Gamma-ray spectral properties of the Galactic globular clusters: constraint on the numbers of millisecond pulsars

Abstract

We study the gamma-ray spectra of 30 globular clusters (GCs) thus far detected with the Fermi Gamma-ray Space Telescope. Presuming that gamma-ray emission of a GC comes from millisecond pulsars (MSPs) contained in, a model that generates spectra for the GCs is built based on the gamma-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 with arising from MSPs. The spectra of the other three, NGC 7078, 2MS-GC01, and Terzan 1, can not 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 as the comparison cases. The candidate spectrum of NGC 6656 can be fit with that of one MSP, supporting its possible association with the gamma-ray source at its position. The five others do not have detectable gamma-ray emission. Their spectral upper limits set limits of $\le 1$ MSPs in them, consistent with the numbers of radio MSPs found in them. The estimated numbers of MSPs in the gamma-ray GCs generally match well those reported for radio pulsars. Our studies of the gamma-ray GCs and the comparison nearby GCs indicate that the encounter rate should not be the only factor determining the number of MSPs a GC contains.

26 pages, 21 figures, 2 tables. Results largely revised by incorporating referee's comments. Accepted for publication in ApJ

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Cited by in corpus (8)

• Pulse Profiles and Polarization of Terzan 5 Pulsars
• Discovery of gamma-ray pulsations of PSR J1835$-$3259B in the Globular Cluster NGC 6652
• Likely detection of gamma-ray pulsations of PSR~J1717+4308A in NGC~6341 and implication of the gamma-ray millisecond pulsars in globular clusters
• On the Gamma-Ray Emission of the Andromeda Galaxy M31
• How the dynamical properties of globular clusters impact their $\gamma$-ray and X-ray emission
• A blazar candidate for the Fermi source 4FGL J1848.7-0129
• The Effect of Pulsar Geometry on the Observed Gamma-ray Spectrum of Millisecond Pulsars
• Discovery and Timing of the First Millisecond Pulsar in NGC 6316