Astronomers observing a disk of matter spiraling into a black hole have discovered that portions of that disk periodically escape the dark demise by erupting into gaseous jets traveling at nearly the speed of light.
While scientists have observed these jets before, this is the first evidence that the small jets, which emerge about every 30 minutes, actually consist of matter from the inner portion of the disk. The scientists were able to observe the synchronized intervals of vanishing disk and jet emission using X-rays to detect the disappearance of the inner disk, and infrared and radio waves to spot the emergence of the jets.
The observations were made by scientists from MIT, the California Institute of Technology (Caltech) and NASA's Goddard Space Flight Center (NASA/GSFC). Some aspects of the observations were also studied and measured by scientists in the Netherlands and France. The research is being presented at a press conference during a meeting of the American Astronomical Society in Washington, DC this morning at 9:30am.
"When we saw the X-ray dips every half hour, we knew that the inner disk was disappearing," said Dr. Ronald Remillard, a research scientist at MIT's Center for Space Research. "But the real excitement came when we saw the infrared flares -- this showed that the jets were forming at the same time that the disk disappears. The force of gravity pulling on the inner disk is incredibly strong, and it's amazing that the jets can rip this matter away from the black hole."
"We're very excited about these results," said Dr. Stephen Eikenberry of Caltech. "Scientists have studied black hole systems for decades, and many theoretical models have predicted that the jets are linked to the inner accretion disk. However, the direct connection between the disappearance of the inner disk and the jet ejection has never been seen until now."
The jets were observed in a black hole system in our galaxy, called GRS 1915+105, which consists of a companion star orbiting around a black hole. Gaseous matter is drawn from the companion star into a relatively flat accretion disk, where it spirals around, moving ever closer to the black hole, until it reaches what scientists call the event horizon.
The event horizon is an invisible sphere encircling the black hole that works like a line of demarcation. Matter, and even light, crossing this line will never escape the pull of the black hole. The jets observed by the astronomers originate close to the inner edge of the accretion disk, but outside of the event horizon.
Dr. Eikenberry and his collaborators at Caltech observed the jets as infrared flares during three days in August 1997, using the Mt. Palomar 200-inch telescope. During those same three days, Dr. Remillard and Dr. Edward Morgan, another MIT research scientist, monitored dips in X-ray emissions from the disk using NASA's Rossi X-ray Timing Explorer (RXTE) satellite.
Dr. Jean Swank and the NASA/GSFC team observed similar dips using X-ray data obtained by RXTE, and again in September, at which time a team in France, led by Dr. Felix Mirabel, measured radio flares that represent the jets.
CAUSE STILL A MYSTERY
Although the scientists believe their research clearly indicates the link between the vanishing accretion disk and the jets, they do not yet understand what triggers the occurrence.
"There are many fine details in the X-ray dips that we may now seriously investigate to better understand the ejection mechanism," said Dr. Morgan. "In particular, there is a very unusual X-ray flash at the bottom of these dips in which the X-ray spectrum changes significantly. This may be the trigger for the rapid acceleration of the disk material."
"Since the disk-jet interaction is so poorly understood, we're hoping that further analysis of these observations will show us more details of what is happening so close to the black hole," said Dr. Eikenberry. "We're planning more detailed studies for the coming year which should give us even more clues as to the nature of these incredibly powerful events. Right now we still aren't even sure why these dips and ejections occur every half hour or so -- why not every week or every 30 seconds, for instance?"
The black hole in GRS 1915+105 became known to astronomers in 1992. Scientists estimate that this particular black hole has a mass equal to 10 Suns or more, all crushed by its own gravity into a tiny sphere contained within the event horizon, which itself has a radius of about 20 kilometers.
As the black hole pulls gaseous matter from the atmosphere of a companion star into the accretion disk, the matter spirals in toward the event horizon like water going down a drain. The gas in the disk heats up dramatically due to the large acceleration and friction, and just before entering the event horizon, it reaches temperatures of millions of degrees, causing it to glow in X-rays. Dips in the X-ray emissions reveal that matter is vanishing, or being thrown out of the system.
"The repeated ejections are really incredible," said Dr. Craig Markwardt, a member of the NASA/GSFC team. "The system behaves like a celestial version of Old Faithful. At fairly regular intervals, the accretion disk is disrupted and a fast-moving jet is produced."
"But this jet is staggeringly more powerful than a geyser," said Dr. Swank. "Every half-hour, the black hole GRS 1915+105 throws off the mass of an asteroid at near the speed of light. This process clearly requires a lot of energy; each cycle is equivalent to six trillion times the annual energy consumption of the entire United States."
The jets from GRS 1915+105 were first documented in 1994 by Drs. Mirabel and Luis Rodriguez of the Center d'Etudes de Saclay in France. They observed radio emission from jets and determined that the jets were traveling faster than 90 percent of the speed of light, or roughly 600 million miles per hour. Since NASA'S RXTE began monitoring the sky in 1996, the exceptionally chaotic behavior of GRS 1915+105 has been chronicled many times.
"This research may help us understand many other types of systems with jets," said Dr. Robert Nelson of the Caltech team. "Astronomers have found jets in a wide range of objects, from quasars -- incredibly powerful objects seen out to the edge of the observable universe -- to young protostars. The half-hour spacing between the ejections may be telling us that what we had thought were smooth, continuous outflows may in fact be intermittent explosions."
Funding for the MIT portion of this research was provided by NASA.
Web users can listen to the X-ray light curves turned into sound. The low-frequency wind-like noises represent X-ray emissions, the silent interruptions represent jet ejections and the high-pitched whistling represents quasiperiodic oscillations in X-rays. Audio is heard at 100 times actual speed. Dr. Morgan recommends the following tapes: Oct. 30, 1997; Aug. 15, 1997; and Oct. 7, 1996.
A version of this article appeared in MIT Tech Talk on January 7, 1998.
