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X-Ray Background from Early Binaries

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Pop III stars
Artist’s impression of Population III stars, the first generation of stars believed to have formed more than 13 billion years ago. [NASA]

What impact did X-rays from the first binary star systems have on the universe around them? A new study suggests this radiation may have played an important role during the reionization of our universe.

Ionizing the Universe

During the period of reionization, the universe reverted from being neutral (as it was during recombination, the previous period) to once again being ionized plasma — a state it has remained in since then. This transition, which occurred between 150 million and one billion years after the Big Bang (redshift of 6 < z < 20), was caused by the formation of the first objects energetic enough to reionize the universe’s neutral hydrogen.

soft X-ray background

ROSAT image of the soft X-ray background throughout the universe. The different colors represent different energy bands: 0.25 keV (red), 0.75 keV (green), 1.5 keV (blue). [NASA/ROSAT Project]

Understanding this time period — in particular, determining what sources caused the reionization, and what the properties were of the gas strewn throughout the universe during this time — is necessary for us to be able to correctly interpret cosmological observations.

Conveniently, the universe has provided us with an interesting clue: the large-scale, diffuse X-ray background we observe all around us. What produced these X-rays, and what impact did this radiation have on the intergalactic medium long ago?

The First Binaries

A team of scientists led by Hao Xu (UC San Diego) has suggested that the very first generation of stars might be an important contributor to these X-rays.

This hypothetical first generation, Population III stars, are thought to have formed before and during reionization from large clouds of gas containing virtually no metals. Studies suggest that a large fraction of Pop III stars formed in binaries — and when those stars ended their lives as black holes, ensuing accretion from their companions could produce X-ray radiation.

The evolution with redshift the authors find for the mean X-ray background intensities. Each curve represents a different observed X-ray energy (and the total X-ray background is given by the sum of the curves). The two panels show results from two different calculation methods. [Xu et al. 2016]

The evolution with redshift of the mean X-ray background intensities. Each curve represents a different observed X-ray energy (and the total X-ray background is given by the sum of the curves). The two panels show results from two different calculation methods. [Xu et al. 2016]

Xu and collaborators have now attempted to model to the impact of this X-ray production from Pop III binaries on the intergalactic medium and determine how much it could have contributed to reionization and the diffuse X-ray background we observe today.

Generating a Background

The authors estimated the X-ray luminosities from Pop III binaries using the results of a series of galaxy-formation simulations, beginning at a redshift of z ~ 25 and evolving up to z = 7.6. They then used these luminosities to calculate the resulting X-ray background.

Xu and collaborators find that Pop III binaries can produce significant X-ray radiation throughout the period of reionization, and this radiation builds up gradually into an X-ray background. The team shows that X-rays from Pop III binaries might actually dominate more commonly assumed sources of the X-ray background at high redshifts (such as active galactic nuclei), and this radiation is strong enough to heat the intergalactic medium to 1000K and ionize a few percent of the neutral hydrogen.

If Pop III binaries are indeed this large of a contributor to the X-ray background and to the local and global heating of the intergalactic medium, then it’s important that we follow up with more detailed modeling to understand what this means for our interpretation of cosmological observations.

Citation

Hao Xu et al 2016 ApJL 832 L5. doi:10.3847/2041-8205/832/1/L5