An astronomical binary system has two stellar objects (usually stars) which are so close that their gravitational interaction causes them to orbit about a common center of mass. Since black holes have incredible amounts of mass, their binary systems are the focus of many studies. This is due to the systems’ tendency to produce large swings in X-ray emission and blast jets of gas at speeds exceeding one-third that of light. This activity is fuelled by gas, pulled from the normal star and hurled spiraling toward the black hole, eventually piling up in a dense accretion disk.
This latest study, conducted at the University of California, Berkeley, and appearing in the December 10th issue of The Astrophysical Journal Letters, targets GX 339-4, a low-mass X-ray binary. It is located about 26,000 light-years away in the constellation Ara and it was chosen due to its high activity – having four outbursts in the past seven years. The research team, led by John Tomsick, observed that every 1.7 days an evolved star (no more massive than our sun) orbits a black hole estimated at 10 solar masses, indicating irregular gravitational activity.
“When a lot of gas is flowing, the dense disk reaches nearly to the black hole,” said Tomsick. “But when the flow is reduced, theory predicts that gas close to the black hole heats up, resulting in evaporation of the innermost part of the disk.” Never before have astronomers shown an unambiguous signature of this transformation.
The research goes back as far as September 2008, nineteen months after the system’s most recent outburst. GX 339-4 was observed using the orbiting Suzaku X-ray observatory, which is operated jointly by the Japan Aerospace Exploration Agency (JAXA) and NASA. Additional information was gathered using NASA’s Rossi X-ray Timing Explorer satellite. The joint data indicated conclusive results: the system was faint, yet in an active state. Moreover, radio data retrieved by the Australia Telescope Compact Array confirmed the results, according to which the GX 339-4’s jets are powered up.
In order to study GX 339-4, X-ray emission was closely observed. It is known that X-ray photons emitted from disk regions closest to the black hole naturally experience stronger gravitational effects. Furthermore, when X-rays lose energy they produce a characteristic signal. When observing GX 339-4 at its brightest state, the astronomers traced the X-rays to within about 20 miles of the black hole, but at low brightness the inner edge of the accretion disk retreats as much as 600 miles.
Despite the system’s faintness, Suzaku was able to measure a critical X-ray spectral line produced by the fluorescence of iron atoms. Team member Kazutaka Yamaoka, from Japan’s Aoyama Gakuin University, explained how it was possible: “Suzaku’s sensitivity to iron emission lines and its ability to measure the shapes of those lines let us see a change in the accretion disk that only happens at low luminosities,” he said. These observations helped the scientists understand the nature of black holes systems such as GX 339-4.
The team faced other obstacles, including evasive emissions. “We see emission only from the densest gas, where lots of iron atoms are producing X-rays, but that emission stops close to the black hole – the dense disk is gone,” explained Philip Kaaret from the University of Iowa. Another obstacle was the varying temperature: while the dense inner disk has a temperature of about 20 million degrees Fahrenheit, the thin evaporated disk may be more than a thousand times hotter. “What’s really happening is that, at low accretion rates, the dense inner disk thins into a tenuous but even hotter gas, rather like water turning to steam,” said Kaaret.
The study has confirmed the presence of low-density accretion flow in these systems and it has shown that GX 339-4 can produce jets even when the densest part of the disk is far from the black hole. “This doesn’t tell us how jets form, but it does tell us that jets can be launched even when the high-density accretion flow is far from the black hole,” Tomsick said. “This means that the low-density accretion flow is the most essential ingredient for the formation of a steady jet in a black hole system.”
TFOT has covered other stories relating to black holes, such as the detection of black hole eruption remnants by the Chandra X-Ray Observatory, and that of the baffling supernova, which strengthened a theory that all stars end their lifetime as black holes. Other related TFOT stories include a piece about the imaging of a black hole in the Milky Way by an international team led by an MIT astronomer, and the proposition of black holes’ limited mass made by researchers from the US and Chile.
For more information about the GX 339-4 study, see NASA’s press release.