Scaling Up: A Global Optical/X-ray Correlation For Reprocessing Across Black Hole Masses
Abstract
This study extends the optical/X-ray correlation from black hole low-mass X-ray binaries (XRBs) to active galactic nuclei (AGN), bridging scales from stellar-mass to supermassive black holes (SMBHs). In XRBs, optical emission predominantly originates from the thermal reprocessing of high-energy photons on the surface of the geometrically thin, optically thick accretion disk. The reprocessed optical luminosity follows a power-law relation of the form $L_{V} \propto L_{X}^β a$, with $β\sim 0.5$, where $L_X$ is the X-ray luminosity and the orbital separation $a$ serves as a proxy for the disk size. To test the universality of this relationship across all mass scales, we analyzed quasi-simultaneous X-ray and optical flux data from a sample of XRBs and AGN that have estimates of the radius of the optically emitting disk. To bridge the structural differences between these populations, we utilized the characteristic optical/X-ray time lag derived from reverberation mapping as a physical proxy for the reprocessing radius. Our analysis reveals a tight, global correlation across all black hole masses, described by $\log L_V = 0.48\log L_X + 0.96\log a_R + 6.5$ with a scatter of $\sim 0.20$ dex, where $a_R$ is the reprocessing radius derived from the lag. Incorporating a geometric correction for the disk inclination angle ($\cos i$) slightly reduces this scatter to $0.18$ dex. These results suggest that X-ray reprocessing is scale-invariant across accretion disks, from stellar-mass black holes to SMBHs. This scaling relation provides new constraints for models of X-ray emission regions in accreting black holes.
Authors 12
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Affiliation as printed
Department of Astronomy , Yale University , PO Box 208101 , New Haven , CT 06520-8101 , USA
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Brera Astronomical Observatory
Affiliation as printed
INAF-Osservatorio Astronomico di Brera , Via Bianchi 46 , I-23807 Merate LC , Italy
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Brera Astronomical Observatory
Affiliation as printed
INAF-Osservatorio Astronomico di Brera , Via Bianchi 46 , I-23807 Merate LC , Italy
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Affiliation as printed
Center for Astrophysics and Space Science , NYUAD Research Institute , New York University Abu Dhabi , PO Box 129188 , Abu Dhabi , UAE
New York University Abu Dhabi , PO Box 129188 , Abu Dhabi , UAE
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Affiliation as printed
Center for Astrophysics and Space Science , NYUAD Research Institute , New York University Abu Dhabi , PO Box 129188 , Abu Dhabi , UAE
New York University Abu Dhabi , PO Box 129188 , Abu Dhabi , UAE
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Space Research Organisation Netherlands · University of Amsterdam
Affiliation as printed
Anton Pannekoek Institute for Astronomy , University of Amsterdam , Science Park 904 , NL-1098 XH Amsterdam , The Netherlands
SRON Space Research Institute Netherlands , Niels Bohrweg 4 , NL-2333 CA Leiden , The Netherlands
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Affiliation as printed
Center for Astrophysics and Space Science , NYUAD Research Institute , New York University Abu Dhabi , PO Box 129188 , Abu Dhabi , UAE
New York University Abu Dhabi , PO Box 129188 , Abu Dhabi , UAE
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Affiliation as printed
Center for Astrophysics and Space Science , NYUAD Research Institute , New York University Abu Dhabi , PO Box 129188 , Abu Dhabi , UAE
New York University Abu Dhabi , PO Box 129188 , Abu Dhabi , UAE
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Affiliation as printed
Department of Physics and Astronomy , University of Southampton , Southampton SO17 1BJ , UK