Issue |
A&A
Volume 398, Number 3, February II 2003
|
|
---|---|---|
Page(s) | 1103 - 1110 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20021693 | |
Published online | 28 January 2003 |
Aperiodic variability of low-mass X-ray binaries at very low frequencies
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
Corresponding author: P. Reig, pablo@physics.uoc.gr
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 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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