---
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.'
author:
- 'Alessandro Patruno , Anna Watts, Marc Klein Wolt, Rudy Wijnands, Michiel van der Klis'
title: '1 Hz flaring in SAX J1808.4–3658: flow instabilities near the propeller stage'
---

Introduction {#1Hz:intro}
============

The low mass X-ray binary transient SAX J1808.4-3658 (hereafter J1808) was discovered with [[*Beppo*]{}-SAX]{} in 1996 [@int98] and was found to be an accreting millisecond X-ray pulsar (AMXP, @wij98) in 1998 when observed with the [*Rossi X-ray Timing Explorer*]{} ([*RXTE*]{}). Since then, four other outbursts have been observed with [*RXTE*]{}. The X-ray lightcurves of the 5 outbursts under consideration are remarkably similar in shape and duration. The typical outburst duration is several weeks 