X-ray states of redback millisecond pulsars

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${}^{-1}$).

Accepted for publication in The Astrophysical Journal. Minor revision after referee's comments, including a brief discussion of the different variability seen in SAX J1808.4-3658. 10 pages, 5 figures, 1 table

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