# The intrinsic collective X-ray spectrum of luminous high-mass X-ray binaries

## TL;DR

This study constructs the collective X-ray spectrum of luminous high-mass X-ray binaries in the local Universe, revealing a power-law shape dominated by ultraluminous sources, and provides insights into supercritical accretion and cosmic X-ray preheating.

## Contribution

It presents the first comprehensive, bias-corrected collective X-ray spectrum of HMXBs per unit star formation rate in the local Universe, based on a large Chandra sample.

## Key findings

- Spectrum is well fit by a power law with photon index 2.1.
- Ultraluminous X-ray sources dominate the spectrum.
- Hard sources dominate above ~2 keV.

## Abstract

Using a sample of two hundred luminous (L_unabs>10^38 erg/s, where L_unabs is the unabsorbed 0.25-8 keV luminosity) high-mass X-ray binary (HMXB) candidates found with Chandra in 27 nearby galaxies, we have constructed the collective X-ray spectrum of HMXBs in the local Universe per unit star formation rate, corrected for observational biases associated with intrinsic diversity of HMXB spectra and X-ray absorption in the interstellar medium. This spectrum is well fit by a power law with a photon index Gamma=2.1+/-0.1 and is dominated by ultraluminous X-ray sources with L_unabs>10^39 erg/s. Hard sources (those with the 0.25-2 keV to 0.25-8 keV flux ratio of <0.6) dominate above ~2 keV, while soft and supersoft sources (with the flux ratios of 0.6-0.95 and >0.95, respectively) at lower energies. The derived spectrum probably represents the angle-integrated X-ray emission of the near- and super-critically accreting stellar mass black holes and neutron stars in the local Universe. It provides an important constraint on supercritical accretion models and can be used as a reference spectrum for calculations of the X-ray preheating of the Universe by the first generations of X-ray binaries.