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Two \textit{NuSTAR} observations of the luminous X-ray pulsar LMC X-4 inOctober and November 2015 captured several bright accretion flares from thissource, which has a long history of stable pulse and superorbital behavior. Wepresent a timing analysis of these data in which we detect a rapid pulseturn-on in association with the accretion flares, during which the sourcereaches super-Eddington luminosities. Pulsations, which are normally seen fromthis source, are found to only occur for approximately one hour before andduring the bright flares. Beyond one hour before and after the flares, we findpulsations to be weak or nonexistent, with fractional RMS amplitudes of lessthan 0.05. At the onset of the flare, the pulse profiles exhibit a phase shiftof 0.25 cycles that could be associated with a change in the emission geometry.This increase in pulse strength occurring well before the flare cannot beexplained by the propeller effect, and potentially offers a connection betweenthe magnetic properties of pulsars that accrete close to their Eddington limitsand ULX pulsars.

Discovery of pulsation dropout and turn-on during the high state of the accreting X-ray pulsar LMC X-4