Unveiling the Nature of Superorbital Modulation of SMC X-1 Using NinjaSat

Abstract We report a long-term, high-cadence timing and spectral observation of the X-ray pulsar SMC X-1 using NinjaSat—a 6U CubeSat in low-Earth orbit—covering nearly a full superorbital cycle. SMC X-1 is a high-mass X-ray binary exhibiting a 0.7 s X-ray pulsar and a nonstationary superorbital modulation with periods ranging from approximately 40 to 65 days. Its peak luminosity of 1.3 × 10 39 erg s −1 makes it a local analog of ultraluminous X-ray pulsars powered by supercritical accretion. We find that the spinup rate during the high state remains consistent with the long-term average, with no significant correlation between the spinup rate and flux. This result indicates that the modulation is primarily geometric rather than accretion-driven. The hardness ratio and spectral shape are stable throughout the entire superorbital cycle, supporting obscuration by optically thick material or energy-independent scattering. In addition, the 2–20 keV pulse profile varies with superorbital phase, which may be explained either by a variable covering fraction, due to geometric obscuration, or by free precession of the neutron star. This represents the first complete measurement of the spinup rate and spectral evolution across a single superorbital cycle in SMC X-1, highlighting the scientific capability of CubeSat-based observatories.