This discovery raised the question of what could be causing the connection. “High-frequency QPOs are likely from matter racing around the black hole, glowing like lightbulbs on a merry-go-round,” said Homan. “Of course, matter is moving much faster around a black hole than on any amusement park attraction. We see frequencies of hundreds of hertz, or hundreds of revolutions of the disk per second. That’s quite a ride.”
A black hole is a region in space where gravitational forces are so great that not even light can escape. Gas and dust funnel towards a black hole in an accretion disk, swirling around and into the void like water down a drain. This process of accretion generates copious amounts of light – predominantly X-ray radiation, particularly in the innermost (hottest) regions of the accretion disk. Near the black hole, gravity is rather intense, but light still can muster an escape by climbing out of the black hole’s gravitational “well,” losing energy during the climb. Thus, scientists can study black hole activity with X-ray telescopes like the Rossi Explorer.
Lower-frequency QPOs are a deeper mystery. These are typically 1 to 10 hertz, and they’re quite common in many binary star systems with black holes. Miller and Homan say that in the GRS 1915+105 system, the slower QPO could be the frequency of a spacetime warp. In that case, the low-frequency QPO flickering is caused by the fabric of space itself churning around the black hole in a wave. This is known as Lense-Thirring precession, which evolves out of Einstein’s theory of general relativity.
Imagine the accretion disk as a music CD. (The Beach Boys come to mind.) The wave produced by the spacetime warp would increase the surface area of the flat disk. The broadness of iron K lines depends on surface area. So, this momentary increase in surface area, “flickering” at a frequency of 1 to 2 hertz, could explain the repetitive changes observed in the iron K line. Each time the hot iron gas encounters the spacetime warp, the light gets a jolt and the broad iron K line changes its appearance.
Miller and Homan caution that this is only one explanation of their observation, and that other explanations may be possible. What is clear, the scientists say, is that they are seeing a connection between QPOs and the broad iron K line, and this in turn means that scientists are probing more closely to black holes than ever before.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.
Text for this article comes from a Harvard press release.