Pike, Sean N.Sean N.PikeHarrison, Fiona A.Fiona A.HarrisonBACHETTI, MatteoMatteoBACHETTIBrumback, McKinley C.McKinley C.BrumbackFürst, Felix S.Felix S.FürstMadsen, Kristin K.Kristin K.MadsenPottschmidt, KatjaKatjaPottschmidtTomsick, John A.John A.TomsickWilms, JörnJörnWilms2021-01-042021-01-0420190004-637Xhttp://hdl.handle.net/20.500.12386/29427We report on Nuclear Spectroscopic Telescope Array observations of transient pulsations in the neutron star X-ray binary SMC X-1. The transition from nonpulsing to pulsing states was not accompanied by a large change in flux. Instead, both pulsing and nonpulsing states were observed in a single observation during the low-flux super-orbital state. During the high state, we measure a pulse period of P = 0.70117(9) s at T <SUB>ref</SUB> = 56145 MJD. Spectral analysis during nonpulsing and pulsing states reveals that the observations can be consistently modeled by an absorbed power law with a phenomenological cutoff resembling a Fermi-Dirac distribution, or by a partially obscured cutoff power law. The shapes of the underlying continua show little variability between epochs, while the covering fraction and column density vary between super-orbital states. The strength of pulsations also varies, leading us to infer that the absence and reemergence of pulsations are related to changing obscuration, such as by a warped accretion disk. SMC X-1 is accreting near or above its Eddington limit, reaching an unabsorbed X-ray luminosity of L <SUB>X</SUB>(2-10 keV) ≈ 5 × 10<SUP>38</SUP> erg s<SUP>-1</SUP>. This suggests that SMC X-1 may be a useful local analog to ultraluminous X-ray pulsars (ULXPs), which likewise exhibit strong variability in their pulsed fractions, 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 the behavior we observe in SMC X-1. Thus we propose that pulse fraction variability of ULXPs may also be due to variable obscuration.STAMPAenArticle10.3847/1538-4357/ab0f2b2-s2.0-85067180253000465974900013https://iopscience.iop.org/article/10.3847/1538-4357/ab0f2b2019ApJ...875..144PFIS/05 - ASTRONOMIA E ASTROFISICAPE9_10