In The Field

AAS 2009: Fermi finds new pulsar classes

pulsar1.jpg In just a matter of months, the Fermi gamma ray observatory since its launch has found dozens of new pulsars — spinning, magnetized remnants of supernovae — that emit a flashing signal in the gamma-ray part of the spectrum only. This new class of pulsar has revolutionized scientists’ view of its general structure: Early in a pulsar’s lifetime, it blasts a broad gamma-ray signal rather than a narrow polar one, as was previously thought. The findings, announced Tuesday at AAS, also includes a club within a club: a subclass of the gamma-ray-only pulsars that flash exceptionally quickly, an indication that they are revving up as they partially devour their dying partners in binary star systems.

The supernovae that come at the end of a star’s life cast off much of a star’s mass, but what’s left collapses to form a neutron star, a fantastically dense object just shy of being a black hole. The spinning magnetic fields produce a narrow cone of radio waves that jet out from the poles. Some 1,800 radio pulsars have been discovered this way.

But there were only a handful that also produced gamma-rays, and these signals were also thought to emanate from the poles. Fermi has boosted the number of known gamma-ray pulsars up to 38, including 13 that lack a radio signal altogether — an indication that the gamma rays can’t be coming from the poles. The results, says mission scientist Roger Romani of Stanford University, “have put the nail in the coffin of the polar cap model.”

He is still working out the mechanisms and geometries by which the intense magnetic fields create the powerful gamma-ray signal, which is expected to arise only in the first million years or so of the pulsar’s life. His models show a trumpet-like bell shape that emerges and sweeps across the pulsar’s face, higher in its magnetosphere, rather than only at the poles. “It means the gamma rays are being beamed widely across the sky,” he says.

The newest phenomenon — seven so-called ‘millisecond’ pulsars that flash hundreds of times faster than normal — has led mission scientist Alice Harding, of Goddard Space Flight Center, to envision how this happens. In binary stars systems — relatively common in the universe — one may die first and become a pulsar. Before the second star explodes, it expands, and its excess material can be sucked into the spinning maw of the pulsar thereby speeding it up. “These guys must have strong stomachs,” Harding says. That’s the scenario pictured here. After the jump, Fermi’s all sky map, and the new gamma-ray pulsars.


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Images: NASA / Fermi / LAT Collaboration / Dana Berry

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