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 V404 Cyg and the neutron star Cen X-4 , over 6 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 } \propto 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 black hole 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 .