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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

Journal
Date Issued
2016
Author(s)
•
Russell, D. M.
•
Kolojonen, K. I. I.
•
•
Hynes, R. I.
•
Corbel, S.
DOI
Abstract
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}\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 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.
Volume
826
Issue
2
Start page
149
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2016ApJ...826..149B
Rights
open.access
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