arXiv subjects

Soma Tsuchiya

Publications and source records attributed to Soma Tsuchiya.

3 recordsLinked to original sources

arXiv2026

In-orbit Demonstration of X-ray Pulsar Navigation with NinjaSat

This study demonstrated the pulsar navigation capability of the CubeSat X-ray observatory NinjaSat, which is equipped with two Gas Multiplier Counters (GMCs). The GMCs are sensitive to the 2-50 keV energy band and have an effective area of 16 cm^2 per module at 6 keV. We verified the timing accuracy by observing the Crab Pulsar and confirmed stable timing performance within 100 microseconds. To demonstrate pulsar navigation, we applied a method that optimizes orbital parameters to maximize the significance of the pulsar X-ray pulse profile, known as the Significance Enhancement of Pulse-profile with Orbit-dynamics (SEPO) method. We observed the Crab Pulsar with a total exposure of approximately 100 ks at different epochs and analyzed the data transmitted to the ground. By comparing the optimized orbit with the satellite position derived from Global Positioning System data, we quantitatively evaluated the navigation performance. The results show that the position component along the Crab line of sight was consistently constrained within approximately 40 km, and the three-dimensional position error ranged from 27 to 370 km depending on the observation epoch. These results demonstrate the feasibility of applying a CubeSat-class X-ray observatory to pulsar navigation and provide the first experimental verification that the accuracy of the SEPO method depends on the seasonal geometry between the orbital plane and the pulsar direction.

astro-ph.IM↗

arXiv2025

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

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.

astro-ph.HE↗

arXiv2025

Return of the Clocked Burster: Exceptionally Short Recurrence Time in GS 1826-238

We report the discovery of an exceptionally short burst recurrence time in the well-known clocked burster GS 1826$-$238, observed with the CubeSat X-ray observatory NinjaSat. In 2025 May, GS 1826$-$238 underwent a soft-to-hard state transition for the first time in 10 years. On June 23, NinjaSat began monitoring GS 1826$-$238 in the hard state and continued until it returned to a steady soft state. During this period, we detected 19 X-ray bursts: 14 during the hard state, 4 in the transitional state, and 1 in the soft state. In the hard state, we identified a new clocked bursting epoch, during which the burst recurrence time remained highly stable and unprecedentedly short among the clocked bursting phases of GS 1826$-$238, with $t_{\rm rec} = 1.603 \pm 0.040$ hr ($1σ$ error). Previous observations showed that the burst recurrence time in GS 1826$-$238 decreased with increasing mass accretion rate, reached its minimum value of $t_{\rm rec} \sim 3$ hr, and then increased again. The observed 1.6 hr recurrence time is therefore exceptionally short, indicating anomalous ignition conditions. We propose that this phenomenon reflects fuel accumulation over a smaller fraction of the neutron star surface, resulting in a higher local accretion rate compared to earlier epochs. This scenario naturally accounts for the exceptionally short recurrence time, together with the observed reductions during bursts in blackbody normalization (proportional to the emitting area) and fluence. We also discuss possible contributions from residual heat in the neutron star crust or the presence of an additional soft spectral component.

astro-ph.HE↗