Retrograde Accretion Disks in High-Mass Be/X-ray Binaries

TL;DR

This study analyzes X-ray data of Be/X-ray binaries in the Small Magellanic Cloud, revealing that pulsars spinning down are likely accreting from retrograde disks, suggesting a new evolutionary model.

Contribution

It provides the first comprehensive evidence that both prograde and retrograde accretion disks exist in Be/X-ray pulsars, challenging previous assumptions about their spin evolution.

Findings

Spin-down and spin-up pulsars share the same population distribution.

Retrograde disks are present in roughly equal numbers to prograde disks.

A new evolutionary scenario for Be/X-ray pulsars is proposed.

Abstract

We have compiled a comprehensive library of all X-ray observations of Magellanic pulsars carried out by {\it XMM-Newton}, {\it Chandra}, and {\it RXTE} in the period 1997-2014. In this work, we use the data from 53 high-mass Be/X-ray binaries in the Small Magellanic Cloud to demonstrate that the distribution of spin-period derivatives vs. spin periods of spinning-down pulsars is not at all different than that of the accreting spinning-up pulsars. The inescapable conclusion is that the up and down samples were drawn from the same continuous parent population, therefore Be/X-ray pulsars that are spinning down over periods spanning 18 years are in fact accreting from retrograde disks. The presence of prograde and retrograde disks in roughly equal numbers supports a new evolutionary scenario for Be/X-ray pulsars in their spin period-period derivative diagram.