decreasing luminosity (and thus decreasing mass accretion rate).

In this paper, we present the first comprehensive analysis of the 1 Hz modulation and its relation to the re-flares and 401 Hz pulsations in all outbursts of J1808. We discuss possible explanations for the onset of the 1 Hz modulation and possible reasons why it has been observed only in J1808 until now. We suggest the onset of accretion flow instabilities when J1808 enters the propeller stage as the origin of the 1 Hz modulation.

X-ray observations and data reduction
=====================================

[[*RXTE*]{}]{} observations
---------------------------

We reduced all the pointed observations with the [*[RXTE]{}*]{} satellite’s Proportional Counter Array (PCA, Jahoda et al. 2006) that cover the outbursts of J1808.

The aperiodic timing analysis was done using GoodXenon data with a time resolution of $2^{-20}$s and Event data with a time resolution of $2^{-13}$s. The data were binned into 1/8192 s bins including all 256 energy channels. We performed fast Fourier transforms (FFTs) of 128 s data segments, fixing the frequency resolution and the lowest available frequency to $1/128$ Hz; the highest available Fourier frequency (Nyquist frequency) was 4096 Hz. No background subtraction or dead-time correction was made prior to the FFTs. The 