Hot disk of the Swift J0243.6+6124 revealed by Insight-HXMT

TL;DR

This study reports the first direct observation of a radiation pressure dominated accretion disk transition in a magnetized neutron star during a giant outburst, revealing new insights into ultra-luminous X-ray pulsars.

Contribution

It provides the first direct evidence of a disk transition to a radiation pressure dominated state in a magnetized neutron star, linking it to weak magnetic fields in ULX pulsars.

Findings

Detected a sharp state transition at super-Eddington luminosity.

Associated the transition with a radiation pressure dominated accretion disk.

Ruled out magnetar-like fields based on non-detection of propeller state.

Abstract

We report on analysis of observations of the bright transient X-ray pulsar \src obtained during its 2017-2018 giant outburst with Insight-HXMT, \emph{NuSTAR}, and \textit{Swift} observatories. We focus on the discovery of a sharp state transition of the timing and spectral properties of the source at super-Eddington accretion rates, which we associate with the transition of the accretion disk to a radiation pressure dominated (RPD) state, the first ever directly observed for magnetized neutron star. This transition occurs at slightly higher luminosity compared to already reported transition of the source from sub- to super-critical accretion regime associate with onset of an accretion column. We argue that this scenario can only be realized for comparatively weakly magnetized neutron star, not dissimilar to other ultra-luminous X-ray pulsars (ULPs), which accrete at similar rates. Further evidence for this conclusion is provided by the non-detection of the transition to the propeller state in quiescence which strongly implies compact magnetosphere and thus rules out magnetar-like fields.