Echo outbursts in KS 1731-260

Details Echo outbursts in KS 1731-260 Echo outbursts in KS 1731-260

Apr 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010a%26a...513a..71s&link_type=abstract

Astronomy and Astrophysics, Volume 513, id.A71

Astronomy and Astrophysics

Astrophysics

1

Stars: Neutron, Accretion, Accretion Disks, Novae, Cataclysmic, Variables, Circumstellar Matter, X-Rays: Binaries, Stars: Individual: Ks 1731-260

Scientific paper

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 dot m_tr 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 dot m_tr decreased again. Only a slow decrease of dot m_tr 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 dot m_tr 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 dot m_tr. 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.

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