems are disc-accreting, and that 85% are close to spin equilibrium. All systems with P ≥ 100 s are predicted to have B > BQED. This applies to 2/3 systems. These predicted magnetic fields are higher than those of neutron stars in Galactic BeXB that have had their magnetic fields directly measured via cyclotron resonance scattering features (CRSF). I conclude that this is because the CRSF sources are not close to spin equilibrium. In Chapter 3, I look at pulse-profiles for the neutron stars discussed in Chapter 2 and find that they contain an array of features that vary both across and within individual systems. I suggest that BeXB containing neutron stars with relatively longer spin periods transition from a pencil to a fan beam at lower luminosities. In Chapter 4, I apply the methods used in Chapters 2 and 3 to LXP187, a BeXB in the Large Magellanic Cloud (LMC) that is not close to spin equilibrium. Results for LXP187 help confirm the conclusions of Chapter 2 - that ~ 2/3 BeXB contain neutron stars with B > BQED