Simulations suggest importance of X-rays in cosmic reionisation
In their 2009 paper in MNRAS, CAASTRO Associate Investigator Chris Power and his European colleagues started exploring the way in which high-mass X-ray binaries in primordial globular clusters could boost the ionising power of these clusters. The team now has a new publication (in The Astrophysical Journal) that examines the role high-mass X-ray binary systems could play in ionising the neutral gas that dominated the early Universe.
The 2009 simulation was extended to a Monte Carlo model that assumes both black-body and power-law components. The model starts with a stellar population from a single instantaneous burst of star formation and evolves over 250 million years, during which time massive stars form and turn into high-mass X-ray binaries. All limits of stellar mass and formation rates were based on recent published data. The results of the simulations show that high-mass X-ray binaries dominate the ionising power after 20 million years and continue to peak up to 40 million years, after which no more massive stars exist to form such systems and the ionising power declines sharply. In contrast to the massive stars, the binaries are much longer lasting ionising sources, up to 100 million years.
The calculation of how much high-mass X-ray binaries contribute to the present-day X-ray background critically depends on the underlying star formation rate. The model used here determined a contribution of 5 x 10-16 erg s-1 cm-2 deg-2 which is factor 100-1000 smaller than the observed limit. The team has announced a follow-up paper that will further discuss the factors giving rise to this discrepancy.
Chris Power, Gillian F. James, Celine Combet, Graham Wynn in ApJ, 28 January 2013