Weekly Science Update

Wednesday, June 11, 2008
Gamma Rays from a Binary Star
Gamma-rays are the most energetic known form of light (that is, of electromagnetic radiation). A gamma ray is by definition at least a hundred thousand times more energetic than an optical light photon, and some gamma rays have been detected packing energies a billion times this or even more - so called VHE (very high energy) gamma rays. Remarkably, astronomers have recently discovered a handful of unusual stars that radiate these most potent of gamma rays, presumably in extreme environments very different from anything familiar to us in the sun or ordinary stars. As a result, a new window is opening onto the hot and violent universe.

Scientists think that these very high energy gamma rays are somehow produced in the environment of the winds or outflowing jets of compact objects, the ultra-dense ("compact") remains of massive stars that expire in supernova explosions. There are two kinds of compact objects produced in supernovae: black holes and neutron stars - stars made up predominantly of neutrons with the mass of a sun in a volume only about 10 kilometers in radius. The winds or jets from the environments of these objects are known to be able to accelerate charged particles to very close to the speed of light, and when light scatters off such energetic particles it becomes energized as well, often turning into VHE gamma rays. At least this is the most popular current theory.

A team of ten SAO astronomers and sixty-eight of their colleagues, led by a student at SAO, Victor Acciari, has used the four VERITAS (Very Energetic Radiation Imaging Telescope Array System) telescopes of the CfA's Fred L. Whipple Observatory to study a bright cosmic source of VHE gamma rays. The object, discovered two years ago as being a source of VHE gamma rays, is actually a binary star consisting of a massive hot young star with a circumstellar disk and an orbiting compact companion. The precise nature of the companion is a mystery, but scientists hope that a study of its VHE gamma rays will enable them to categorize it accurately. There are only three known binary stars producing VHE gamma rays.

The astronomers observed the object with VERITAS for six months during 2006-2007. Previous optical and radio research had found that the compact object orbits the massive companion star every 26.5 days, yielding the time-varying emission that we see. The VERITAS observations found that the VHE gamma rays also time-vary, but in a way that produces detectable VHE gamma rays only when the two objects are farthest apart from each other in their elliptical orbits. The scientists had hoped that this curious behavior would enable them to discriminate between models, but with the current sensitivity either jets from a black hole or shocked winds from a neutron star could reproduce these observations. Scheduled improvements to VERITAS should enable the team to detect VHE gamma rays throughout the full period of the orbit, however, and thus to eliminate one or the other -- or both -- candidate models. The new results thus highlight the amazing strides being made in this new field of astronomy, and our ability to understand some of the most extreme and unusual conditions in the universe.

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