Jet astronomy: tracing the fire from black holes.

Jets of high-speed particles riddle the celestial canvas. They are generated by dramatically different objects: nascent stars still in the process of coalescing, massive stars that have collapsed to form the smallest of black holes, and supermassive black holes weighing as much as a billion suns. Astronomers have long dreamed of having one theory that could explain the origin and evolution of all these jets. New observations are bringing that vision one step closer to reality. For the first time, scientists have traced the slowing and dimming of X-ray-emitting jets from a small black hole. Monitoring the jets with the orbiting Chandra X-ray Observatory over the past 2 years, researchers have viewed the jets as they traveled at half the speed of light, slowed down, and faded.

The jets emanate from the region surrounding a small black hole within the Milky Way that is about 10 times as massive as the sun. Compared with supermassive black holes, which can weigh as much as a billion suns and last millions of years, small black holes have a limited fuel supply and their jets have a much shorter lifetime.

"We watched, in a few years, developments that would have taken thousands of years to occur around a supermassive black hole in a distant galaxy," says Stephane Corbel of the University of Paris VII and the French Atomic Energy Commission in Saclay, France. Corbel and his colleagues report their findings in the Oct. 4 Science. They also describe details of their study in two upcoming articles in the Astrophysical Journal.

The observations, Corbel notes, are like a time-lapse movie of the evolution of the jets. Moreover, theorists have calculated that the processes producing the jets from small, nearby black holes are the same as those that generate longer-lived, higher-energy jets associated with more-distant supermassive black holes, notes astronomer Cole Miller of the University of Maryland in College Park. The brilliant beacons whose radiation streams out of faraway galaxies are known as quasars.

Corbel's team relied on Chandra and radio telescopes to study two jets shooting out in opposite directions from a double-star system that lies in the Milky Way some 17,000 light-years from Earth. Scientists have classified one member of this stellar partnership as a black hole, the ultimate ember of a long-dead star; the other is an ordinary star from which the black hole steals matter.

Though smaller and more rapidly changing, the jets of this system resemble those emanating from much bigger black holes. In fact, because these jets form and fade over just a few years, they can serve as a Rosetta stone for deciphering the evolution of quasars, which would take thousands of generations of astronomers to directly observe.

JET SPOTTING Observations of the jet-emitting black hole began in 1998, when the Rossi X-ray Timing Explorer (XTE) spacecraft detected an X-ray flare from this stellar system. The flare, which lasted for a day, was a sign that the system's black hole had been dining voraciously on its companion star. As gas from the companion star spirals onto the so-called accretion disk surrounding the black hole, the material emits X rays and other radiation.

In a process that's still not well understood, jets may also shoot out from the vicinity of an accretion disk. Twin jets emitting radio waves, each moving in the opposite direction, were found within 4 light-days of the black hole just days after the observation of the flare.

Astronomers theorize that the accretion disk sculpts the jets. According to this scenario, energetic particles spewing outward from the neighborhood of the black hole take the path of least resistance. Rather than plowing through the material of the accretion disk, these particles shoot out as twin beams perpendicular to the disk's plane.

Corbel and his collaborators, who include John Tomsick of the University of California, San Diego and Philip E. Kaaret of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., became interested in the stellar system XTE J1550-564 when XTE observations earlier this year recorded new X-ray activity. The team then examined the black hole system with a radio telescope, the Australia Telescope Compact Array in Narrabri, and searched for the system in X-ray images taken by Chandra.

The team found a pair of oppositely directed X-ray jets about a light-year away from the location of the radio-wave jets detected by other researchers in 1998. Corbel's team compared X-ray images taken by Chandra in 2000 with new images from last March and June. A component of the motion of one of the jets points toward Earth, while the other jet is moving away from Earth. During the 2 years between the Chandra observations, the X-ray jets moved about 3 light-years apart and both gradually decelerated. The 2002 images show hot spots, which represent places where the jets have slowed and given up energy as they crash into dense interstellar gas.

The gradual slowing of the jets, the distance they've traveled, and the relatively recent development of X-ray hot spots suggest that for most of their journey, the jets have passed through remarkably low-density regions of space, comments Michael P. Rupen of the National Radio Astronomy Observatory in Socorro, N.M. Following the stellar collapse that formed the black hole, a massive wind may have cleared out material, creating a virtually empty bubble, he suggests.

In most other systems that astronomers have observed, X-ray jets shoot out and fade without gradually decelerating, Rupen says. If there were much of anything in the vicinity of XTE J1550-54, its jets would have immediately slowed down and shown the hot spots seen in jets from supermassive black holes. These larger jets shoot out from galaxies into the intergalactic medium.

BLIND SPOTS Although the theory of general relativity holds that a jet moving toward Earth at a substantial fraction of the speed of light should appear brighter than its equal but oppositely directed counterpart, just the reverse shows up in the Chandra images.

One explanation is that the black hole has poured more energy into the jet headed away from Earth. Another possibility is that this jet, once it journeyed past the low-density region surrounding its parent black hole, encountered a denser interstellar medium than did the jet that's getting closer to Earth. That would have caused the jet moving away from Earth to radiate more X rays. The cometlike shape of this brighter jet suggests that it is indeed interacting strongly with the interstellar medium, says Kaaret.

Both jets now have dimmed, and the one pointing has all but disappeared. "This is the first time we have observed a jet from the initial explosion until it slowed and faded," says Tomsick.

"Astrophysical jets are an extremely common phenomenon, and they provide a way for us to understand the workings of enigmatic black holes," comments Kimberly A. Weaver of NASA's Goddard Space Flight Center in Greenbelt, Md. "Having a case where we can watch the entire life cycle of the jets is critical to helping us understand jets in general, and this can only be done for these nearly stellar-mass black hole systems."

One thing the new findings don't do, Weaver notes, is to shed light on the origin of the jets. "Unfortunately, we didn't actually see the jets being produced. We saw a bright X-ray flash, which signaled the flaring of the black hole, and then later saw the jets appear far away from the black hole," she says. "We didn't have the opportunity to watch what was happening when the jets were forming close to the black hole."

The observations, notes Rupen, "illustrate the final stages of the rapid evolution [of these jets], as they crash into the interstellar medium, expiring in a blaze of glory only a few years after their birth."

To take pictures of the births of the jets, Kaaret says, astronomers will need to continuously monitor sources like XTE J1550-54 with future X-ray telescopes that are even sharper than Chandra. Such observations, adds Weaver, may reveal the role that a black hole's magnetic field and other features play in the origin and acceleration of the jets. And that in turn may give astronomers the data they need to develop a single theory to explain all the black hole spitfires, from the weakest jets to the most powerful quasars.