ON THE OPTICAL X-RAY CORRELATION FROM OUTBURST TO QUIESCENCE IN LOW-MASS X-RAY BINARIES: THE REPRESENTATIVE CASES OF V404 CYG AND CEN X-4

Low-mass X-ray binaries (LMXBs) show evidence of a global correlation of debated origin between X-ray and optical luminosity. We study for the first time this correlation in two transient LMXBs, the black hole (BH) V404 Cyg and the neutron star Cen X-4, over six orders of magnitude in X-ray luminosity, from outburst to quiescence. After subtracting the contribution from the companion star, the Cen X-4 data can be described by a single power law correlation of the form L-opt alpha L-X(0.44) consistent with disk reprocessing. We find a similar correlation slope for V404 Cyg in quiescence (0.46) and a steeper one (0.56) in the outburst hard state of 1989. However, V404 Cyg is about 160-280 times optically brighter, at a given 3-9 keV X-ray luminosity, compared to Cen X-4. This ratio is a factor of 10 smaller in quiescence, where the normalization of the V404 Cyg correlation also changes. Once the bolometric X-ray emission is considered and the known main differences between V404 Cyg and Cen X-4 are taken into account (a larger compact object mass, accretion disk size, and the presence of a strong jet contribution in the hard state for the BH system), the two systems lie on the same correlation. In V404 Cyg, the jet dominates spectrally at optical infrared frequencies during the hard state but makes a negligible contribution in quiescence, which may account for the change in its correlation slope and normalization. These results provide a benchmark to compare with data from the 2015 outburst of V404 Cyg and, potentially, other transient LMXBs as well.