EGRET observations suggest this process might be occurring in the magnetosphere of a pulsar in the constellation Vela. The LAT should thus be able to tell scientists about this rate of decline, which in turn will yield precious clues about the particle-acceleration mechanism.įinding new gamma-ray pulsars will be nice, but as LAT science team member Alice Harding of NASA Goddard notes, "GLAST is really about studying the physics of these sources." For example, GLAST will probably be able to determine whether pulsar magnetic fields are so strong that gamma-ray photons packing more than about 4 or 5 GeV of energy can transform themselves into pairs of particles and antiparticles. Pulsars spin-down as they age, and this should weaken particle acceleration, which in turn should cause their gamma-ray flux to weaken. The LAT will be able to see much fainter pulsars, many of which will be much older than Geminga. If it weren’t so close to Earth (about 500 light-years), EGRET would not have seen it. Geminga is roughly 300,000 years old, which makes it middle-aged in the pulsar life cycle. Among these discoveries, scientists hope to find pulsars similar to Geminga, which is relatively bright in gamma rays but is strangely quiet in radio waves, perhaps because its radio beam doesn't point toward Earth. The EGRET instrument on NASA's Compton Gamma-ray Observatory saw six pulsars, but the LAT has the sensitivity to find dozens or perhaps hundreds. The neutron star contains about a Sun's worth of mass packed in a sphere the size of a large city. Image right: A neutron star is the dense, collapsed core of a massive star that exploded as a supernova. "Getting a much more complete sample of the Milky Way's population of neutron stars is one of the most important ways that GLAST will advance our understanding of the life cycle of stars." "Because pulsar beams are much broader in gamma rays, GLAST will allow us to detect some of the youngest, most energetic pulsars in our galaxy," says GLAST Interdisciplinary Scientist Stephen Thorsett of the University of California, Santa Cruz. But even many young pulsars are invisible to us with radio telescopes because of their narrow lighthouse beams. Without much available energy to power emissions at various wavelengths, they have faded to near invisibility. One is age: most neutron stars are billions of years old, which means they have plenty of time to cool and spin down. There are two reasons for this shortfall. But as GLAST Project Scientist Steve Ritz of NASA Goddard points out, "With magnetic fields trillions of times stronger than Earth's, pulsar magnetic fields are high-energy particle accelerators." The magnetospheres of some pulsars accelerate particles to such high energies that they are relatively bright gamma-ray sources.Īstronomers have found less than 2,000 pulsars, yet there should be about a billion neutron stars in our Milky Way Galaxy. They beam radio waves in narrow cones, which periodically sweep across Earth like lighthouse beacons. These relatively young objects rotate extremely rapidly, with some spinning faster than a kitchen blender. Most known neutron stars belong to a subclass known as pulsars. They are laboratories for extreme physics and conditions that we cannot reproduce here on Earth," says Large Area Telescope (LAT) science team member David Thompson of NASA's Goddard Space Flight Center in Greenbelt, Md. "With neutron stars, we're seeing a combination of strong gravity, powerful magnetic and electric fields, and high velocities. Matter is packed so tightly that a sugar-cube-sized amount of material would weigh more than 1 billion tons, about the same as Mount Everest! Neutron stars cram roughly 1.3 to 2.5 solar masses into a city-sized sphere perhaps 20 kilometers (12 miles) across. When the core of a massive star undergoes gravitational collapse at the end of its life, protons and electrons are literally scrunched together, leaving behind one of nature's most wondrous creations: a neutron star.