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

TL;DR

This study uses NinjaSat CubeSat observations to analyze the superorbital modulation of SMC X-1, revealing that the modulation is mainly geometric, with stable spectral properties and variable pulse profiles, demonstrating CubeSat's scientific potential.

Contribution

First complete measurement of spin-up rate and spectral evolution across a superorbital cycle in SMC X-1 using CubeSat data, indicating geometric obscuration as the primary modulation mechanism.

Findings

Spin-up rate remains consistent during high states.

Spectral shape is stable throughout the cycle.

Pulse profile varies with superorbital phase.

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 non-stationary superorbital modulation with periods ranging from approximately 40 to 65 days. Its peak luminosity of $1.3\times10^{39}$~\lumcgs\ makes it a local analogue of ultraluminous X-ray pulsars powered by supercritical accretion. We find that the spin-up rate during the high state remains consistent with the long-term average, with no significant correlation between spin-up 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 variable covering fraction due to geometric obscuration, or by free precession of the neutron star. This represents the first complete measurement of spin-up rate and spectral evolution across a single superorbital cycle in SMC X-1, highlighting the scientific capability of CubeSat-based observatories.