Sudden end of X-ray outbursts around periastron of Circinus X-1 observed with MAXI

Abstract

Abstract MAXI/GSC (Monitor of All-sky X-ray Image/Gas Slit Camera) observed 21 outbursts from Circinus X-1 between 2009 August and 2013 December. Although 14 outbursts showed ordinary gradual decays, in seven others we found a sudden drop in luminosity on a time scale of a few hours around the periastron, and then the outbursts terminated. These sudden drops started at the estimated luminosity of a few times 1037 erg s−1 and their luminosities dropped to ≲3 × 1036 erg s−1. We offer three interpretations of the sharp luminosity decrease: (1) the end of the outburst during its dip, (2) the propeller effect, and (3) the stripping effect by the stellar wind of the companion star. It is difficult to explain the phenomenon by any of these interpretations alone. The interpretation of (1) is possible for only two outbursts on the assumption of rapid decay. The propeller effect (2) is expected to occur at a constant luminosity, which is incompatible with the observed facts. In the stripping effect (3), the ram pressure of a typical stellar wind is not sufficient to blow most of the accretion disk off. In this paper, we discuss the possibility of a modified stripping effect by assuming other additional conditions such as wind clumping and disk instability.