Radiation Backgrounds at Cosmic Dawn: X-Rays from Compact Binaries

Abstract

We compute the expected X-ray diffuse background and radiative feedback on the intergalactic medium (IGM) from X-ray binaries prior and during the epoch of reionization. The cosmic evolution of compact binaries is followed using a population synthesis technique that treats separately neutron stars and black hole binaries in different spectral states and is calibrated to reproduce the observed X-ray properties of galaxies at z ∼ < 4. Together with an updated empirical determination of the cosmic history of star formation, recent modeling of the stellar mass-metallicity relation, and a scheme for absorption by the IGM that accounts for the presence of ionized H II bubbles during the epoch of reionization, our detailed calculations provide refined predictions of the X-ray volume emissivity and filtered radiation background from "normal" galaxies at z ∼ > 6. Radiative transfer effects modulate the background spectrum, which shows a characteristic peak between 1 and 2 keV. Because of the energy dependence of photoabsorption, soft X-ray photons are produced by local sources, while more energetic radiation arrives unattenuated from larger cosmological volumes. While the filtering of Xray radiation through the IGM slightly increases the mean excess energy per photoionization, it also weakens the radiation intensity below 1 keV, lowering the mean photoionization and heating rates. Numerical integration of the rate and energy equations shows that the contribution of X-ray binaries to the ionization of the bulk IGM is negligible, with the electron fraction never exceeding 1%. Direct He I photoionizations are the main source of IGM heating, and the temperature of the largely neutral medium in between H II cavities increases above the temperature of the cosmic microwave background (CMB) only at z ∼ < 10, when the volume filling factor of H II bubbles is already ∼ > 0.1. Therefore, in this scenario, it is only at relatively late epochs that neutral intergalactic hydrogen may be observable in 21-cm emission against the CMB.

Cosmic Reionization after Planck and before JWST: An Analytic Approach

“…In the case of a galaxydominated reionization scenario, however, X-rays from compact binaries are expected to keep the electron fraction of the IGM between H II cavities below 1 percent (Madau & Fragos 2017). (12) and (13), and writing the total average opacity as κ ν ≡ (dτ /dz)|dz/cdt|, one then recovers Equation (14).…”

Reflection Spectroscopy of the Black Hole Binary XTE J1752−223 in Its Long-stable Hard State

“…Supermassive black holes at the centers of galaxies shape their hosts through their powerful outflows (Fabian 2012), and their smaller cousins, black hole binaries (BHBs), may have played an important role during the epoch of ionization (e.g., Madau & Fragos 2017). Accretion processes in supermassive black holes are, however, hard to study because of their long variability timescale and large distances, which result in low fluxes.…”

X-Ray Background at High Redshifts From Pop Iii Remnants: Results From Pop Iii Star Formation Rates in the Renaissance Simulations

“…Any strong accretion onto these massive Pop III seed BHs would lead to X-ray radiation at high redshifts (Kuhlen & Madau 2005;Alvarez et al 2009;Tanaka et al 2012;Visbal et al 2012). X-rays from Pop III binaries have been suggested to produce a pre-heated IGM (e.g., Mirabel et al 2011;Haiman 2011;Visbal et al 2012;Mesinger et al 2013;Madau & Fragos 2016), and to partially ionize the IGM in large volumes (e.g., Ostriker & Gnedin 1996;Pritchard & Furlanetto 2007). Xu et al (2014) showed that Pop III binaries might be the dominant X-ray sources at high redshifts and discussed how the IGM might be heated locally by hundred eV photons and globally by the X-ray background from keV photons.…”