Abstract
The X-ray binary with the neutron star (NS), KS 1731-260, displays superorbital cycle similar to that in Her X-1. The accretion disk had the memory of the cycle-length even when this modulation sometimes disappeared in the main outburst of KS 1731-260, and during anomalous low state in Her X-1. The disk still existed during such seasons. Although irradiation of the disk by X-rays is a viable explanation for the disk precession and warping (see model of Foulkes et al.), the mechanisms which give rise to the observed X-ray modulation are quite dierent for each of these systems. Variable absorption can explain this cycle only in Her X-1. We propose a variable mass accretion rate onto the NS in KS 1731-260 due to a highly variable impact of the inflowing mass stream with the changing phase of the cycle.
Highlights
KS 1731–260 (e.g. [2, 32]) and Her X-1 (e.g. [10]) are mass exchanging X-ray binary systems, with the neutron star (NS) accreting matter from a low-mass companion
We present arguments in favor of the superorbital nature of the cycle in the main outburst of KS 1731–260
The profile of the unique optical emission plays an important role in distinguishing between the character of the active and inactive states in Her X-1
Summary
KS 1731–260 (e.g. [2, 32]) and Her X-1 (e.g. [10]) are mass exchanging X-ray binary systems, with the neutron star (NS) accreting matter from a low (or medium)-mass companion. [10]) are mass exchanging X-ray binary systems, with the neutron star (NS) accreting matter from a low (or medium)-mass companion They can be considered specific type of transient systems as regards their activity (On/Off transients in the notation of [13] (rapid rise, very long (years) lasting state of activity, rapid return to the inactive state)). Their active X-ray states were accompanied by intense brightenings by more than a magnitude in the optical [12] and near infrared bands [21, 32]. Since the observations of KS 1731– 260 in quiescence showed that the donor is of type G5V (with a contribution of a synchrotron jet) [32], it is possible that the cycle is superorbital, too, if this star fills its Roche lobe
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