Washington, June 30: Every 3.4 years, a unique combination of stars pass each other at a distance closer than Venus orbits the Sun, and the event is marked by a sharp increase in gamma rays, the most extreme form of light.

Late last year, the closest encounter yet took place between a pair of mismatched stars in the southern constellation Crux

Now, a team using NASA's Fermi Gamma-ray Space Telescope to observe the 2010 encounter reports that the system displayed fascinating and unanticipated activity.

Few pairings in astronomy are as peculiar as high-mass binaries, where a hot blue-white star many times the Sun's mass and temperature is joined by a compact companion no bigger than Earth -- and likely much smaller.

Depending on the system, this companion may be a burned-out star known as a white dwarf, a city-sized remnant called a neutron star (also known as a pulsar) or, most exotically, a black hole.

Just four of these "odd couple" binaries were known to produce gamma rays, but in only one of them did astronomers know the nature of the compact object. That binary consists of a pulsar and a 10th-magnitude Be-type star. The pair lies 8,000 light-years away.

Late last year, as the pulsar headed toward its massive companion, the Large Area Telescope (LAT) aboard Fermi discovered faint gamma-ray emission.

Stranger still, the system's output at radio and X-ray energies showed nothing unusual as the gamma-ray flares raged.

"The most intense days of the flare were Jan. 20 and 21 and Feb. 2, 2011," said Aous Abdo, a Research Assistant Professor at George Mason University in Fairfax, Va., and a leader of the research team.

"What really surprised us is that on any of these days, the source was more than 15 times brighter than it was during the entire month-and-a-half-long first passage," he added.

The study has been published in the Astrophysical Journal Letters. (ANI)