'Quiet' star wasn't quiet after all, say scientists

by Marshall Space Flight Center

For more than two years the star was "quiet." Or so scientists 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. A study by scientists at the National Space Science and Technology Center (NSSTC) in Huntsville Ala., offers new insight into this star system, particularly during 32 months it appeared to be inactive. The NSSTC is a partnership with the Marshall Center, Alabama universities and federal agencies.

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.

A study by scientists at the National Space Science and Technology Center (NSSTC) in Huntsville Ala., the University of Southampton in England 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.

This 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 - optical emission lines caused by glowing material blown off the star into a disk around its equator.

"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 fellow NSSTC researcher Dr. Mark Finger, they now believe it was as active as ever.

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 infared 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.

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 wave of density 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.

"The new study, titled "A decade in the life of EXO 2030+375," was published in the May 1 issue of The Astrophysical Journal.

A collaboration that enables scientists, engineers and educators to share research and other facilities, the NSSTC is a partnership with NASA's Marshall Space Flight Center in Huntsville, Alabama universities and federal agencies. It focuses on space science, Earth sciences, materials science, biotechnology, propulsion, information technology and optics.