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

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.

Keywords: accretion; accretion discs—stars; binaries; magnetic fields—stars; neutron—X-rays.