Extracting common signal components from the X-ray and optical light curves of GX 339-4: new view for anti-correlation


Simultaneous X-ray and optical observations of black hole X-ray binaries have shown that the light curves contain multiple correlated and anti-correlated variation components when the objects are in the hard state. In the case of the black hole X-ray binary, GX 339-4, the cross correlation function (CCF) of the light curves suggests a positive correlation with an optical lag of 0.15$s$ and ant-correlations with an optical lag of $1s$ and X-ray lag of $4 s$. This indicates the two light curves have some common signal components with different delays. In this study, we extracted and reconstructed those signal components from the data for GX 339-4. The results confirmed that correlation and anti-correlation with the optical lag are two common components. However, we found that the reconstructed light curve for the anti-correlated component indicates a positively correlated variation with an X-ray lag of $\sim +1 s$. In addition, the CCF for this signal component shows anti-correlations not only with the optical lag, but also with the X-ray lag, which is consistent with the CCF for the data. Therefore, our results suggest that the combination of the two positively correlated components, that is, the X-ray preceding signal with the $0.15 s$ optical lag and the optical preceding signal with the $1 s$ X-ray lag, can make the observed CCF without anti-correlated signals. The optical preceding signal may be caused by synchrotron emission in a magnetically dominated accretion flow or in a jet, while further study is required to understand the mechanism of the X-ray time lag.

Publications of the Astronomical Society of Japan