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 HII 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. While the filtering of X-ray 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 HeI photoionizations are the main source of IGM heating, and the temperature of the largely neutral medium in between HII cavities increases above the temperature of the cosmic microwave background (CMB) only at z<10, when the volume filling factor of HII bubbles is already >0.1. Therefore, in this scenario, it is only at relatively late epochs that the bulk of neutral intergalactic hydrogen may be observable in 21-cm emission against the CMB.

• The paper demonstrates that X-ray binaries, particularly HMXBs, contribute less than 1% electron fraction, indicating a minimal role in direct IGM ionization.
• It employs detailed population synthesis models, calibrated against observed galactic X-ray properties, to analyze the cosmic evolution of compact binaries.
• Results suggest that X-rays preheat the IGM above the CMB at z≈10, supporting a delayed reionization scenario consistent with recent observational constraints.

X-rays from Compact Binaries During Cosmic Dawn: An Examination of Their Role in Reionization

Overview

The paper by Piero Madau and Tassos Fragos presents a comprehensive analysis of the contribution of X-ray binaries (XRBs) to the ionization and thermal histories of the intergalactic medium (IGM) prior to and during the epoch of reionization. Through a detailed population synthesis model, calibrated against observed X-ray properties of galaxies, the authors assess the influence of X-ray emissions from high-mass X-ray binaries (HMXBs) on the IGM, with a keen focus on their potential role during the cosmic dawn.

X-ray Emissivity and Cosmic Evolution

The research utilizes a robust population synthesis approach to trace the cosmic evolution of compact binaries, considering separately neutron star binaries and black hole binaries in different spectral states. By aligning their models with observed data up to redshift $z \approx 4$ and incorporating recent advancements in the empirical determination of the cosmic history of star formation, the authors estimate the X-ray volume emissivity and filtered radiation background from galaxies at redshifts $z \approx 6$. The paper emphasizes how HMXBs, due to weaker stellar winds in low-metallicity environments, dominate the X-ray output in young stellar populations at high redshifts, outshining the fading active galactic nuclei (AGN) contribution at $z > 5$.

Numerical Insights

The study reports that the HMXB population at $z > 6$ is not sufficient to significantly ionize the bulk IGM, with the electron fraction generated being less than 1%. This suggests a minimal impact on the ionization front compared to UV emissions from early stars. The calculations indicate that while the energy deposited by XRBs during these epochs is non-negligible, it does not lead to early ionization or thermal domination of the IGM as previously hypothesized in some models which assume unrealistically large X-ray luminosities per SFR.

Implications for Reionization and Future Prospects

The findings imply that while X-rays contribute to the pre-heating of the IGM, they do so at a later stage, notably raising the IGM temperature above the cosmic microwave background at $z \approx 10$. This delayed reheating contrasts with scenarios proposing much earlier X-ray-driven ionization. Nevertheless, such moderate impact aligns with more recent observational constraints that advocate for a late reionization era, as suggested by Planck satellite measurements and other cosmological probes.

Looking ahead, the nuanced role of XRBs in cosmic reionization underscores the necessity for more refined models that integrate a broader spectrum of galactic data and account for local environmental conditions, such as metallicity and interstellar medium composition. Continuous advancements in X-ray astronomy, combined with further data from upcoming 21-cm surveys, will enhance our understanding of the interplay between high-energy phenomena and large-scale structure formation in the early universe.

Conclusion

Madau and Fragos's study offers a critical perspective on the role of XRBs during the epoch of reionization, challenging previous assumptions of their dominant influence. Their work highlights the importance of incorporating detailed population synthesis and radiative transfer calculations in exploring cosmic histories, setting a precedent for future research aimed at unraveling the complexities of reionization dynamics. While the direct contribution of XRBs to reionization may be limited, their study paves the way for further integrative approaches to cosmic dawn research, alongside other astrophysical sources.