A Synthetic Population of Ultra-Luminous X-ray Sources: Optical-X-ray Correlation

Abstract

This paper presents an analysis of the predicted optical-to-X-ray spectral index (${\alpha }_{\mathrm{ox}}$) within the context of ultra-luminous X-ray sources (ULXs) associated with stellar mass black holes and neutron stars. We use the population synthesis code COSMIC to simulate the evolution of binary systems and investigate the relationship between UV and X-ray emission during the ULX phase, namely the ${\alpha }_{\mathrm{ox}}$ relation. The study investigates the impact of metallicity on ${\alpha }_{\mathrm{ox}}$ values. Notably, it predicts a significant anti-correlation between ${\alpha }_{\mathrm{ox}}$ and UV luminosity ($L_{\mathrm{UV}}$), consistent with observations, with the slope of this relationship varying with metallicity for BH-ULXs. The NS-ULX population shows a relatively consistent slope around $-0.33$ across metallicities, with minor variations. The number of ULXs decreases with increasing metallicity, consistent with observational data, and the X-ray luminosity function shows a slight variation in its slope with metallicity, exhibiting a relative excess of high-luminosity ULXs at lower metallicities. Inclusion of beaming effect in the analysis shows a significant impact on the XLF and ${\alpha }_{\mathrm{ox}}$, particularly at high accretion rates, where the emission is focused into narrower cones. Furthermore, the study finds that UV emission in ULXs is predominantly disk-dominated, which is the likely origin of the ${\alpha }_{\mathrm{ox}}$ relation, with the percentage of disk-dominated ULXs increasing as metallicity rises.

16 pages, 18 figures, accepted for publication in Astronomy & Astrophysics

References in corpus (22)

• Modules for Experiments in Stellar Astrophysics (MESA)
• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13
• A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source
• Masses, Beaming and Eddington Ratios in Ultraluminous X-ray Sources
• A spectral-timing model for ULXs in the super-critical regime
• The role of mass transfer and common envelope evolution in the formation of merging binary black holes
• Ultraluminous X-ray sources
• POSYDON: A General-Purpose Population Synthesis Code with Detailed Binary-Evolution Simulations
• X-ray irradiation in XTE J1817-330 and the inner radius of the truncated disc in the hard state
• Reprocessing of X-rays in the outer accretion disc of the black hole binary XTE J1817--330
• Towards a Realistic Explosion Landscape for Binary Population Synthesis
• The Metallicity Dependence of the High-Mass X-ray Binary Luminosity Function
• The 2019 super-Eddington outburst of RX J0209.6-7427: Detection of pulsations and constraints on the magnetic field strength
• New Hubble Space Telescope imaging of the counterparts to six ultraluminous X-ray sources
• XMM-Newton Campaign On Ultraluminous X-ray Source NGC 1313 X-1: Wind vs. State Variability
• Pulsating ULXs: large pulsed fraction excludes strong beaming
• Chandra reveals a possible ultra-fast outflow in the super-Eddington Be/X-ray binary Swift J0243.6+6124
• Orbital decay in M82 X-2
• The Binary Black Hole Merger Rate from Ultraluminous X-ray Source Progenitors
• The largest bright ULX population in a galaxy: X-ray variability and Luminosity Function in the Cartwheel ring Galaxy