Sudden End of X-Ray Outbursts around Periastron of Circinus X-1 Observed with MAXI

TL;DR

This study reports on 21 X-ray outbursts from Circinus X-1 observed by MAXI, highlighting 7 with sudden luminosity drops near periastron and exploring potential physical mechanisms behind these abrupt terminations.

Contribution

The paper presents detailed observations of sudden outburst terminations and proposes modified wind stripping effects as a plausible explanation, advancing understanding of accretion dynamics in X-ray binaries.

Findings

7 outbursts showed sudden luminosity drops near periastron

Sudden decreases started at a few times 10^{37} erg s^{-1} and dropped below 3×10^{36} erg s^{-1}

Modified wind stripping effects may explain the phenomena

Abstract

MAXI/GSC observed 21 outbursts from Circinus X-1 between 2009 August and 2013 December. Although 14 outbursts showed ordinary gradual decays, in 7 outbursts we found sudden luminosity decrease in a time scale of a few hours around the periastron, and then the outbursts terminated. These sudden decreases started at the estimated luminosity of a few times $10^{37}$ erg s$^{-1}$ and reached to $\lesssim3\times10^{36}$ erg s$^{-1}$. We propose three interpretations for the sudden luminosity decrease: (1) the end of the outburst during the 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 with any of these interpretations alone. The interpretation of (1) is possible for only two outbursts assuming rapid decay. The propeller effect (2) is expected to occur at a constant luminosity, which is incompatible with the observed facts. In wind stripping effect (3), the ram pressure of a typical stellar wind is not sufficient to blow out most of the accretion disk. In this paper, we discuss a possibility of a modified effect of (3) assuming other additional conditions such as wind clumping and disk instability.