X-ray variability of transitional millisecond pulsars: a faint, stable, and fluctuating disc

Transitional millisecond pulsars (tMSPs) have emerged in the last decade as a unique class of neutron stars at the crossroads between accretion- and rotation-powered phenomena. In their (sub-luminous) accretion disc state, with X-ray luminosities of order 10³³-10³⁴ erg s^-1, they switch rapidly between two distinct X-ray modes: the disc-high (DH) and disc-low (DL) states. We present a systematic XMM-Newton and Chandra analysis of the aperiodic X-ray variability of all three currently known tMSPs, with a main focus on their disc state and separating DH and DL modes. We report the discovery of flat-topped broad-band noise in the DH state of two of them, with break frequencies of 2.8 mHz (PSR J1023 + 0038) and 0.86 mHz (M28-I). We argue that the lowest frequency variability is similar to that seen in disc-accreting X-ray binaries in the hard state, at typical luminosities at least two orders of magnitude higher than tMSPs. We find strong variability in the DH state around 1 Hz, not typical of hard state X-ray binaries, with fractional rms amplitudes close to 30 per cent. We discuss our results and use them to constrain the properties of the accretion disc, assuming that the X-ray variability is produced by fluctuations in mass accretion rate, and that the break frequency corresponds to the viscous time-scale at the inner edge of the disc. In this context, we find that the newly found break frequencies are broadly consistent with a disc truncated close to the light cylinder with $\dot{M}\simeq 10^{13}-5\times 10^{14}$ g s^-1 and a viscosity parameter α ≳ 0.2.