Fundamental Physics Measurements and the Gamma-Ray Emission of Pulsars and Globular Clusters

Abstract

Globular clusters (GCs) with high encounter rates and dense cores favour the production of millisecond pulsars (MSPs) which are gamma-ray sources and are thought to account for GC emission overall. The evidence for this is largely indirect and no detailed spectral comparison between GCs and MSPs has been attempted.

I analyse the gamma-ray emission of 111 GCs, detecting 34 (100 MeV10GeV), with emission from inside the tidal radius. GC luminosity is positively correlated with encounter rate and mass-encounter rate product as expected for MSP emission but uncorrelated with metallicity which is at odds with MSPs being the emission source. Most GCs have a spectrum consistent with MSPs but there are exceptions. I find that GC unresolved diffuse X-ray emission is correlated with GC gamma-ray emission, possibly due to unresolved sources or relativistic electron propulation from MSPs.

I exclude a shock front emission mechanism in Terzan 5. I stack the spectrum of 98 MSPs, which is a good fit of one to two-thirds of GCs, but this fit is uncorrelated with GC characteristics impacting MSP formation. A spectral colour comparison shows GC emission is harder than the MSP model at 48 GeV, suggesting an additional non-MSP component. I conclude MSPs are important but not the only emission source in GCs.

I derive upper limits to the gamma-ray emission from 17 pulsars, and use a previously reported axion emissivity model, to determine an upper limit to the mass () of the Axion of 9.610 eV which is strongly temperature dependent. I obtain an upper limit of 8.0510 eV by applying an axion power model to magnetars.

Finally, I show that GCs with hard spectral models up to 100 GeV are prime future targets for the Cherenkov Telescope Array and such observations will be vital to determine sources of GC emission.