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

Xu, H., Ahn, K., Norman, M.L., Wise, J.H., & O’Shea, B.W., 2016, ApJL, 832, L5.

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Summary

Abstract

Xu and collaborators use the Renaissance Simulations to estimate the X-ray background produced by Population III binary systems between redshifts z = 20 and 7.6. Because Population III stars continue forming throughout this period, their remnants may steadily produce X-rays until near the end of reionization. The resulting background could heat the IGM to around 1,000 K and ionize a few percent of its neutral hydrogen.

1. Introduction

  • X-rays travel farther than ultraviolet radiation and can heat and partially ionize large regions of the early IGM.
  • Population III binaries may contain accreting black holes that produce strong X-ray emission.
  • The paper investigates how this emission builds a background and affects the IGM near the end of reionization.

2. Renaissance Simulations and X-Ray Models

  • The authors use Population III star-formation histories from the Rarepeak, Normal, and Void Renaissance simulations.
  • The model assumes that 50 percent of Population III stars form X-ray binaries that remain active for 30 million years.
  • Two methods estimate X-ray emission outside the highly resolved regions, with the density-based second method treated as more reliable.

3. Results

  • Population III star formation continues at a low, nearly constant rate, allowing X-rays to be produced throughout reionization.
  • High-energy X-rays travel long distances, redshift to lower energies, and gradually accumulate into a broad X-ray background.
  • The model produces approximately 6 eV of X-ray energy per hydrogen atom, enough to heat the IGM and ionize a small fraction of its hydrogen.

4. Summary and Discussion

  • Population III binaries may remain an important X-ray source until the end of reionization.
  • The X-ray background could heat the IGM to roughly 1,000 K and raise its free-electron fraction by a few percent.
  • These changes could affect the 21 cm signal, the Lyman-alpha forest, and measurements of the cosmic microwave background.

Best Figures

Figure 2
Evolution of X-ray luminosity density from Population III binaries
The X-ray luminosity increases as Population III binaries continue forming. The different curves compare the simulated environments and the two methods used to estimate the total luminosity.
Figure 5
Synthetic X-ray background spectra at redshifts 12 and 7.6
High-energy X-rays accumulate and redshift into a broad background spectrum. By z = 7.6, much of this radiation has shifted toward the energies where it can efficiently heat and ionize the IGM.