Discovery of a New Spectral Transition in Swift J0243.6+6124 in the Sub-Eddington Regime

TL;DR

This study identifies a new spectral transition in the ultraluminous X-ray pulsar Swift J0243.6+6124 during sub-Eddington accretion, revealing complex magnetic field configurations and spectral evolution through detailed analysis of multiple outbursts.

Contribution

It reports the discovery of a fifth transition in Swift J0243.6+6124's spectral behavior, linking it to multipolar magnetic fields and advancing understanding of accretion physics in neutron stars.

Findings

Discovery of a new spectral transition at L_t ≈ 4.5×10^37 erg/s.

Spectral parameters, especially blackbody normalization, show systematic evolution.

Magnetic field configuration involves multipolar poles with different strengths.

Abstract

We conduct a detailed spectral analysis of the Galactic ultraluminous X-ray pulsar Swift J0243.6+6124 in its sub-Eddington regime, using Insight-HXMT and NICER observations during multiple outbursts including the 2018 giant outburst. We discover a new transition at $L_{\rm t} \approx 4.5 \times 10^{37}\ {\rm erg\ s^{-1}}$, accompanied by systematic evolution of spectral parameters, in particular a significant turnover in the blackbody normalization. This transition luminosity in the sub-Eddington regime represents the fifth transition identified so far in Swift J0243.6+6124, further highlighting the complexity of its accretion-powered emission. We interpret the transition in terms of a multipolar magnetic-field configuration, where weak ($\sim 2.8 \times 10^{12}\ {\rm G}$) and strong ($\sim 1.6 \times 10^{13}\ {\rm G}$) magnetic poles dominate the emission at different accretion rates. On the magnetospheric scale, this configuration is equivalent to an effective dipole field of $\sim 6.6 \times 10^{12}\ {\rm G}$, while allowing the local surface field to exceed $10^{13}\ {\rm G}$.