What can bright neutron star binaries tell us about ultra-luminous X-ray pulsars?

One of the many puzzling observations of ultra-luminous X-ray (ULX) pulsars is pulsation transience. The non-detection of pulsations from typical bright pulsar X-ray binaries is usually accompanied by a decrease in flux and is therefore attributed to the propellor regime, obscuration, magnetic field variations, or irregular densities in the accreted material. Here we present the detection of a rapid pulse "turn-on" in NuSTAR observations of the high mass X-ray pulsar LMC X-4. The pulsations are weak or nonexistent for parts of the observation, but dramatically turn on in association with a super-Eddington accretion flare. Pulse dropout of this nature is the first of its kind detected in a bright X-ray pulsar and cannot be explained by the propeller regime. Changes in the emission geometry of the source during the flare could be responsible for observed changes in pulse phase and shape. These super-Eddington accretion effects make LMC X-4 an exciting new analogue to ULX pulsars, which exhibit similar forms of pulsation dropout and superorbital variations despite accreting at hundreds of times the neutron star Eddington limit.