Gamma-Ray Spectral Properties of the Galactic Globular Clusters: Constraint on the Number of Millisecond Pulsars

We study the γ-ray spectra of 30 globular clusters (GCs) thus far detected with the Fermi Gamma-ray Space Telescope. Presuming that γ-ray emission of a GC comes from millisecond pulsars (MSPs) contained within the GC, a model that generates spectra for the GCs is built based on the γ-ray properties of the detected MSP sample. We fit the GCs’ spectra with the model, and for 27 of them, their emission can be explained as arising from MSPs. The spectra of the other three, NGC 7078, 2MS-GC01, and Terzan 1, cannot be fit with our model, indicating that MSPs’ emission should not be the dominant one in the first two and the third one has a unique hard spectrum. We also investigate six nearby GCs that have relatively high encounter rates compared to the comparison cases. The candidate spectrum of NGC 6656 can be fit with that of one MSP, supporting its possible association with the γ-ray source at its position. The five others do not have detectable γ-ray emission. Their spectral upper limits set limits of ≤1 MSPs in them, consistent with the numbers of radio MSPs found in them. The estimated numbers of MSPs in the γ-ray GCs generally match well those reported for radio pulsars. Our studies of the γ-ray GCs and the comparison nearby GCs indicate that the encounter rate should not be the only factor determining the number of γ-ray MSPs a GC contains.