Meta-analysis of electron cyclotron resonance absorption features detected in high-mass X-ray binaries

Using recent compilations of detailed X-ray observations and spectral models of exceptional quality, we record the electron cyclotron resonance absorption (ECRA) features that have been detected in 45 pulsating high-mass X-ray binaries (HMXBs) and ultraluminous X-ray (ULX) sources harboring neutron stars, although seven of these detections are still questionable and another 21 are single and/or not independently confirmed. From the comprehensive catalogs of Jaisawal & Naik and Staubert et al. and from several additional recent observations, we produce two lists of HMXB ECRA sources: a list of 17 sources in which multiple ECRA lines or single very low-energy lines are seen, in which we can reasonably assume that the lowest energy reveals the fundamental cyclotron level for each source; and a contaminated list of 38 sources including the 21 detections of single ECRA lines that may (not) be higherlevel harmonics. Both lists confirm a previous result that we have obtained independently by modeling the propeller lines of Magellanic HMXB pulsars: the surface dipolar magnetic fields B of HMXB neutron stars are segregated around five distinct values with B = 0.28 +/- 0.08, 0.55 +/- 0.11, 1.3 +/- 0.37, 3.0 +/- 0.68 and 7.9 +/- 3.1, in units of TG. However, an explanation of this phenomenon is currently lacking. We have found no correlation between these B values and the corresponding observed spin periods, spin period derivatives, orbital periods, maximum X-ray luminosities, neutron star masses or companion star masses.