Issue |
A&A
Volume 405, Number 1, July I 2003
|
|
---|---|---|
Page(s) | 199 - 206 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20030514 | |
Published online | 16 June 2003 |
Long-term X-ray activity of the ultra-compact binary 4U 1820–30*
Astronomical Institute, Academy of Sciences of the Czech Republic, 251 65 Ondřejov, Czech Republic
Corresponding author: simon@asu.cas.cz
Received:
15
May
2002
Accepted:
11
March
2003
An analysis of the long-term X-ray activity of the ultra-compact binary
4U 1820–30 within the years 1996–2002, using the data, is
presented. The X-ray light curve displays complicated large-amplitude
variations, even on short time scales of a few days. Episodes of brief low
states (BLS), lasting for several days, are superposed on the general course
of the cycle, the length of which we determined to be
days by the method of the O–C residuals. We found no significant secular
trend in T C; this is consistent with the stable period found over
longer time intervals by Chou & Grindlay ([CITE]) and is thus consistent with
the triple model. The statistical distribution of 1.5–12 keV intensity,
Isum, for all data is not far from Gaussian although some deviations
exist. The Isum curve, folded with the 172 day cycle and smoothed,
shows a rapid rise, double-peaked maximum, and a slow decline. The course of
the smoothed residuals of this curve, the so called
curve,
is more complicated than the Isum curve, with several minima and
maxima. Two sharp maxima of
coincide with the rising
branch and the dip after the maximum of the Isum curve. We have
shown that the scatter of the folded Isum curve is dominated by
the BLS. The activity is discussed in terms of several models. Thermal
instability of the disk can be excluded. The increases of the mass transfer
from the donor star are likely to explain the 172 day cycle of activity.
However, we argue that the highly asymmetric X-ray light curve of this cycle,
with the long and complicated profile of the decay branch, cannot be
explained purely by the influence of a third body via the Mazeh & Shaham
([CITE]) mechanism. Instead, a hybrid model in which the irradiation-driven
instability of the donor star governs the complicated profile of the decay
with the superposed BLS appears to be more viable.
Key words: stars: neutron / stars: binaries: close / stars: circumstellar matter / stars: individual: 4U 1820–30
© ESO, 2003
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