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

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.

22 pages, 7 figures, ApJ accepted. Two figures removed, one typo in the Timing parameters corrected (Table 2)

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• The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440
• NICER Discovery of Millisecond X-ray Pulsations and an Ultracompact Orbit in IGR J17494-3030
• A broadband look of the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 using AstroSat and XMM-Newton
• The INTEGRAL view of the pulsating hard X-ray sky: from accreting and transitional millisecond pulsars to rotation-powered pulsars and magnetars
• The 2015 outburst of the accreting millisecond pulsar IGR J17511-3057 as seen by INTEGRAL, Swift and XMM-Newton
• The AstroSat observation of accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst
• Timing of the accreting millisecond pulsar IGR J17591-2342: evidence of spin-down during accretion
• Broadband X-ray timing and spectral characteristics of the accretion-powered millisecond X-ray pulsar MAXI J1816$-$195
• Broad-band X-ray spectral and timing properties of the accreting millisecond X-ray pulsar IGR J17498$-$2921 during the 2023 outburst
• Updated Spin and Orbital Parameters and Energy Dependent Pulse Behaviors of the Accreting Millisecond X-ray Pulsar IGR J17591$-$2342
• An apparent positive relation between spin and orbital angular momentum in X-ray binaries
• Continuous gravitational waves from thermal mountains on accreting neutron stars: effect of the nuclear pasta phase
• An Elaborate Search for Coherent Pulsations from Intermittent-AMXPs
• The 2025 outburst of IGR J17511-3057: timing and spectral insights from NICER and NuSTAR