EDP Sciences
Free access
Volume 398, Number 3, February II 2003
Page(s) 1103 - 1110
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20021693

A&A 398, 1103-1110 (2003)
DOI: 10.1051/0004-6361:20021693

Aperiodic variability of low-mass X-ray binaries at very low frequencies

P. Reig1, 2, I. Papadakis1, 3 and N. D. Kylafis1, 3

1  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

(Received 31 July 2002 / 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 $1 \times 10^{-7}{-}5 \times 10^{-6}$ 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 (~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 ( $\alpha
\la0.9$ ) and low variability ( $\rm rms \la20\%$). Type II systems comprise 4U 1636-53, 4U 1735-44 and GX3+1. They are more variable ( $20\% \la\rm rms \la30\%$) and display steeper power spectra ( $0.9 \la\alpha \la1.2$) than type I sources. Type III systems are the most variable ( $\rm rms > 30\%$) 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.

Key words: stars: neutron -- binaries: close -- X-rays: binaries -- accretion: accretion discs

Offprint request: P. Reig, pablo@physics.uoc.gr

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