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We report on NuSTAR observations of transient pulsations in the neutron starX-ray binary SMC X-1. The transition from non-pulsing to pulsing states was notaccompanied by a large change in flux. Instead, both pulsing and non-pulsingstates were observed in a single observation during the low-flux super-orbitalstate. During the high-state, we measure a pulse period of $P =0.70117(9)\,\mathrm{s}$ at $T_{ref} = 56145\,\mathrm{MJD}$. Spectral analysisduring non-pulsing and pulsing states reveals that the observations can beconsistently modeled by an absorbed power law with a phenomenological cutoffresembling a Fermi-Dirac distribution, or by a partially obscured cutoff powerlaw. The shapes of the underlying continua show little variability betweenepochs, while the covering fraction and column density vary betweensuper-orbital states. The strength of pulsations also varies, leading us toinfer that the absence and reemergence of pulsations are related to changingobscuration, such as by a warped accretion disk. SMC X-1 is accreting near orabove its Eddington limit, reaching an unabsorbed X-ray luminosity of $L_{\rmX}({\rm 2-10~keV}) \approx 5 \times 10^{38}\, {\rm erg}\, {\rm s}^{-1}$. Thissuggests that SMC X-1 may be a useful local analog to ultraluminous X-raypulsars (ULXPs), which likewise exhibit strong variability in their pulsedfractions, as well as flux variability on similar timescales. In particular,the gradual pulse turn-on which has been observed in M82 X-2 is similar to thebehavior we observe in SMC X-1. Thus we propose that pulse fraction variabilityof ULXPs may also be due to variable obscuration.

Observing the transient pulsations of SMC X-1 with NuSTAR