Scientists at NASA's Jet Propulsion Laboratory have observed strong gamma-ray emission emanating from black hole candidate, Cygnus X-1, providing the clearest evidence yet that anti-matter is produced in large quantities by that type of object.
"Many theoretical models have predicted this effect, but this is the first clear evidence supporting such predictions," said Dr. James Ling, who presented an invited paper on the results before workshop on Nuclear Spectroscopy of Astrophysical Sources, in Washington D.C., Dec. 13-16.
The observed gamma rays are highly penetrating, with an energy of about one million electron volts, similar to those used medically for the treatment of cancer with radioactive cobalt.
"They can travel the entire diameter of our galaxy without being deflected," added Ling, "and thus provide us with direct glimpse of conditions in the emission environment at the source."
Discovery of the gamma rays was recently described in paper published in the Oct. 15 issue of the Astrophysical Journal, with co-authors Drs. William A. Mahoney, William Wheaton and Allan S. Jacobson, all of JPL.
Cygnus X-1. about 7,500 light years from Earth in our own galaxy, the Milky Way, has long been considered prime candidate for black hole.
Such objects are believed to form when certain massive stars exhaust their nuclear energy and collapse. Cygnus X-1's tremendous gravity squeezes mass about 10 times that of our sun into sphere about 40 miles across, the size of the Los Angeles Basin.
The intense gravity of the black hole draws off matter from supergiant companion star, with mass about 30 times that of the sun. As the matter spins around the black hole, it forms hot disk -- referred to as the accretion disk -- of very high energy plasma observable as X-rays.
The gamma-rays, however, are strong evidence for the creation of anti-matter in the form of positrons, or anti- electrons, which have positive charge, in the hot innermost region of the accretion disk.
"Most likely," said Drs. Charles Dermer and Edison Liang of the Lawrence Livermore Laboratory, who presented theoretical paper on the subject at the workshop, "these gamma-rays were produced in spherical volume of about 300 miles across surrounding the black hole."
The Livermore scientists further suggested that the region must be highly packed with positrons, about 100 million billion per cubic centimeter, and heated to temperature of few billion degrees, nearly one million times hotter than the surface of our sun.
The observation of gamma-rays was made by the JPL Gamma Ray Spectroscopy experiment aboard the satellite HEAO 3 in 1979 and 1980. The JPL scientists said that the emission appeared only during one 14 day period when the X-ray intensity was unusually low.
During total of 170 days of observation of the system, strong variability of the X-ray emission was also observed, behavior which has not yet been explained.
"Cygnus X-1 is just fascinating object," Ling said. "Just when we get better understanding of the system, it hits us with more stuff to chew on."
X-ray astronomy previously established class of dozens of neutron stars, collapsed stars whose strong gravitational fields attract, and then intensely heat, gas from companion stars. This reaction produces X-rays with energy few tens of thousands of electron volts, the scientists said.
But such neutron stars generally do not produce gamma-rays similar to those observed on Cygnus X-1, Ling said, perhaps because they typically have strong magnetic fields. Such field would force accreting matter to rotate rigidly with the star and reduce the friction of spinning gases which is responsible for heating the matter enough to emit gamma-rays.
Also, strong magnetic fields are opaque to gamma- rays. The emission of the gamma-rays is further evidence Cygnus X-1 is black hole, Ling said, because black holes should not have magnetic fields. The gamma rays thus provide new observational test for distinguishing black holes from neutron stars.
The scientists said that in this regard, Cygnus X-1 is similar not just to other double-star systems, but to the energy source at the center of the Milky Way galaxy itself, and to other galaxies.
The research was carried out by JPL/Caltech under contract with NASA.
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