It seems all but certain that black holes lurk near the centers of distant galaxies, possibly even our own Milky Way. Since the early part of the 20th century, when astronomer Heber Curtis first trained his telescope on the object M87, scientists have observed distant objects in the sky with jets of material (Curtis called them "Straight Rays") being spewn out at velocities approaching the speed of light. But within the past few years, astronomers have discovered that powerful accelerators of material are also found close to home, in our own galactic neighborhood, as nearby black holes with jets.
Dr. B. Alan Harmon of NASA's Marshall Space Flight Center will be presenting the results of multi-wavelength studies of these "Superluminal Sources" - so-named because relativity effects cause the jets of material to appear to be moving faster than the speed of light - in an invited presentation on Wednesday, June 11, at the 190th meeting of the American Astronomical Society in Winston-Salem, North Carolina.
In order to make the jets like the one shown at the upper-right of the picture above, deep in the nucleus of a distant galaxy, something must accelerate particles to very high energies. The only way we know of to supply that energy is from the immense gravitational energy reservoir of a black hole. However, it can be difficult to see down into the center of a galaxy (the bright spot at the lower left of the above picture) to get the details of what is going on due to the surrounding stars and dust and the great distance these objects are away from Earth.
What astronomers needed was a way to observe such jets at close range. This opportunity came in 1994 with the discovery that two X-ray sources, which became very bright that year, were also accelerating matter in the form of jets. The most surprising aspect of these sources is that they were in our own Galaxy, practically in the neighborhood compared to distant galaxies. In addition, the jets, which can be seen in radio telescopes as a string of bright "blobs", actually appear to move on the sky in a few hours. Traveling at near the speed of light, these blobs carry huge amounts of energy away from the central object, which astronomers believe is a black hole several times more massive than the Sun.
These X-ray sources are known by the technical names GRS 1915+105 and GRO J1655-40, after the space-based Russian Granat and U.S. NASA Compton Gamma Ray Observatory that discovered them, respectively, and their location in the sky. These sources do not represent the powerful central engine in the nucleus of a distant galaxy, but in fact are a special breed of binary X-ray source which we knew little about until recently.
It seems that GRS 1915+105 and GRO J1655-40 may be scaled-down versions of the distant galactic nuclei (not a few million solar masses, but more like 3 or 10 times the mass of our Sun), which reside in a binary system, along with a low mass star from which matter is accreted onto the black hole. The material which is drawn off the star doesn't fall straight into the black hole, however, but instead forms in a frisbee-like shape surrounding the black hole, in an objects astronomers call an "accretion disk." One unusual thing about the jets is that they do not seem to be common to all black hole binaries. If they were commonplace, we probably would have known about them earlier. Another unusual thing is that, occasionally, whenever the X-ray source flares into a bright state, the jets are detected in the radio band of the electromagnetic spectrum, while at other times they are not seen.
The transient production of jets is puzzling, and is still not understood by scientists. What it means is the "Galactic Accelerator" can turn itself on and off. This would be very difficult to observe in an extragalactic source because of the restriction imposed by the speed at which light travels. Consequently, the distant extra-galactic jets are observed to change very slowly from one year to the next.
If a black hole binary can make jets occasionally, what are the conditions necessary for jets to be produced? The answer may be in the accretion disk and surrounding regions, which shine in X-rays during outburst. The trigger for jet production clearly seems to be connected to the appearance of X-rays and gamma rays, which tells us that material has been, or is flowing from the star into the accretion disk.
Observers have found that the X-ray and gamma ray radiation has special properties when jets are present that do not look quite the same as in other black hole systems. The patterns seen in observation suggest that accreting material may be feeding the jets. The ability to turn jets on and off could be related to how much material is accreted, and how much energy can be stored and for how long.
It remains for theorists to test models for the disk-jet system against the observations. These models have many features which are not directly observable, such as the size of the accretion disk and the strength of electromagnetic fields near the black hole. Once we have a good idea of these physical parameters, models can yield precise predictions. For example, even a modest magnetic field would be able to release the energy stored in the disk, and produce the collimated outflow we see as a jet.
We are fortunate, at least for a while, that GRO J1655-40 and GRS 1915+105 remain active and are being monitored daily from the ground and in space by a host of observatories. Observers around the world are also looking for new objects with jets, and in the very near future may clear the way for understanding of these unusual binary systems, and possibly even the central engines of active galaxies. Having such objects nearby in our "neighborhood" for a close look makes a big difference in helping us understand what powers these exotic jets.