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 ∼1.5 × 10 31 erg s −1 (NGC 6752) to ∼2.2 × 10 32 erg s −1 (M28). The lowest limiting luminosity for X-ray source detections amongst these four clusters was 1.1 × 10 30 erg 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 blackbody plus power law, or a thermal bremsstrahlung model plus blackbody 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.

The X-ray emissivity of low-density stellar populations

“…Eger et al (2010) and Wu et al (2014) identify likely diffuse emission from the outskirts of Terzan 5 and 47 Tuc, but the flux is <10% of the total from these clusters, and is not attributed to the kind of point sources we study, so we do not include it. Hui et al (2009) do not find evidence for diffuse X-ray emission in Chandra observations of the globular clusters M5, M13, M3, M71, M53, and M4. On the other hand, Bassa et al (2004) find roughly equal L X from M4's core below L X (0.5-2.5)= 6×10 30 ergs/s, vs. sources from 6×10 30 up to = 6×10 31 erg/s, indicating that unresolved emission can produce a significant contribution in the analysis of X-ray faint clusters.…”

Modelling gas evacuation mechanisms in present-day globular clusters: stellar winds from evolved stars and pulsar heating

“…Observations of globular clusters with accompanying X-ray haloes support this idea (Mirabal 2010). In addition, it has been suggested by Hui et al (2009) that while the luminous X-ray pulsar wind nebulae have been detected from pulsars in the Galaxy, there is no evidence of a contribution to the diffuse X-ray emission by pulsar wind nebulae within globular clusters. Searching for low and high energy diffuse emission within and around pulsar-hosting globular clusters could, in principle, help uncovered the heating structures from these objects and provide a much clearer understanding of the underlying processes at work.…”

High energy γ-ray emission from compact galactic sources in the context of observations with the next generation Cherenkov Telescope Arrays

“…The γ-ray emission from GCs shows some interesting correlations with the GC content. For example, Abdo et al [6] and Hui et al [187] find a clear correlation of the observed GeV γ-ray luminosity with the stellar encounter rate which determines the number of MSPs and Cataclysmic Variables within the specific GC. It is believed that the encounter rate determines the formation rate of compact binaries which are responsible for the origin of the MSPs inside GCs.…”

“…The encounter rate can be estimated from the observed density of stars in the core of GC and the core radius [166]. Moreover, Hui et al [187] report the positive correlation of the γ-ray luminosity with the metallicity [Fe/H] of the GC. They also find a tendency of the correlation of the γ-ray flux with the energy density of soft photon field at the GC location.…”