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

TL;DR

This study uses Insight-HXMT data to analyze how the pulse fractions of Swift J0243.6+6124 evolve during a super-Eddington outburst, revealing a luminosity-dependent transition in pulsation strength likely linked to accretion disk physics.

Contribution

First to confirm the luminosity-dependent evolution of pulse fractions in a Galactic ULX during super-Eddington accretion using Insight-HXMT data.

Findings

Pulse fraction increases with luminosity and energy at super-critical levels.

A transition luminosity marks a change in pulsation strength behavior.

The transition luminosity is energy independent.

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 $7.71\times10^{38}$~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.