Average bolometric corrections and optical to X-ray flux measurements as a function of accretion rate for X-ray binaries

TL;DR

This study analyzes X-ray and optical flux ratios and bolometric corrections in X-ray binaries and ULXs, revealing their dependence on accretion rates and disk geometry, with implications for understanding accretion physics.

Contribution

It provides average bolometric corrections as a function of accretion rate and examines optical to X-ray flux ratios, incorporating ULX pulsars for the first time.

Findings

Black-hole and pulsar BCs behave similarly when ULX pulsars are included.

Optical to X-ray flux ratio remains nearly constant across accretion rates.

Thicker disk models are supported by observational data.

Abstract

In this paper we use an RXTE library of spectral models from 10 black-hole and 9 pulsar X-ray binaries, as well as model spectra available in the literature from 13 extra-galactic Ultra-luminous X-ray sources (ULXs). We compute average bolometric corrections (BC=$\mathrm{L_{band}/L_{bol}}$) for our sample as a function of different accretion rates. We notice the same behaviour between black-hole and pulsars BCs only when ULX pulsars are included. These measurements provide a picture of the energetics of the accretion flow for an X-ray binary based solely on its observed luminosity in a given band. Moreover it can be a powerful tool in X-ray binary population synthesis models. Furthermore we calculate the X-ray (2-10 keV) to optical (V-band) flux ratios at different Eddington ratios for the black-hole X-ray binaries in our sample. This provides a metric of the maximum contribution of the disk to the optical emission of a binary system and better constraints on its nature (donor type etc). We find that the optical to X-ray flux ratio shows very little variation as a function of accretion rate, but testing for different disk geometries scenarios we find that the optical contribution of the disk increases as the $p$ value decreases ($T(r)\sim r^{-p}$). Moreover observational data are in agreement with a thicker disk scenario ($p<0.65$), which could also possibly explain the lack of observed high-inclination systems.