X-ray observations of a new black hole candidate contain what appears to be a unique signature that was predicted three years ago by physics Professor Robert V. Wagoner's astrophysics group at Stanford University. The signature may allow scientists to determine the mass and rotation rate of these exotic cosmological objects with an unprecedented level of accuracy.
Observations of the object - GRS 1915+105 - were announced today at the annual meeting of the American Astronomical Society in Toronto. They were made by Edward Morgan and Ronald Remillard of the Massachusetts Institute of Technology with Jochen Greiner of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, using the Rossi X-ray Timing Explorer satellite.
Their observations include a feature - a 67 Hertz (cycles per second) peak in the power spectrum - that is consistent with predictions made by the Stanford group. The researchers proposed that the extremely powerful gravity produced by a massive black hole should allow the disk of dust and gas that forms around it to vibrate in unique ways. Further, these vibrations should affect the radiation coming from the object in a detectable fashion.
"The new observation fits very well with our model," said Stanford graduate student Dana E. Lehr, who will discuss this interpretation on Jan. 15 at the AAS meeting.
Black holes are thought to be collapsed stars with a gravitational pull so strong that nothing can escape, not even light. Astronomers have detected about 20 celestial objects that they think might contain black holes, including the center of the Milky Way galaxy.
Before it is swallowed by the black hole, the matter that comes within its gravitational grasp should first form into a flattened disk in much the same fashion that the planet Saturn has pulled the material in its vicinity into a series of rings.
By modeling the gravitational field of a black hole in three dimensions, Wagoner - working with Lehr and senior research associate Alexander S. Silbergleit at Stanford and Michael A. Nowak and Mitchell C. Begelman at the University of Colorado, Boulder - determined that such an "accretion disk" should oscillate in certain ways that differ distinctly from the vibrations that characterize similar disks surrounding stars and other celestial objects. According to their analysis, these vibrations are made possible by unusual relativistic effects that result from the extreme distortions of space and time that occur in the vicinity of a black hole.
For stellar-sized black holes the physicists estimate that these fluctuations should appear as X-rays and vary on a time scale of milliseconds. For supermassive black holes - those weighing in at 100 million times the mass of the sun, which are thought to be the power source for quasars and other highly luminous galactic cores - the disk oscillations should appear in the visible to ultraviolet portion of the spectrum and vary daily.
Unlike other proposed signatures for black holes, the period of these vibrations is determined almost solely by the mass and the rotation rate of the central black hole, Wagoner said. So they provide a relatively direct way to calculate these two defining characteristics.
Based on their model, the vibrations seen in GRS 1915+105 correspond to a black hole with a mass ranging between 10 and 36 times the mass of the sun, depending on how fast the black hole is rotating. If the black hole is not rotating at all, its mass would equal 10 solar masses. If it is rotating as fast as relativity allows, its mass would be more than three times greater. The scientists need to see a second vibrational mode before they can separate mass from rotational effects.
Because the frequency of these vibrations is determined by the mass and rotation rate of the black hole, the scientists expect them to remain constant despite major changes in the conditions in the accretion disk. That is how the 67 Hertz feature in GRS 1915+105 behaves: It remains at a constant frequency through major variations in the object's X-ray brightness.
The phenomenon is similar in many ways to the oscillations that have been seen on the surface of the sun and a few other stars. Such observations have provided valuable information about the interior of these objects and have spawned new fields of study now called "helioseismology" and "asteroseismology." As a result, the scientists have dubbed their approach "diskoseismology."
According to Lehr, the researchers have learned of another unpublished black hole candidate observation that also appears consistent with their diskoseismology model. In this case the mass of the black hole has been inferred from the motion of its companion star. The vibration mode that has been detected predicts that the black hole is spinning at a rate close to the maximum allowed by general relativity, which corresponds to a rotation rate of about 30,000 revolutions per minute.
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