A Striking Confluence Between Theory and Observations of High-Mass X-ray Binary Pulsars

TL;DR

This study analyzes high-mass X-ray binary pulsars and ultraluminous X-ray sources, finding a strong correlation between observed outbursts and theoretical propeller line models, confirming the role of magnetic fields in their emission states.

Contribution

It provides the first comprehensive comparison of observed X-ray outbursts with theoretical propeller lines, confirming the magnetic field strength variations among pulsars.

Findings

Most outbursts reach the Eddington luminosity level.

Sources align with predicted propeller lines based on magnetic fields.

Higher-lying pulsars have stronger surface magnetic fields.

Abstract

We analyse the most powerful X-ray outbursts from neutron stars in ten Magellanic high-mass X-ray binaries and three pulsating ultraluminous X-ray sources. Most of the outbursts rise to $L_{max}$ which is about the level of the Eddington luminosity, while the rest and more powerful outbursts also appear to recognize that limit when their emissions are assumed to be anisotropic and beamed toward our direction. We use the measurements of pulsar spin periods $P_S$ and their derivatives $\dot{P_S}$ to calculate the X-ray luminosities $L_p$ in their faintest accreting ("propeller") states. In four cases with unknown $\dot{P_S}$, we use the lowest observed X-ray luminosities, which only adds to the heterogeneity of the sample. Then we calculate the ratios $L_p/L_{max}$ and we obtain an outstanding confluence of theory and observations from which we conclude that work done on both fronts is accurate and the results are trustworthy: sources known to reside on the lowest Magellanic propeller line are all located on/near that line, whereas other sources jump higher and reach higher-lying propeller lines. These jumps can be interpreted in only one way, higher-lying pulsars have stronger surface magnetic fields in agreement with empirical results in which $\dot{P_S}$ and $L_p$ values were not used.