Gamma-ray spectra of globular clusters in MW

However, the relative contributions of two separate emission processes - curvature radiation from millisecond pulsar magnetospheres versus inverse Compton emission from relativistic pairs launched into the globular cluster environment by millisecond pulsars - have long been unclear. To address this, we search for evidence of inverse Compton emission in 8-yr Fermi-LAT data from the directions of 157 Milky Way globular clusters.

We find a mildly statistically significant (3.8{sigma}) correlation between the measured globular cluster gamma-ray luminosities and their photon field energy densities. However, this may also be explained by a hidden correlation between the photon field densities and the stellar encounter rates of globular clusters. Analysed in toto, we demonstrate that the gamma-ray emission of globular clusters can be resolved spectrally into two components: (i) an exponentially cut-off power law and (ii) a pure power law.