X-ray states of redback millisecond pulsars

TL;DR

This study systematically analyzes X-ray states of eight redback millisecond pulsars, revealing universal mode switching in the disk state and insights into accretion processes and magnetospheric interactions.

Contribution

It provides the first comprehensive analysis of X-ray mode switching across multiple redback pulsars, establishing its universality in the disk state.

Findings

Mode switching observed in two redbacks' disk states.

Magnetospheric radius lies outside the light cylinder.

High X-ray luminosity in pulsar state correlates with disk development.

Abstract

Compact binary millisecond pulsars with main-sequence donors, often referred to as "redbacks", constitute the long-sought link between low-mass X-ray binaries and millisecond radio pulsars, and offer a unique probe of the interaction between pulsar winds and accretion flows. We present a systematic study of eight nearby redbacks, using more than 100 observations obtained with Swift's X-ray Telescope. We distinguish between three main states: pulsar, disk and outburst states. We find X-ray mode switching in the disk state of PSR J1023+0038 and XSS J12270-4859, similar to what was found in the other redback which showed evidence for accretion: rapid, recurrent changes in X-ray luminosity (0.5-10 keV, L$_\mathrm{X}$), between [6-9]$\times$10$^{32}$ erg s$^{-1}$ (disk-passive state) and [3-5]$\times$10$^{33}$ erg s$^{-1}$ (disk-active state). This strongly suggests that mode switching $-$which has not been observed in quiescent low-mass X-ray binaries$-$ is universal among redback millisecond pulsars in the disk state. We briefly explore the implications for accretion disk truncation, and find that the inferred magnetospheric radius in the disk state of PSR J1023+0038 and XSS J12270-4859 lies outside the light cylinder. Finally, we note that all three redbacks which have developed accretion disks have relatively high L$_\mathrm{X}$ in the pulsar state ($>$10$^{32}$ erg s$^{-1}$).