The nature of the companion star in Circinus X-1

TL;DR

This study analyzes optical spectra and light curves of the X-ray binary Circinus X-1, revealing a variable companion star likely affected by a recent supernova, with implications for understanding its nature and orbital dynamics.

Contribution

The paper provides the first detailed optical spectral analysis and modeling of the companion star in Circinus X-1, suggesting it is over-luminous and under-dense due to a recent supernova event.

Findings

Optical light curve shows strong variability over days.

H-alpha emission lines are variable and phase-dependent.

The companion star is likely over-luminous and under-dense.

Abstract

We present optical spectra and images of the X-ray binary Circinus X-1. The optical light curve of Cir X-1 is strongly variable, changing in brightness by 1.2 magnitudes in the space of four days. The shape of the light curve is consistent with that seen in the 1980s, when the X-ray and radio counterparts of the source were at least ten times as bright as they are currently. We detect strong, variable H-alpha emission lines, consisting of multiple components which vary with orbital phase. We estimate the extinction to the source from the strength of the diffuse interstellar bands and the Balmer decrement; the two methods give A_V = 7.6 +/- 0.6 mag and A_V > 9.1 mag respectively. The optical light curve can be modelled as arising from irradiation of the companion star by the central X-ray source, where a low-temperature star fills its Roche lobe in an orbit of moderate eccentricity (e ~ 0.4). We suggest that the companion star is over-luminous and under-dense, due to the impact of the supernova which occurred less than 5000 yr ago.