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

TL;DR

This study presents a detailed timing analysis of the 2019 outburst of SAX J1808.4-3658, revealing pulse phase evolution linked to accretion rate changes and enabling precise astrometry through observed spin frequency modulation.

Contribution

First coherent timing analysis of SAX J1808.4-3658 during its 2019 outburst using NICER data, linking pulse phase shifts to hot-spot drift and Earth's orbital modulation.

Findings

Pulse phase shifts correlate with source flux.

Hot-spot drift explains phase evolution.

Detected modulation in spin frequency at Earth's orbital period.

Abstract

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