  Globular clusters are dense stellar systems that have typical ages of ~13
billion years, implying that they formed at redshifts of z>~6. Massive stars in
newly formed or primordial globular clusters could have played an important
role during the epoch of cosmological reionisation (z>~6) as sources of
energetic, neutral hydrogen ionising UV photons. We investigate whether or not
these stars could have been as important in death as sources of energetic X-ray
photons as they were during their main sequence lives. Most massive stars are
expected to form in binaries, and an appreciable fraction of these (as much as
~30%) will evolve into X-ray luminous (L_X~10^38 erg/s) high-mass X-ray
binaries (HMXBs). These sources would have made a contribution to the X-ray
background at z>~6. Using Monte Carlo models of a globular cluster, we estimate
the total X-ray luminosity of a population of HMXBs. We compare and contrast
this with the total UV luminosity of the massive stars during their main
sequence lives. For reasonable estimates, we find that the bolometric
luminosity of the cluster peaks at ~10^42 erg/s during the first few million
years, but declines to ~10^41 erg/s after ~5 million years as the most massive
stars evolve off the main sequence. From this time onwards, the total
bolometric luminosity is dominated by HMXBs and falls gradually to ~10^40 erg/s
after ~50 million years. Assuming a power-law spectral energy distribution for
the HMXBs, we calculate the effective number of neutral hydrogen ionisations
per HMXB and show that HMXBs can be as important as sources of ionising
radiation as massive stars. Finally we discuss the implications of our results
for modelling galaxy formation at high redshift and the prospects of using
globular clusters as probes of reionisation.
