New class of pure gamma-ray pulsars discovered

Cosmic lighthouses send broadly scattered gamma rays instead of tightly focused radio pulses

The new class of gamma-ray pulsars shows that the gamma rays (pink) do not emerge at the poles like the radio rays (green), but are generated high above the neutron star in space. © NASA / Fermi / Cruz de Wilde
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Astronomers have discovered more than a dozen new-type pulsars using the Fermi gamma-ray observatory. The fast-rotating neutron stars send out bursts of high-energy, broad-spread gamma rays instead of narrowly focused radio pulses, as reported by an international research team in "Science Express". This opens up new possibilities for exploring these cosmic phenomena. Already some old theories have been refuted.

Pulsars are fast-rotating, highly magnetized neutron stars that emit radiation across the electromagnetic spectrum. So far, they have been discovered mainly with the help of radio telescopes. The downside: The radio beam of these pulsars is sharply focused. Like a lighthouse, it passes through space and can only be detected if it coincidentally hits the earth in its orbit. For the first time, the gamma-ray observatory Fermi has also located pulsars based on their gamma radiation, thus opening up new possibilities for exploring these phenomena.

Fermi discovers new gamma ray sources

The orbiting gamma-ray observatory Fermi with its Large Area Telecope (LAT) registers high-energy photons of energies between 20 megaelectronvolts and 300 gigaelectronvolts. These particles are million to billion times more energy-rich than the components of the light visible to us. In the observation data from five months over these frequency ranges the astronomers searched with the help of new evaluation software specifically for periodically repeating gamma ray bursts - and were able to find.

They discovered twelve new pulsating gamma sources, which turned out to be pulsars with rotation periods of 48 to 444 milliseconds. "We know 1, 800 pulsars, but until Fermi we could only catch tiny energy bolts from some of them, " explains Roger Romani of Stanford University. "Now we see the actual power of these cosmic machines for dozens of pulsars."

The Fermi Gamma Ray Observatory discovered twelve previously unknown pulsars (orange) and gamma rays from known radio pulsars (magenta, cyan) and previously unidentified gamma-ray sources (green). NASA / Fermi / LAT Collaboration

Wide emission field instead of narrow beam

In contrast to the radio pulsars, the gamma radiation of the new pulsars is less strongly bundled. The probability of capturing such a ray from the earth is thus much greater. The new pulsars therefore help to better understand the geometry of pulsar emissions and also provide valuable information about the distribution and assembly of these cosmic beacons. In addition, a whole series of hitherto unidentified gamma ray sources in our galaxy could now be determined as a pulsar. display

Radiation pulses are not produced at the poles but in the magnetic field

"We always thought that the gamma rays emitted near the polar caps of a neutron star, where the radio beams form, " explains Alice Harding of NASA's Goddard Space Flight Center. "The new pure gamma-ray pulsars refute this theory."

The astronomers now assume that the pulsating gamma rays arise high above the surface of the neutron star. There, particles are accelerated along the open lobes of the star's magnetic field, thus generating the radiation. For the Vela-Pulsar, the brightest known gamma-ray source, this emission region is located approximately 480 kilometers outside the neutron star, which is just over 30 kilometers long.

(Naval Research Laboratory / NASA, 11.09.2009 - NPO)