1 Hz flaring in SAX J1808.4--3658: flow instabilities near the propeller stage

TL;DR

This study investigates 1 Hz flares in SAX J1808.4-3658, linking them to disk instabilities near the propeller stage, based on systematic timing analysis of multiple outbursts.

Contribution

First comprehensive analysis of 1 Hz flares in SAX J1808.4-3658, proposing their origin in hydrodynamic instabilities at the disk-magnetosphere boundary.

Findings

Possible link between pulsations and 1 Hz flares.

Flares triggered by hydrodynamic disk instabilities.

Flares occur near the transition to the propeller regime.

Abstract

We present a simultaneous periodic and aperiodic timing study of the accreting millisecond X-ray pulsar SAX J1808.4-3658. We analyze five outbursts of the source and for the first time provide a full and systematic investigation of the enigmatic phenomenon of the 1 Hz flares observed during the final stages of some of the outbursts. We show that links between pulsations and 1 Hz flares might exist, and suggest they are related with hydrodynamic disk instabilities that are triggered close to the disk-magnetosphere boundary layer when the system is entering the propeller regime.