ROSAT Status Report # 25
,May 22nd 1992
SPECIAL SCIENCE BULLETIN
U.S. scientists have solved a 20-year-old mystery about the nature of Geminga, one of the brightest emitters of high-energy gamma rays in the sky. Scientists were unclear about the source of Geminga's power and why it shines brightly in gamma rays. Using data from two different spacecraft, scientists now know that the power plant in Geminga is a rotating, 300,000- year-old neutron star.
Dr. Jules Halpern, Columbia University, New York City, and Dr. Stephen Holt, NASA's Goddard Space Flight Center, Greenbelt, Maryland, report in today's issue of the weekly science journal, Nature, that they have observed x-ray pulsations from Geminga using data from the German/American Roentgen Satellite (ROSAT). These observations firmly establish Geminga as a close cousin of the Crab and Vela nebulae, which also have pulsating neutron stars at their cores. The rotating neutron star produces focussed beams of radiation much like the periodic flashing or pulsating lighthouse beacon.
In a companion paper, an investigation led by Goddard's Dr. David Bertsch confirms the pulsations using gamma ray data from the Energetic Gamma Ray Experiment (EGRET) on NASA's Compton Gamma Ray Observatory and further estimate the age of Geminga as 300,000 years.
Geminga was discovered 20 years ago in the first high-energy gamma ray survey of the sky conducted with NASA's Small Astronomical Satellite-2. Located only a few degrees from the Crab nebula, one of the brightest x-ray and gamma ray sources in the sky.
Geminga is even brighter than the Crab in gamma rays. Because it has no obvious optical or x-ray counterpart like the Crab nebula for example, it was given the name Geminga, which means "it isn't there" in Milanese Italian dialect. With virtually all of its power emitted in gamma rays, its nature has been a true mystery for 20 years.
Several years ago, a very weak x-ray source was suggested as a potential identification for Geminga on the basis of approximate positional coincidence. Using the ROSAT observatory, Halpern and Holt discovered that the x-ray intensity of this source is modulated with a period of 237 milliseconds, more than seven times longer than the period of the pulsar in the Crab nebula and three times longer than the pulsar in the Vela nebula.
The pulsations are the key to understanding the nature of Geminga. The Crab and Vela nebulae are supernova remnants powered by converting the kinetic energy of pulsars in their centers into radiation that generally includes the whole electromagnetic spectrum. Why Geminga is powerful only in gamma rays is not certain, but the newly discovered pulsar firmly establishes Geminga as a close cousin of the Crab and Vela nebulae.
Bertsch and the EGRET team confirmed that the gamma ray emission from the source modulated with the same period. Their observations also allowed the scientists to measure the rate at which the neutron star is slowing, providing an estimate of the age of Crab and Vela to be 1,000 and 10,000 years, respectively. Geminga was estimated to have an age of 300,000 years.
This discovery not only explains the nature of Geminga, but suggests that many of the remaining unidentified gamma ray sources in the Milky Way galaxy may also be neutron stars. Although nearly all pulsars are discovered because of their strong radio signals, Geminga is apparently silent in the radio band. It is possible that Geminga is not unique in this regard. The ROSAT and Compton observatories will search for additional members of this emerging class of gamma ray pulsars.
ROSAT, a cooperative project between NASA and Germany, was launched on a Delta launch vehicle from Cape Canaveral Air Force Station, Florida, on June 1, 1990. The Compton Gamma Ray Observatory was launched on the Space Shuttle Atlantis from the Kennedy Space Center, Fla., on April 5, 1991. The Goddard Space Flight Center manages the U.S. participation in the ROSAT program and the Compton Gamma Ray Observatory for NASA's Office of Space Science and Applications, Washington, D.C.
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