Long-Term Evolution in Neutron Star HMXBs: the Cases of 4U 1538-522 and 4U 1907+09

Neutron star X-ray binaries present useful laboratories for a number of fieldsof study, including stellar and binary evolution, due to their status as theendpoints of stellar life, and the behavior of matter under extreme conditions,due to the accretors' compactness. The high-mass X-ray binaries 4U 1538-522 and4U 1907+09 are somewhat similar sources: both are slow-rotating accretingpulsars with cyclotron lines, pseudo-absorption features which characterize themagnetic field strength near the neutron star surface. Our NuSTAR and Suzakuobservations of 4U 1538-522 suggest that its cyclotron line energy increased by~5% over the late 2000s, as evidenced by a comparison to RXTE datasets from theearly 2000s. In comparison, our preliminary results from 4U 1907+09 suggest nosignificant change in the magnetic field strength compared toobservations from the past two decades. We additionally place the best limits yet on orbital periodevolution in 4U 1538-522, which has implications for the evolutionary state ofthe donor star, which I will discuss.