Accretion in the spin-down regime

TL;DR

This paper proposes that thick accretion disks around millisecond X-ray pulsars enable matter to accrete even during spin-down phases, explaining sustained pulsations and brightness across various mass flow rates.

Contribution

It introduces a model where thick disk regions allow accretion in the spin-down regime, accounting for observed pulsations at low accretion rates.

Findings

Fractional accretion explains persistent pulsations.

Model fits the lightcurve of SAX J1808.4-3658 during outburst.

Rapid decay stage modeled by spin-down regime accretion.

Abstract

Accretion driven millisecond X-ray pulsars can accrete over a wide range of mass flow rates. The pulsations persist even at small accretion rates at which these objects would be expected to be in the propeller stage. We argue that the inner regions of disks around millisecond X-ray pulsars are sufficiently thick that a fraction of the inflowing matter can accrete even in the spin-down regime from regions of the disk away from the disk plane. This allows these systems to be bright throughout a wide range of mass flow rates. We model the lightcurve of SAX J1808.4-3658 during an outburst and show that the rapid decay stage can be modeled with fractional accretion in the spin-down regime.