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Gamma-Ray Pulsars Old and New

The Fermi Gamma-ray Space Telescope is discovering neutron stars that are high-energy gamma-ray emitters. A flood of results from the Fermi Gamma-ray Space Telescope, launched on 11 June 2008, is changing the way that we view neutron stars. The Large Area Telescope (LAT), the main instrument on Fermi, detects photons of energy between 0.1 and 100 GeV emitted from spinning neutron stars known as radio pulsars, from supermassive black holes in the “blazar” class of active galactic nuclei, and from other high-energy sources. By timing the arrival of photons from gamma ray–bright points in the Galaxy, Fermi is discovering new pulsars whose existence was only conjectured. Sixteen such “gamma ray–only” pulsars, rotating between 2 and 20 times per second, are reported on page 840 in this issue (1). In a companion paper on page 848, the detection of pulsed gamma rays from eight nearby radio “millisecond pulsars” rotating faster than 200 times per second is described (2). Complementing these discoveries, Fermi has detected a steady glow of gamma rays from the globular star cluster 47 Tucanae, as described on page 845 (3); this region is thought to harbor dozens of millisecond pulsars.