Phase-coherent timing of the accreting millisecond pulsar SAX J1748.9-2021

TL;DR

This paper presents a detailed phase-coherent timing analysis of the intermittent accreting millisecond pulsar SAX J1748.9-2021, revealing complex timing behavior, pulse profile characteristics, and energy dependence, while ruling out a direct link to Type I X-ray bursts.

Contribution

The study provides the first phase-coherent timing solution for SAX J1748.9-2021, characterizes its pulse profiles and energy dependence, and investigates the causes of its intermittency.

Findings

Timing solution shows erratic behavior due to timing noise.

Pulse profiles are sinusoidal with fractional amplitude increasing with energy.

No direct correlation between Type I X-ray bursts and pulsation appearance.

Abstract

We present a phase-coherent timing analysis of the intermittent accreting millisecond pulsar SAX J1748.9-2021. A new timing solution for the pulsar spin period and the Keplerian binary orbital parameters was achieved by phase connecting all episodes of intermittent pulsations visible during the 2001 outburst. We investigate the pulse profile shapes, their energy dependence and the possible influence of Type I X-ray bursts on the time of arrival and fractional amplitude of the pulsations. We find that the timing solution of SAX J1748.9-2021 shows an erratic behavior when selecting different subsets of data, that is related to substantial timing noise in the timing post-fit residuals. The pulse profiles are very sinusoidal and their fractional amplitude increases linearly with energy and no second harmonic is detected. The reason why this pulsar is intermittent is still unknown but we can rule out a one-to-one correspondence between Type I X-ray bursts and the appearance of the pulsations.