We have obtained discrete Fourier power spectra of a sample of persistent low-mass neutron-star X-ray binaries using long-term light curves from the All Sky Monitor on board the Rossi X-ray Timing Explorer .
Our aim is to investigate their aperiodic variability at frequencies in the range 1 \times 10 ^ { -7 } -5 \times 10 ^ { -6 } Hz and compare their properties with those of the black-hole source Cyg X–1 .
We find that the classification scheme that divides LMXBs into Z and atoll sources blurs at very low frequencies .
Based on the long-term ( \sim years ) pattern of variability and the results of power-law fits ( P ( \nu ) \propto \nu ^ { - \alpha } ) to the 1 \times 10 ^ { -7 } -5 \times 10 ^ { -6 } Hz power density spectra , low-mass neutron-star binaries fall into three categories .
Type I includes all Z sources , except Cyg X–2 , and the atoll sources GX9+1 and GX13+1 .
They show relatively flat power spectra ( \alpha \mathbin { \lower 3.0 pt \hbox { $ \hbox to 0.0 pt { \raise 5.0 pt \hbox { $ \char 60 $ } % } \mathchar 29208 $ } } 0.9 ) and low variability ( rms \mathbin { \lower 3.0 pt \hbox { $ \hbox to 0.0 pt { \raise 5.0 pt \hbox { $ \char 60 $ } } % \mathchar 29208 $ } } 20 % ) .
Type II systems comprise 4U 1636–53 , 4U 1735–44 and GX3+1 .
They are more variable ( 20 % \mathbin { \lower 3.0 pt \hbox { $ \hbox to 0.0 pt { \raise 5.0 pt \hbox { $ \char 60 $ } } % \mathchar 29208 $ } } rms \mathbin { \lower 3.0 pt \hbox { $ \hbox to 0.0 pt { \raise 5.0 pt \hbox { $ \char 60 $ } } % \mathchar 29208 $ } } 30 % ) and display steeper power spectra ( 0.9 \mathbin { \lower 3.0 pt \hbox { $ \hbox to 0.0 pt { \raise 5.0 pt \hbox { $ \char 60 $ } } % \mathchar 29208 $ } } \alpha \mathbin { \lower 3.0 pt \hbox { $ \hbox to 0.0 pt { \raise 5.0 % pt \hbox { $ \char 60 $ } } \mathchar 29208 $ } } 1.2 ) than Type I sources .
Type III systems are the most variable ( rms > 30 % ) and exhibit the steepest power spectra ( \alpha > 1.2 ) .
The sources 4U 1705–44 , GX354–0 and 4U 1820–30 belong to this group .
GX9+9 and Cyg X–2 appear as intermediate systems in between Type I and II and Type II and III sources , respectively .
We speculate that the differences in these systems may be caused by the presence of different types of mass-donor companions .
Other factors , like the size of the accretion disc and/or the presence of weak magnetic fields , are also expected to affect their low-frequency X-ray aperiodic varibility .