Pulsars More Complicated Than Previously Thought
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453April 14, 1999
Charleston, S.C. -- The "lighthouse" model for pulsars, with a single beam of light sweeping through space, may be as antiquated as the old lighthouse itself, according to a multi-wavelength study of the oldest known gamma-ray pulsar by an international team of astronomers.
A pulsar is a rapidly rotating neutron star, the tightly packed remains of an exploded, massive star. This city-sized sphere can spin about its axis in a fraction of a second and is surrounded by super-strong magnetic fields and electric voltages. Like a spinning top, a pulsar gradually loses energy as it slows down -- energy astronomers can see as radio waves, visible light and X-rays. Seven pulsars are known to produce gamma rays, the most energetic form of radiation.
A team led by Dr. David Thompson, an astrophysicist at NASA's Goddard Space Flight Center, Greenbelt, Md., found that the oldest of these gamma-ray pulsars, PSR B1055-52, produces flashes of light that arrive at different times and with different appearance, depending on what type of telescope is used. Dr. Thompson discusses work on all gamma-ray pulsars at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in Charleston, S.C., April 14, 1999.
"If the simple lighthouse model were correct, all these flashes would arrive simultaneously," said Dr. Thompson. "Instead, this pulsar shows great variety going from wavelength to wavelength."
The group found that while all the pulses from PSR B1055-52 repeat every two-tenths of a second, the different types of radiation arrive at slightly different times. This diversity indicates that the pulsar has a more complex geometry than the simplest models predict, with radio, X-rays and gamma rays probably produced at different places in the pulsar's whirling environment.
Another curious feature of PSR B1055-52 is that it produces gamma rays more efficiently than younger, faster pulsars through a mechanism not yet understood. For most pulsars, gamma-ray flashes are weak, comprising only a small percentage of the total spin energy. Yet the 500,000-year-old PSR B1055-52 can convert 20 percent of its spin energy into gamma rays.
In comparison, the mighty 1,000-year-old Crab pulsar -- the archetypal "lighthouse" pulsar and the first known to emit gamma rays -- has an energy output nearly 10,000 times than of PSR B1055-52, but only a tenth of a percent of that energy appears in the form of gamma rays.
"What we've found is that each pulsar is unique, and seeing one with one type of telescope is not enough," said Dr. Thompson. "A comparison of multi-wavelength properties in pulsars is crucial in attempting to construct models for these objects."
The results of the international team's analysis are published in the May 1 issue of The Astrophysical Journal. Analysis was conducted using data from NASA's Compton Gamma Ray Observatory, the German-operated ROSAT satellite (a German/US/UK joint venture), and the Parkes radio telescope in Australia. The research team comprises scientists from universities in the United States, Europe, Japan and Australia, from the Max Planck Institute of Extraterrestrial Physics, and from NASA Goddard.
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