A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope

By A. Abdo, M. Ackermann, M. Ajello, L. Baldini, J. Ballet, G. Barbiellini, D. Bastieri, R. Bellazzini, R. Blandford, E. Bloom and 133 more. 2010

145 citations recorded by the source corpus.

Context. Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gammaray emission. The observation of this emission provides a powerful tool to assess the millisecond pulsar population of a cluster, is essential for understanding the importance of binary systems for the evolution of globular clusters, and provides complementary insights into magnetospheric emission processes. Aims. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. Methods. We use 546 days of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular clusters. Results. Steady point-like high-energy gamma-ray emission has been significantly detected towards 8 globular clusters. Five of them (47 Tucanae, Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices (0.7 < Γ < 1.4) and clear evidence for an exponential cut-off in the range 1.0−2.6 GeV, which is the characteristic signature of magnetospheric emission from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral indices (1.0 < Γ < 1.7), however the presence of an exponential cut-off can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC 6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral properties. From the observed gamma-ray luminosities, we estimate the total number of MSPs that is expected to be present in these globular clusters. We show that our estimates of the MSP population correlate with the stellar encounter rate and we estimate 2600−4700 MSPs in Galactic globular clusters, commensurate with previous estimates. Conclusions. The observation of high-energy gamma-ray emission from globular clusters thus provides a reliable independent method to assess their millisecond pulsar populations.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/e87f7592b7b6a2516b7ccfe38978e571a603c977

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Citations in the corpus

THE SECOND FERMI LARGE AREA TELESCOPE CATALOG OF GAMMA-RAY PULSARS (2013, background): “The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope did not just confirm the expectation: by discovering dozens of radio-quiet gamma-ray pulsars and millisecond pulsars (MSPs, thought to be old pulsars spun up to rapid periods via accretion from a companion, Alpar et al. 1982), the”
Fermi Large Area Telescope Fourth Source Catalog (2019, methodology): “Two were extensions of the Cen A lobes, which appear larger in γrays than the WMAP template that we use following Abdo et al. (2010d).”
The Consistency of Fermi-LAT Observations of the Galactic Center with a Millisecond Pulsar Population in the Central Stellar Cluster (2010, methodology): “ the HG Central stellar cluster region ( = 0 :29 0:12, E c = 1:34 0:12) is consistent with four of the eight globular clusters that have been observed by Fermi-LAT and seen to have gamma-ray emission [3]: Omega Cen: = 0 :7+0:7+0:4 0:6 0:4 ;E c = 1:2 +0:7+0:2 0:4 0:2 ; NGC 6388: = 1 :1+0:7+0:8 0:5 0:”
FERMI LARGE AREA TELESCOPE FIRST SOURCE CATALOG (2010, methodology): “The association between GeV γ-ray PSRs and the PWNs visible in TeV γ-rays seems well established, as has already been discussed in Abdo et al. (2010m).”