An alternative interpretation of the timing noise in accreting millisecond pulsars

TL;DR

This paper proposes that the observed timing noise in accreting millisecond pulsars is largely due to flux-correlated pulse phase offsets caused by accretion rate variations affecting hot spot locations, which biases spin frequency measurements.

Contribution

It introduces a new interpretation linking flux variations to timing noise, challenging previous spin change assumptions in AMXPs.

Findings

Timing noise correlates with X-ray flux changes.

Revised spin frequencies for six AMXPs are presented.

Hot spot location shifts depend on accretion rate.

Abstract

The measurement of the spin frequency in accreting millisecond X-ray pulsars (AMXPs) is strongly affected by the presence of an unmodeled component in the pulse arrival times called 'timing noise'. We show that it is possible to attribute much of this timing noise to a pulse phase offset that varies in correlation with X-ray flux, such that noise in flux translates into timing noise. This could explain many of the pulse frequency variations previously interpreted in terms of true spin up or spin down, and would bias measured spin frequencies. Spin frequencies improved under this hypothesis are reported for six AMXPs. The effect would most easily be accounted for by an accretion rate dependent hot spot location.