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Mystery gamma-ray source pinned to vampire stars

by Staff Writers
Paris (AFP) Jan 9, 2008
An intriguing source of gamma rays linked to the high-energy collision of fundamental particles in the centre of our galaxy has been traced to vampire-like binary stars, a study says.

The big smash comes from negatively-charged electrons colliding with their corresponding positively-charged "antiparticle," known as positrons.

When electrons and positrons meet, the event is very brief, for they destroy -- "annihilate" -- each other in a flash of energy.

In 1970s, astronomers discovered a narrow blast of electron-positron annihilation emanating from near the centre of the Milky Way, detectable in the 511 kiloelectron volt (keV) range of gamma-ray energy.

But the source of the phenomenon has been strongly debated.

Some astrophysicists suggested it could be stellar nucleosynthesis, the seething, searing reaction within stars by which the nuclei of heavier elements are forged.

Others proposed the annihilation stream came from neutron stars, black holes and white dwarves. A minority of opinion ventured it came from "dark matter" -- a strange and elusive mass that is believed to wash across the cosmos.

European and American skywatchers, poring over four years of spectroscopic data from the European Space Agency's orbital gamma-ray observatory, Integral, now believe they have the answer.

The main source appears to be low-mass X-ray binaries (LXMBs) -- two stars that spin around each other in a sort of vampirical death dance.

One of the stars is a normal star, but the other one is a star that has collapsed upon itself, becoming a neutron star or a black hole, one of the mightiest forces in the Universe.

The gravitional pull from the collapsed star is such that it sucks material from the normal star, via a so-called accretion disk.

Trillions of positrons escape each second from so-called "hard" LXMBs, according to the study, released on Wednesday by Nature, the weekly British science journal.

This would explain the stream of collisions, which happens when positrons ultimately collide with electrons in the vast distances between stars.

"There may be no necessity to invoke exotic explanations such as the annihilation of dark matter," it says.

Previous research has suggested there could be as many as a hundred LXMBs in the Milky Way.

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LIGO Sheds Light On Cosmic Event
Pasadena CA (SPX) Jan 04, 2008
An analysis by the international LIGO (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration has excluded one previously leading explanation for the origin of an intense gamma-ray burst that occurred last winter. Gamma-ray bursts are among the most violent and energetic events in the universe, and scientists have only recently begun to understand their origins.

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