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

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...

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Bibliographic Details
Published in:The Astrophysical journal 2016-08, Vol.826 (2), p.149-149
Main Authors: Bernardini, F., Russell, D. M., Kolojonen, K. I. I., Stella, L., Hynes, R. I., Corbel, S.
Format: Article
Language:English
Subjects:
ACCRETION DISKS
accretion, accretion disks
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
BENCHMARKS
BINARY STARS
BLACK HOLES
BOLOMETERS
Companion stars
COMPARATIVE EVALUATIONS
Correlation
CORRELATIONS
EMISSION
KEV RANGE 01-10
LUMINOSITY
MASS
NEUTRON STARS
Outbursts
REPROCESSING
Slopes
stars: black holes
stars: individual (V404 Cyg, Cen X-4)
stars: neutron
X RADIATION
X rays
X-ray stars
X-rays: binaries
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Summary: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 , 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.
ISSN:0004-637X
1538-4357
DOI:10.3847/0004-637X/826/2/149