Energetic gammas from beyond the galaxy.

For the first time, astronomers have detected high-energy gamma rays photons millions of times more energetic than the most powerful X-rays - from an object outside our galaxy,

The source, a quasar-like object at the center of an elliptical galaxy called Markarian 421, lies some 400 million light-years from Earth. Although the core of this compact object has roughly the diameter of the solar system, its gamma ray output is about 10 million times the sun's total luminosity at all wavelengths, researchers say,

Using ground-based telescopes that detect visible light produced when high-energy gamma rays self-destruct in Earth's atmosphere, astronomers had previously examined likely extragalactic sources of this radiation, including quasars and active galaxies. But such gamma rays - with energies of about 1 trillion electron-volts - turned up only in the Milky Way, most notably in the Crab nebula (SN: 4/28/90, p.270).

When the Earth-orbiting Compton Gamma Ray Observatory (GRO) recently detected lower-energy gamma rays from 14 objects outside our galaxy, Trevor C. Weekes of the Whipple Observatory in Amado, Ariz. and his colleagues decided to examine several of the sources with a telescope that could infer the presence of gammas about 1,000 times more energetic.

Because gamma rays can't survive in Earth's atmosphere, only satellites such as the GRO can detect them directly, But the small detectors aboard such craft have difficulty recording the relatively low abundance of very energetic gammas, Weekes notes.

Though the data suggested that Markarian 421 was not the most intense high-energy gamma ray emitter among the 14 Sources GRO had identified, that galaxy does reside closest to Earth. And in observing this galaxy with a gamma ray telescope at the Whipple Observatory last spring, Weekes and his colleagues found the extragalactic emissions they had long been searching for. They report their results in the Aug. 6 NATURE.

Researchers have suggested that previous searches for trillion-electron-volt gamma rays from more distant galaxies had failed because such radiation is easily absorbed by the fog of infrared starlight in the intergalactic medium. Weekes notes that the gamma radiation generated by Markarian 421 probably comes from the edges of a jet believed to emanate from a quasar-like entity, called a BL Lac object, at the galaxy's center. Energetic protons colliding with other particles in the jet may generate the gammas, he speculates.

In a commentary accompanying the NATURE article, Francis Halzen of the University of Wisconsin-Madison says that Markarian's gamma ray output suggests that the galaxy may emit an even higher intensity of elusive subatomic particles called neutrinos. -R. Cowen