Gamma-ray properties of globular clusters and the 'fundamental planes'

arXiv:1111.1045v1 [astro-ph.HE] 4 Nov 2011 2011 Fermi Symposium, Roma., May. 9-12 1 Gamma-ray properties of globular clusters and the “fundamental planes” P. Tam, A. Kong Institute of Astronomy and Department of Physics, National Tsing Hua University, Hsinchu, Taiwan C. Y. Hui Department of Astronomy and Space Science, Chungnam National University, Daejeon, Republic of Korea K S. Cheng Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong We report on the discovery of gamma-ray emission from several globular clusters (GCs), including Terzan 5, the second known gamma-ray GCs. By now, more than a dozen GCs are known to emit gamma-rays of energies above 100 MeV, thus enabling us to carry out the first detailed correlation study with several cluster properties. We found strong correlations between the observed gamma- ray luminosities and four cluster parameters: stellar encounter rate, metallicity [Fe/H], and energy densities of the soft photons at the cluster locations. These “fundamental planes” of gamma-ray GCs put an intimate relation of the observed gamma-rays to the underlying millisecond pulsar population and have important implications on the origin of the gamma-ray emission of GCs. I. INTRODUCTION Radio and X-ray observations have revealed about 140 millisecond pulsars (MSPs) in 26 globular clus- ters [GCs; 1]. However, the presence of much stronger X-ray emitters can contaminate the X-ray observa- tions of MSPs. Because MSPs are the only known steady γ-ray sources in GCs [2], γ-ray observations of GCs serve as an alternative channel in studying the underlying MSP populations in GCs. Using the Large Area Telescope (LAT), γ-rays from 8 GCs [3] have been discovered, including 47 Tu- canae [4] and Terzan 5 [5]. II. MODELS OF γ-RAYS FROM GLOBULAR CLUSTERS The radiation mechanism of γ-rays is unclear. In the pulsar magnetosphere model, e.g. [6], γ-rays up to a few GeV come from the MSPs through curva- ture radiation. On the other hand, inverse Compton (IC) processes resulted from energetic particles up- scattering low-energy photons, such as starlight and infrared light, may give rise to γ-rays of MeV to TeV energies, e.g. [7]. In either model, it is expected that the γ-ray luminosity of a GC is proportional to the stellar encounter rate, a measure of the number of MSPs in a GC. III. NEW γ-RAY GLOBULAR CLUSTERS UNCOVERED Terzan 5 contains the largest number of known MSPs among all GCs. It was discovered as the sec- ond known γ-ray emitting GC after 47 Tucanae [5] (see Figure 1). We note that 47 Tucanae was discov- ered in the bright source list [8], while the discovery of Terzan 5 in γ-rays was announced [5] before the re- lease of the first Fermi/LAT catalog [9] and the report of the 8 GCs [3]. Like 47 Tucanae, the γ-ray spectrum of Terzan 5 also shows a cutoffat ∼3 GeV [3, 5]. After the dis- covery of other six γ-ray emitting GCs [3], we also identified a group of GCs with high encounter rate. Using more than two years of data taken from LAT, we found γ-ray emission from the directions of Liller 1, NGC 6624, and NGC 6752 [10]. The test-statistic maps of the regions around these 3 GCs are shown in Figures 2 and 3. For M80, NGC 6139, and NGC 6541, the detection is marginal (4−5σ) when it was first re- ported [10]. For the cases where the γ-ray emission is offset from the core (i.e. Liller 1 and NGC 6624), the γ-ray spec- tra in the energy range of 200 MeV to 100 GeV are presented in Figure 4. The photons above ∼20 GeV are detected at significance levels of 3–4. Once the existence of these high-energy photons is established, it will be easier to be reconciled in the IC models than in the pulsar magnetosphere model. In the latter case, spectral cut-offs at several GeV are expected. IV. THE FUNDAMENTAL PLANES OF γ-RAY GLOBULAR CLUSTERS We have investigated the properties of the γ- ray emitting globular clusters [11]. By correlating the observed γ-ray luminosities with various cluster prop- erties, we probe the origin of the high energy photons from these GCs. We found that the γ-ray luminosity is positively correlated with the encounter rate and the metalicity [Fe/H] which places an intimate link eConf C110509 2 2011 Fermi Symposium, Roma., May. 9-12 FIG. 1: The count maps of the 5◦× 5◦region centered on Terzan 5. The insets show the test-statistic maps [5]. FIG. 2: The test-statistics map of Liller 1 [10] between the γ-ray emission and the MSP population. We also found that the γ-ray luminosity increases with the energy densities of the soft photons at the clus- ter location. When combining two parameters at the same time, the correlation is even stronger. The edge- on fundamental plane relations of γ-ray GCs are de- picted in Figure 5. This finding strongly suggests that models that in- corporate optical or infrared photons should be taken into considerations in explaining the γ-ray emission from GCs, e.g. the IC models [7]. Acknowledgments P. Tam acknowledges the support of the Formosa Program o

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