NuSTAR observations of SMC X-1 at two different superorbital phases

SMC X-1 is a Roche-lobe accreting 0.71 s X-ray pulsar with a supergiant companion. The system has a 3.9 day orbital period and is eclipsing. Its X-ray lightcurve also shows a superorbital cycle, on a varying timescale of 40 to 70 d. This variability is thought to be dueto a precessing, warped accretion disk which leads to varying obscuration. NuSTAR observed SMC X-1 twice in 2012, once during a minimum of its superorbital cycle and once during the decline of the following peak. The fluxes of the two observations differ by a factor of 10. We present a detailed broadband spectral analysis and comparison of the two spectra. They are well described by empirical cutoff power law models as well as by thermal Comptonization. The previously reported soft excess from the accretion disk is also tentatively detected. One spectral difference between the two superorbital phases is that the low flux spectrum shows additional absorption as well as a higher equivalent width iron line. This confirms the warped disk picture and refines sparse earlier broadband results obtained with BeppoSax and RXTE. Above 3 keV the pulse profile is only mildly energy dependent. For the high flux observation we perform a detailed pulse phase resolved analysis. No strong changes in spectral shape are detected. To our knowledge this is the first such spectral study extending to above 10 keV reported for SMC X-1.