1 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 Detection of High-Energy Gamma-Ray Emission from the Globular Cluster 47 Tucanae with Fermi N.A. Webb, J. Kn¨ odlseder Universit´e de Toulouse, UPS, CESR, 9 Avenue du Colonel Roche, F-31028 Toulouse Cedex 9, France on behalf of the Fermi Large Area Telescope Collaboration Globular clusters are known to harbour a significant population of neutron star X-ray binaries that could be responsible for delaying the inevitable core collapse of these dense clusters. As a result, their progeny, namely millisecond pulsars, are also present in large numbers. Following the confirmation using the Fermi Gamma-ray Space Telescope that millisecond pulsars are indeed gamma-ray emitters, we report on the detection of the Galactic globular cluster 47 Tuc with the Fermi Large Area Telescope. This is the first detection of a Galactic globular cluster in the gamma- ray domain. The gamma-ray spectrum is consistent with gamma-ray emission from a population of millisecond pulsars. The observed gamma-ray luminosity implies an upper limit of 60 millisecond pulsars present in 47 Tucanae. rays. 47 Tucanae (NGC 104) is one of the most promis- ing candidates for high-energy gamma-ray emission because of the large number of known MSPs in the cluster and its relative proximity (4 kpc) [10]. So far, 23 MSPs have been detected in 47 Tuc through radio and/or X-ray observations, and the total population is estimated to be between 30 and 60 [9, 11, 12], al- though claims in the past reached up to 200 [13]. With their typical ages of ∼ 10 10 years, globular clusters form the most ancient constituents of our Galaxy. They are seen throughout the electromag- netic spectrum, from radio waves to X-ray energies, revealing their various stellar components. As an ex- ample, X-ray observations have shown that globular clusters contain considerably more close binary sys- tems per unit mass than the Galactic disc [1]; this finding is interpreted as a result of frequent stellar en- counters in their dense stellar cores [2]. This scenario is strengthened by the observation that the number of low-mass X-ray binary systems containing neutron stars is directly correlated with the stellar encounter rate [3, 4]. These close binary systems may provide a source of internal energy stabilizing the cluster against the inevitable core collapse [5]. Another consequence of this scenario is the presence of many millisecond pulsars (MSPs, also known as recycled pulsars); these are pulsars that were spun up to millisecond periods by mass accretion from a low-mass X-ray binary com- panion [6]. We have observed 47 Tuc with the LAT telescope aboard Fermi. Our data amount to 194.3 days of con- tinuous sky survey observations over the period Au- gust 8th 2008 - April 3rd 2009 during which a total exposure of ∼ 2 × 10 10 cm 2 s (at 1 GeV) has been obtained for 47 Tuc. Events for the data taking pe- riod satisfying the standard low-background event se- lection ( Diffuse events [14]) and coming from zenith angles < 105 ◦ (to greatly reduce the contribution from Earth albedo gamma rays) were used. To further re- duce the effect of Earth albedo backgrounds, the time intervals when the Earth was appreciably within the field of view (specifically, when the center of the field of view was more than 47 ◦ from the zenith) were ex- cluded from this analysis, and all events taken when the spacecraft was within the South Atlantic Anomaly were also excluded. The data analysis presented in this paper has been performed using the LAT Sci- ence Tools package, which is available from the Fermi Science Support Center, using P6 V3 post-launch in- strument response functions (IRFs). These take into account pile-up and accidental coincidence effects in the detector subsystems that are not considered in the definition of the pre- launch IRFs. Using a maxi- mum likelihood model fitting procedure we determine the position of the gamma-ray source to be ( α 2000 = 0 $^{h}$24 . $^{m}$3, δ 2000 = -72 $^{◦}$03 . $^{m}$8) with a 95% confidence er- ror radius of 4.2’. Systematic uncertainties in the po- The only domain in which globular clusters have so far eluded detection is gamma rays. Recent observa- tions with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope have revealed gamma-ray pulsations from eight MSPs, establishing these objects as a class of high-energy gamma-ray sources [7, 8]. Most of the MSPs detected in gamma rays are within a distance of only a few hundred par- secs of the Sun, which implies that MSPs are rather faint objects with isotropic gamma-ray luminosities that generally do not exceed 10 33 ergs s − 1 [8]. Placed at a distance of a few kiloparsecs (the distance to the nearest globular clusters), it is unlikely, although not impossible, that individual MSPs are being detected in gamma rays. Globular clusters, however, may con- tain tens to several hundreds of MSPs [9], and their cumulative magnetospheric emission is probably the first signature that would be picked out in gamma arXiv:1001.1855v1 [astro-ph.HE] 12 Jan 2010 I. INTRODUCTION II. DATA ANALYSIS eConf C091122