Diffuse X-ray emission in globular cluster cores

Abstract

The unresolved X-ray emission in the cores of 10 globular clusters hosting millisecond pulsars is investigated. Subtraction of the known resolved point sources leads to detectable levels of unresolved emission in the core region of M28, NGC 6440, M62, and NGC 6752. The X-ray luminosities in the 0.3-8 keV energy band of this emission component were found to lie in the range $\sim 1.5 \times 10^{31}$ ergs s$^{-1}$ (NGC 6752) to $\sim 2.2 \times 10^{32}$ ergs s$^{-1}$ (M28). The lowest limiting luminosity for X-ray source detections amongst these four clusters was $1.1 \times 10^{30}$ ergs s$^{-1}$ for NGC 6752. The spectrum of the unresolved emission can be fit equally well by a power-law, a thermal bremsstrahlung model, a black body plus power-law, or a thermal bremsstrahlung model plus black body component. The unresolved emission is considered to arise from the cumulative contribution of active binaries, cataclysmic variables, and faint millisecond pulsars with their associated pulsar wind nebulae. In examining the available X-ray data, no evidence for any pulsar wind nebular emission in globular clusters is found. It is shown that the X-ray luminosity contribution of a faint source population based on an extrapolation of the luminosity function of detected point sources is compatible with the unresolved X-ray emission in the cores of NGC 6440 and NGC 6752. Adopting the same slope for the luminosity function for M62 as for NGC 6440 and NGC 6752 leads to a similar result for M62. For M28, the contribution from faint sources in the core can attain a level comparable with the observed value if a steeper slope is adopted. The characteristics on the faint source population as constrained by the properties of the unresolved X-ray emission are briefly discussed.