Insight-HXMT observations of Swift J0243.6+6124: the evolution of RMS pulse fractions at super-Eddington luminosity

Abstract

Based on Insight-HXMT data, we report on the pulse fraction evolution during the 2017-2018 outburst of the newly discovered first Galactic ultraluminous X-ray source (ULX) Swift J0243.6+6124. The pulse fractions of 19 observation pairs selected in the rising and fading phases with similar luminosity are investigated. The results show a general trend of the pulse fraction increasing with luminosity and energy at super-critical luminosity. However, the relative strength of the pulsation between each pair evolves strongly with luminosity. The pulse fraction in the rising phase is larger at luminosity below $7.71\times 10^{38}$~erg~s${}^{-1}$, but smaller at above. A transition luminosity is found to be energy independent. Such a phenomena is firstly confirmed by Insight-HXMT observations and we speculate it may have relation with the radiation pressure dominated accretion disk.

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• Beam pattern evolution of accreting X-ray pulsar 1A 0535+262 during its 2020 giant outburst
• Probing spectral and timing properties of the X-ray pulsar RX J0440.9+4431 in the giant outburst of 2022-2023
• X-Ray Views of Galactic Accreting Pulsars in High-Mass X-Ray Binaries
• Timing analysis of Swift J0243.6+6124 with NICER and Fermi/GBM during the decay phase of the 2017-2018 outburst
• X-ray spectral-timing variability of 1A 0535+262 during the 2020 giant outburst
• Searching for the highest energy of pulsation and critical luminosity of Swift J0243.6+6124 observed by Insight-HXMT
• Studying magnetic fields of ultraluminous X-ray pulsars using different accretion torques