FERMI DISCOVERY OF GAMMA-RAY EMISSION FROM THE GLOBULAR CLUSTER TERZAN 5

We report the discovery of gamma-ray emission from the Galactic globular cluster (GC) Terzan 5 using data taken with the Fermi Gamma-ray Space Telescope, from 2008 August 8 to 2010 January 1. Terzan 5 is clearly detected in the 0.5-20 GeV band by Fermi at similar to 27 sigma level. This makes Terzan 5 the second gamma-ray-emitting GC seen by Fermi after 47 Tuc. The energy spectrum of Terzan 5 is best represented by an exponential cutoff power-law model, with a photon index of similar to 1.9 and a cutoff energy at similar to 3.8 GeV. By comparing Terzan 5 to 47 Tuc, we suggest that the observed gamma-ray emission is associated with millisecond pulsars and is either from the magnetospheres or inverse Compton scattering between the relativistic electrons/positrons in the pulsar winds and the background soft photons from the Galactic plane. Furthermore, it is suggestive that the distance to Terzan 5 is less than 10 kpc and >10 GeV photons can be seen in the future.