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`Quiet` star wasn`t quiet after all, say astronomers

July 10, 2002

For more than two years the star was `quiet`. Or so astronomers thought. But the X-ray pulsar EXO 2030+375 was abuzz with activity. Scientists simply lacked the ability to `hear` it over the hum of a nearby black hole.

Now a study by scientists at the University of Southampton, the National Space Science and Technology Center (NSSTC) in Huntsville USA, and the University of Valencia in Spain, offers new insight into the EXO 2030+375 star system, particularly during a period of 32 months, from August 1993 to April 1996, when it appeared to be inactive.

Pulsars are tiny rapidly rotating neutron stars - the material left over after normal stars burn out and collapse under their own gravity, until just a sugar cube-sized piece can weigh as much as 200,000 elephants. The type of pulsar studied, a transient accreting X-ray pulsar, orbits a massive star eight to 15 times the mass of our Sun, with a distinctive signature - emission lines caused by glowing material blown off the star into a disk around its equator. Unfortunately, observations of this system are made difficult by the close proximity on the sky of the famous black hole binary Cygnus X-1.

`The nearby black hole, Cygnus X-1, makes a lot of noise,` said Dr Colleen A Wilson-Hodge, a NASA astrophysicist at the NSSTC. `If this black hole were in the frequency range where we could hear it, it would hum.`

This `humming` of the black hole, scientists now believe, was loud enough to give the false impression that the X-ray pulsar EXO 2030+375 was inactive. But thanks to a new technique developed by Wilson-Hodge and collaborators, they now believe it was as active as ever. `We thought it had gone to sleep, but actually it was just hidden by its noisy neighbour,` said Dr Malcolm Coe, University of Southampton.

The new technique analyzes existing data, but uses different mathematical equations to compensate for the hum of the nearby black hole.

The researchers used data from a special device - the Burst and Transient Source Experiment, better known as BATSE. BATSE was designed to observe gamma rays, a powerful form of energy invisible to the naked eye and undetectable by most telescopes that - unlike BATSE - `see` only visible light.   

This instrument was mounted on NASA`s Compton Gamma Ray Observatory, which orbited Earth from 1991 to 2000. Unlike many telescopes that monitor cosmic objects one at a time, BATSE had a wider range, observing the entire sky for nearly a decade, what Wilson-Hodge believes was a key factor in making the surprise discovery.

`After we accounted for the hum of Cygnus X-1, we realized the pulsar was active during this time frame after all,` said Wilson-Hodge. `The signal was merely fainter, as we can now observe using the new techniques.`

Correlating the gamma ray observations with optical and infrared observations taken by Dr Malcolm Coe of the University of Southampton and Dr Juan Fabregat of the University of Valencia in Spain showed the researchers why the pulsar became fainter - the disk of material around the companion star became less dense, giving the pulsar less material to consume.

`Our data (taken from telescopes in the Canary Islands) suggest the star decided to have a rest for a while and produced a much weaker stellar wind for a couple of years. We don`t yet exactly know why this happened, but our own Sun goes through activity cycles every 11 years and this may give us a clue,` said Coe.

In addition to discovering the pulsar wasn`t `quiet` during those 32 months, the research led to another discovery about the star system. `For the first time ever, we`ve been able to observe a density wave in gamma-rays,` Wilson-Hodge said.

Density waves - compression waves, like sound, that travel through cosmic objects and cause a collection of gas - have been observed in the optical wavelength, but never in the X-ray wavelength, until now.

Optical observations of emission lines from hydrogen by Coe and his collaborators also show evidence for a density wave in EXO2030+375. As this density wave moves throughout the disk of material around the companion star, the optical emission lines change shape and the X-ray activity occurs at a different place in the orbit.

These new techniques have the potential to unlock secrets of other star systems. `With every advance - whether in how we obtain the data, or how we analyze the data - we obtain yet another piece of the cosmic puzzle,` Wilson-Hodge said. `Every breakthrough brings us that much closer to understanding star activity across the universe.`

Southampton, University of