The ionizing and heating power of ultraluminous X-ray sources under the geometrical beaming model

TL;DR

This study models the ionizing and heating effects of ultraluminous X-ray sources under a geometrical beaming framework, revealing their potential role in intergalactic medium heating and explaining certain nebular emissions.

Contribution

It introduces theoretical spectra of ULX populations considering different accretion models, assessing their impact on nebular emission and intergalactic medium heating.

Findings

ULX spectra can partially explain observed nebular emissions.

Soft X-ray emission from ULXs significantly contributes to IGM heating.

ULX contribution varies with accretion and compact-object mass models.

Abstract

While there is now a consensus that X-ray binaries (XRBs) are the dominant X-ray sources in the early Universe and play a significant role during the epoch of heating of the intergalactic medium (IGM), recent studies report contradicting results regarding their contribution in the nebular emission of local Universe galaxies. Ultraluminous X-ray sources (ULXs), which dominate the X-ray budget of normal galaxies, may be important interstellar-medium (ISM) ionizing sources. However, their output in the extreme UV (EUV) and soft--X-ray part of the spectrum remains observationally unconstrained. In this paper, we predict the ionizing and heating power from ULX populations under the geometrical beaming scenario, and three models describing the emission from super-critical accretion disks. We find that our theoretical spectra for ULX populations cannot (can) explain the HeII (NeV) emission observed in some galaxies, with their contribution being less (more) important than the underlying stellar population. Stochastic fluctuations in the number of ULXs may allow for equal contributions in the HeII emission, in a fraction of galaxies. We provide average spectra of ULX populations as an input to local, and early-Universe studies. We find that the soft--X-ray emission arising from super-critical accretion is significant for the heating of the IGM, and consistent with recent constraints from the 21-cm cosmic signal. Based on the dependence on the adopted compact-object (CO) mass and accretion model, we encourage efforts in modeling ULX spectra via simulations, and their combination with detailed binary population synthesis models.