Echo outbursts in X-ray binaries—the case of KS 1731-260

Abstract

The thermal instability of 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 like KS 1731‐260 goes into its low state. In our interpretation, the irradiation of the donor during the main outburst lead to an increase of the mass transfer rate ṁtr, which prolonged this event; ṁtr became a significant fraction of the mass accretion rate from the disk onto the NS. After the end of the main outburst, the still continuing ṁtr via the mass stream impinging the outer disk rim gave rise to a series of outside‐in echo outbursts. We argue that introducing any bursts of matter from the donor as the cause of the echo outbursts is superfluous. We find a non‐uniform time evolution of the recurrence time of the echo outbursts, which can be explained by a variable disk viscosity. The long‐term activity of KS 1731‐260 provides a link between persistent and transient LMXBs.