Searching for the highest energy of pulsation and critical luminosity of Swift J0243.6+6124 observed by Insight-HXMT

TL;DR

This study used Insight-HXMT to detect the highest pulsation energy exceeding 200 keV from the PULX Swift J0243.6+6124, revealing correlations with luminosity and identifying a critical luminosity separating accretion regimes.

Contribution

The paper introduces a new method to determine the critical luminosity and provides the highest energy pulsation measurements for this PULX, advancing understanding of accretion regimes.

Findings

Detected pulsation energy >200 keV, the highest for PULXs.

Found positive correlation between pulsation energy and luminosity.

Identified critical luminosity at 4×10^38 erg/s separating accretion regimes.

Abstract

Owing to the broad energy coverage of Insight-HXMT in the hard X-ray band, we detected the highest energy of pulsation exceeding 200 keV around the 2017-2018 outburst peak of the first Galactic pulsating ultraluminous X-ray source (PULX) Swift J0243.6+6124, which is the highest energy detected from PULXs to date. We also obtained the highest energy of pulsation of every exposure during the outburst in 2017-2018, and found the highest energy is roughly positively correlated with luminosity. Using our newly developed method, we identified the critical luminosity being $4\times 10^{38}\, \rm erg\,s^{-1}$ when the main peaks of the low and high energy pulse profiles became aligned, which separates the fan-beam dominated and pencil-beam dominated accretion regimes. Above the critical luminosity, the phase of the main peak shifted gradually from 0.5 to 0.8 until the outburst peak in all energy bands is reached, which is in agreement with the phase shift found previously at low energies. Our result is consistent with what is derived from spectral analysis.