X-ray coherent pulsations during a sub-luminous accretion disk state of the transitional millisecond pulsar XSS J12270-4859

TL;DR

This paper reports the first detection of X-ray coherent pulsations from the transitional millisecond pulsar XSS J12270-4859 in a sub-luminous accretion disk state, indicating matter channeled onto the neutron star's poles during low luminosity phases.

Contribution

It provides the first evidence of X-ray pulsations in a sub-luminous accretion state of a transitional millisecond pulsar, revealing details of accretion processes at low luminosity.

Findings

Detected X-ray pulsations at 7.7% rms amplitude

Pulsations observed without flaring or dipping activity

Pulsations linked to matter accretion onto neutron star poles

Abstract

We present the first detection of X-ray coherent pulsations from the transitional millisecond pulsar XSS J12270-4859, while it was in a sub-luminous accretion disk state characterized by a 0.5-10 keV luminosity of 5E33 erg/s (assuming a distance of 1.4 kpc). Pulsations were observed by XMM-Newton at an rms amplitude of (7.7 +/- 0.5)% with a second harmonic stronger than the the fundamental frequency, and were detected when the source is neither flaring nor dipping. The most likely interpretation of this detection is that matter from the accretion disk was channelled by the neutron star magnetosphere and accreted onto its polar caps. According to standard disk accretion theory, for pulsations to be observed the mass in-flow rate in the disk was likely larger than the amount of plasma actually reaching the neutron star surface; an outflow launched by the fast rotating magnetosphere then probably took place, in agreement with the observed broad-band spectral energy distribution. We also report about the non-detection of X-ray pulsations during a recent observation performed while the source behaved as a rotationally-powered radio pulsar.