Accretion torques and motion of the hot spot on the accreting millisecond pulsar XTE J1807-294

TL;DR

This paper analyzes the 2003 outburst of XTE J1807-294, revealing hot spot motion and flux influence on pulse timing, with implications for measuring neutron star spin changes.

Contribution

It provides the first detailed timing analysis of XTE J1807-294's outburst, highlighting hot spot motion and flux effects on pulse timing.

Findings

Highest observed pulsation amplitudes among accreting millisecond pulsars.

Anti-correlation between pulse residuals and flux suggests accretion rate influence.

Hot spot motion affects pulse timing, complicating spin measurements.

Abstract

We present a coherent timing analysis of the 2003 outburst of the accreting millisecond pulsar XTE J1807-294. We find an upper limit for the spin frequency derivative of 5E-14 Hz/s. The sinusoidal fractional amplitudes of the pulsations are the highest observed among the accreting millisecond pulsars and can reach values of up to 27% (2.5-30 keV). The pulse arrival time residuals of the fundamental follow a linear anti-correlation with the fractional amplitudes that suggests hot spot motion over the surface of the neutron star both in longitude and latitude. An anti-correlation between residuals and X-ray flux suggests an influence of accretion rate on pulse phase, and casts doubts on the use of standard timing techniques to measure spin frequencies and torques on the neutron star.