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In this paper we present a coherent timing analysis of the 401 Hz pulsationsof the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019outburst. Using observations collected with the Neutron Star InteriorComposition Explorer (NICER), we establish the pulsar spin frequency andorbital phase during its latest epoch. We find that the 2019 outburst shows apronounced evolution in pulse phase over the course of the outburst. Thesephase shifts are found to correlate with the source flux, and are interpretedin terms of hot-spot drift on the stellar surface, driven by changes in themass accretion rate. Additionally, we find that the long-term evolution of thepulsar spin frequency shows evidence for a modulation at the Earth's orbitalperiod, enabling pulsar timing based astrometry of this accreting millisecondpulsar.

Timing the pulsations of the accreting millisecond pulsar SAX J1808.4-3658 during its 2019 outburst