A1 Refereed original research article in a scientific journal
Pulse profile modelling of the accretion-powered millisecond pulsar SAX J1808.4−3658 using NICER data from its 2019 and 2022 outbursts
Authors: Dorsman, Bas; Salmi, Tuomo; Watts, Anna L.; Ng, Mason; Bobrikova, Anna; Loktev, Vladislav; Poutanen, Juri; Wilms, Joern
Publication year: 2026
Journal: Monthly Notices of the Royal Astronomical Society
Article number: staf1983
Volume: 545
Issue: 2
ISSN: 0035-8711
eISSN: 1365-2966
DOI: https://doi.org/10.1093/mnras/staf1983
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.1093/mnras/staf1983
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/508947279
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
Pulse profile modelling is a relativistic ray-tracing technique that has provided constraints on parameters, with a focus on mass and radius, of five rotation-powered millisecond pulsars. While the technique can also be applied to accretion-powered millisecond pulsars (AMPs), this requires accounting for the X-rays from the accretion disc and has only been applied to archival data from the Rossi X-ray Timing Explorer. Here, we apply a previously developed neutron star and accretion disc model to the NICER (Neutron star Interior Composition Explorer) data of the 2019 and 2022 outbursts of SAX J1808.4–3658. We find that a single circular hotspot model is insufficient to explain the data. Modelling with two hotspots and an accretion disc model provides better phase-residuals, but a spectral residual at around 1 keV remains. In contrast, we find a good fit with a flexible background approach, replacing the accretion disc. However, the inferred parameters are not robust due to a degeneracy in the origin of the non-pulsed radiation, which can be caused either by the background or a hotspot that is at least partially in view throughout a full rotation. This work represents an important next step in pulse profile modelling of AMPs by analysing NICER data and underlines the need for more accurate accretion disc and hotspot modelling to achieve robust parameter constraints. We expect the inclusion of higher energy and polarimetric data will provide complementary constraints on inclination, hotspot colatitude, and hotspot size, improving the accuracy of pulse profile modelling of AMPs.
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Funding information in the publication:
BD thanks Nathan Rutherford for discussions of the EoS informed approach, Niek Bollemeier for assistance with XSPEC, and Duncan Galloway for discussions related to X-ray bursts and distance estimates. BD, TS, and ALW acknowledge support from ERC Consolidator grant No. 865768 AEONS (PI: Watts). MN is a Fonds de Recherche du Quebec-Nature et Technologies (FRQNT) postdoctoral fellow. This work was supported in part by NASA through the NICER mission. This work was sponsored by NWO Domain Science for the use of supercomputer facilities. This work used the Dutch national e-infrastructure with the support of the SURF Cooperative using grant no. EINF-5867 and is subsidized by NWO Domain Science. Part of the work was carried out on the HELIOS cluster including dedicated nodes funded via the above mentioned ERC CoG. We acknowledge extensive use of NASA's Astrophysics Data System (ADS) Bibliographic Services and the ArXiv.
