Probing the accretion geometry of the transient accreting millisecond pulsar SAX J1808.4-3658: transitions to the propeller regime

TL;DR

This study uses NuSTAR and NICER data to analyze the accretion geometry of SAX J1808.4-3658, revealing disk truncation and hints of a transition to the propeller regime during outbursts.

Contribution

It provides detailed spectral analysis showing disk truncation, reflection features, and evidence for the propeller regime transition in a transient accreting millisecond pulsar.

Findings

Inner disk radius moves outward over observations.

Reflection features indicate disk ionization decreases.

Evidence of propeller regime transition with disk truncation at ~23R_g.

Abstract

We analyze three NuSTAR observations and two NICER observations of the transient accreting millisecond pulsar SAX J1808.4-3658 in the hard spectral state during its most recent outbursts in 2022 and 2025. The spectral analysis of the persistent emission shows that the continuum is well described by an absorbed thermal Comptonization model with a high plasma temperature of ~25-90 keV. A prominent iron emission line around 5-8 keV and a Compton hump around 15-30 keV have been detected from all NuSTAR observations, indicating the reflection of the hard X-ray photon from the accretion disk. We employ the relativistic reflection model relxillCP to describe the reflection phenomena. The spectral fit of three NuSTAR observations shows that the inner disk radius moves outward, the Comptonized thermal emission decreases in flux, the mass accretion rate decreases, and the disk becomes less ionized as we proceed from the 2022 to the 2025 observations. Reflection studies also reveal a moderate inclination of the source within ~30-50 degrees. During the 2025 September observation, the inner radius of the disk is significantly truncated (~23R_g), and the corresponding magnetospheric radius is comprehensively larger than the disk's co-rotation radius, suggesting a hint of the transition to the propeller regime. Although the disk is truncated at the larger radius, accreted material is still reaching the surface of the neutron star, which is confirmed through the detection of a Type-I X-ray burst during this NuSTAR observation. The spectral analysis of the burst suggests helium burning at a low ignition depth.