More than 40 years ago, a mysterious gamma-ray emitting object was seen in the constellation of Gemini by the SAS-2 and COS-B satellite observatories. The source, however, is invisible in the radio and optical bands. Giovanni Bignami, its discoverer, named it Geminga, for "Gemini gamma-ray source" and after a word meaning "it's not there" in the Milanese dialect. The source remained mysterious for more than twenty years after its discovery, until observations with the ROSAT X-ray observatory found X-ray pulsations from Geminga, establishing it as a nearby, rapidly spinning neutron star. The image above shows a detailed, high-resolution X-ray image of Geminga obtained by the Chandra X-ray observatory. The Chandra observations show that the X-ray emission from Geminga is highly structured, with emission from the neutron star itself, extended emission from a jet associated with the spin axis of the neutron star, and an "anti-tail" directly behind the pulsar. As shown in the illustration below the X-ray image, the emission near Geminga is believed to arise from a disk of hot material around the neutron star, while extended emission from the jet is swept back behind the star as Geminga flies through the Galaxy at a speed of nearly 1 million kilometers per hour. The "tail" is a bit of a mystery still: astronomers think this tail might be produced by enormously powerful magnetic explosions similar to the flares produced by instabilities in the Sun's magnetic field.
Chandra Images Show That Geometry Solves a Pulsar Puzzle
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