2003A&A...398.1103R

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 1x10^–7-5x10^–6Hz 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 (∼years) pattern of variability and the results of power-law fits (P(ν)∝ν^–α) to the 1x10^–7-5x10^–6Hz 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 (α≲0.9) and low variability (rms≲20%). Type II systems comprise 4U 1636-53, 4U 1735-44 and GX3+1. They are more variable (20%≲rms≲30%) and display steeper power spectra (0.9≲α≲1.2) than type I sources. Type III systems are the most variable (rms>30%) and exhibit the steepest power spectra (α>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.