Optical Multicolor WBVR-Observations of the X-Ray Star V1341 Cyg = Cyg X-2 in 1986-1992

TL;DR

This study presents extensive multicolor optical observations of the X-ray binary V1341 Cyg (Cyg X-2) over several years, analyzing its variability, accretion disk activity, and stellar components through detailed photometry and comparison with X-ray data.

Contribution

First comprehensive multicolor optical monitoring of V1341 Cyg over multiple years, linking optical variability with X-ray activity and clarifying the nature of the optical star.

Findings

Ellipsoidal brightness variations due to gravitational distortion.

No evidence of eclipses during observed phases.

Optical star identified as a red giant, not a blue straggler.

Abstract

We present the results of $WBVR$ observations of the low-mass X-ray binary V1341 $\textrm{Cyg} = \textrm{Cyg}$ X--2. Our observations include a total of 2375 individual measurements in four bands on 478 nights in 1986-1992. We tied the comparison and check stars used for the binary to the $WBVR$ catalog using their $JHK$ magnitudes. The uncertainty of this procedure was 3$%$ in the $B$ and $V$ bands and 8%-10% for the $W$ and $R$ bands. In quiescence, the amplitude of the periodic component in the binary's $B$ brightness variations is within $0.265^{m}{-}0.278^{m}$ ($0.290^{m}{-}0.320^{m}$ in $W$); this is due to the ellipsoidal shape of the optical component, which is distorted with gravitational forces from the X-ray component. Some of the system's active states (long flares) may be due to instabilities in the accretion disk, and possibly to instabilities of gas flows and other accretion structures. The binary possesses a low-luminosity accretion disk. The light curves reveal no indications of an eclipse near the phases of the upper and lower conjunctions in quiescence or in active states during the observed intervals. We conclude that the optical star in the close binary V1341 $\textrm{Cyg} = \textrm{Cyg}$ X-2 is a red giant rather than a blue straggler. We studied the long-term variability of the binary during the seven years covered by our observations. The optical observations presented in this study are compared to X-ray data from the Ginga observatory for the same time intervals.