Volume 398, Number 3, February II 2003
|Page(s)||1103 - 1110|
|Section||Stellar structure and evolution|
|Published online||28 January 2003|
Aperiodic variability of low-mass X-ray binaries at very low frequencies
Foundation for Research and Technology-Hellas, 711 10 Heraklion, Crete, Greece
2 G.A.C.E, Departament d'Astronomia i Astrofísica, Universitat de Valéncia, 46071 Paterna-Valencia, Spain
3 Physics Department, University of Crete, PO Box 2208, 710 03 Heraklion, Crete, Greece
Corresponding author: P. Reig, email@example.com
Accepted: 13 November 2002
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 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 (~years) pattern of variability and the results of power-law fits () to the 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 () and low variability (). Type II systems comprise 4U 1636–53, 4U 1735–44 and GX3+1. They are more variable () and display steeper power spectra () than type I sources. Type III systems are the most variable () and exhibit the steepest power spectra (). 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.
Key words: stars: neutron / binaries: close / X-rays: binaries / accretion: accretion discs
© ESO, 2003
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