Radio jets and the 'great annihilator.' (gamma ray research) (Brief Article)

More than a decade ago, when the space-borne Einstein Observatory detected X-rays from a body near the center of our galaxy, that object seemed a rather ordinary radiation source. However, researchers later identified it as one of the most powerful emitters of high-energy photons, called gamma rays, in our galaxy (SN: 5/11/91, p.294). Known as 1E1740.7-2942, this body appears to intermittently spit out a torrent of gamma rays. But the exact origin of these emissions has remained a puzzle.

Now, French astronomers examining this body at radio wavelengths say their work can explain how some of these gamma rays are produced. Using the Very Large Array radiotelescope near Socorro, N.M., the team found that 1E1740.7-2942 has a compact, radio-wave-emitting core that radiates in sync with the gamma rays. In addition, the core appears to send out beams of charged particles along two equal, opposite jets.

Both the jets and the intense gamma rays suggest that this enigmatic Milky Way resident is a burned-out "miniquasar" that may have a star-size black hole lurking at its center, the astronomers note. "The mass and other parameters may differ from a quasar, but the physics is the same," says I. Felix Mirabel at the Centre d'Etudes de Saclay in Gifsur-Yvette. He and his colleagues detail their findings in the July 16 NATURE.

In a commentary accompanying the NATURE article, E. Sterl Phinney of the California Institute of Technology in Pasadena cautions that the jets may not belong to 1E1740.7-2942. Instead, they may represent part of a distant radio galaxy that happens to reside in the same part of the sky. But if the jet finding does prove correct, it would account for some of the unusual properties of gamma rays emitted by 1E1740.7-2942, Phinney notes.

The Milky Way object sporadically spews out two patterns of gamma rays: high-energy photons that cover a broad band of wavelengths and those that cover a much narrower band. Both patterns peak at the wavelength of photons created when an electron collides with and annihilates its antimatter counterpart, a positron. That earlier finding prompted scientists to dub 1E1740.7-2942 the "Great Annihilator."

The broad-band emission represents the collision of positrons with nearby, warm electrons that have a wide range of kinetic energies. In contrast, the narrower-band emission requires a colder medium. The extended radio jets found by Mirabel's team suggest that beams of positrons emitted by the Great Annihilator slam into electrons located in a cold, dense molecular cloud about three light-years away. Gamma rays produced in such collisions would have the narrow-wavelength feature observed.

Despite extensive studies, astronomers have found similar radio jets in only one other X-ray source in the Milky Way. But Mirabel contends that jets could have eluded detection if they are weak and relatively diffuse. His team recently found indications that another strong Milky Way source, known as GRS1758-258, also has radio jets, Mirabel says. "Perhaps we have [previously] been missing this kind of phenomenon," he notes.