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 Article · Mon Aug 01 04:00:00 EDT 2016 · Astrophysical Journal

DOI:https://doi.org/10.3847/0004-637X/826/2/149· OSTI ID:22868859

Bernardini, F.; Russell, D. M.; Kolojonen, K. I. I. ^[1]; Stella, L. ^[2]; Hynes, R. I. ^[3]

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1. New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi (United Arab Emirates)
2. INAF—Osservatorio astronomico di Roma, Via Frascati 44, Monteporzio I-00040, Catone (Roma) (Italy)
3. Department of Physics and Astronomy, Louisiana State University, 202 Nicholson Hall, Tower Drive, Baton Rouge, LA 70803 (United States)

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{sub opt}∝ L{sub X}{sup 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.

• OSTI ID: 22868859
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 826; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCRETION DISKS
BENCHMARKS
BINARY STARS
BLACK HOLES
BOLOMETERS
COMPARATIVE EVALUATIONS
CORRELATIONS
EMISSION
KEV RANGE 01-10
LUMINOSITY
MASS
NEUTRON STARS
REPROCESSING
X RADIATION