Echo outbursts in KS 1731–260

Astronomical Institute, Academy of Sciences of the Czech Republic, 251 65 Ondřejov, Czech Republic e-mail: simon@asu.cas.cz

Received: 10 March 2009
Accepted: 17 December 2009

Abstract

Context. The thermal-viscous instability of the accretion disk is thought to play an important role in the activity of low-mass X-ray binaries (LMXBs). It should also appear when a (quasi)persistent LMXB goes into its low state.

Aims. We analyze the long-term X-ray activity of the neutron star LMXB KS 1731–260 during a part of its long main outburst and a complicated transition back to the low state (quiescence). We pay special attention to the variations during the state transition.

Methods. We make use of the ASM/RXTE observations for a timeseries analysis of the longterm variations and investigation of the X-ray color changes in the 1.5–12 keV passband.

Results. In our interpretation, the mass transfer rate between the donor and neutron star significantly increased during the main outburst of KS 1731–260, which prolonged this event by keeping the disk ionized to its outer rim. Later on, a considerable fraction of the outer part of the disk was rapidly brought out of steady-state when decreased again. Only a slow decrease of continued even after the end of the main outburst. This lead to the formation of the central disk region, which was kept thermally stable, ionized by irradiation, and was surrounded by the outer, thermally unstable disk region. This configuration triggered the series of the echo outbursts. These outbursts are outside-in, caused by the still continuing via the mass stream impinging the outer rim of the disk without overflow. We argue that introducing any strong bursts of matter from the donor to the disk as the cause of the echo outbursts is superfluous. The spectral evolution suggests that the disk returns to steady-state during the maxima of the echo outbursts. The X-ray emission between the echo outbursts is much higher than in the true quiescence and can be due to the ongoing . We place the peculiar long-term activity of KS 1731–260 in the context of the systems with the echo outbursts, both soft X-ray transients and dwarf novae.