RX J1131-1231, a monster black hole, has been observed rotating at over half the speed of light. The object lies six billion light years from Earth. This is the first time direct measurements have been taken of the rotational speed of a distant black hole.
As supermassive black holes spin, they draw material toward their center, through gravity. This material forms into a plate-shaped object around the object, called an accretion disk. As gas and dust spirals in, it heats, emitting energy in the form of X-rays. The form of this radiation is characteristic of black holes, acting as a method of identifying the bodies. Sometimes, as in the case of RX J1131, this heating can produce a brilliant, energetic object called a quasar.
Observations were carried out on the object using the Chandra X-ray Observatory, as well as the XMM-Newton telescope, operated by the European Space Agency (ESA). A giant elliptical galaxy caught between that quasar and the Earth acts as a type of natural telescope, through a process known as gravitational lensing. Just as a lens or mirror bends and directs light, objects the size of galaxies can warp light passing near them.
"Because of this gravitational lens, we were able to get very detailed information on the X-ray spectrum - that is, the amount of X-rays seen at different energies - from RX J1131. This in turn allowed us to get a very accurate value for how fast the black hole is spinning," Mark Reynolds, co-author of the study, said.
Mass and spin are two of the most important characteristics to determine when classifying these objects. It is fairly easy to determine the mass of a black hole, but spin is more difficult to measure.
For RX J1131, spin was calculated by looking at the X-rays coming from the inner edge of the accretion disk. Gravity from the black hole is so great that it bends space around it enough to significantly alter the wavelength of radiation emanating from the disk. By measuring the severity of that change, astronomers can measure the distance between the inner edge of the disk and the center of the black hole.
"We estimate that the X-rays are coming from a region in the disk located only about three times the radius of the event horizon, the point of no return for infalling matter. The black hole must be spinning extremely rapidly to allow a disk to survive at such a small radius," Jon Miller, who co-authored the paper, said.
A rotational speed of over half the speed of light suggests RX J1131 grew through collisions of galaxies. One that developed through random accretion of material would likely exhibit a much slower rotational rate.
Astronomers believe studying the black hole will provide information on the life cycles of these mysterious objects. The light we see from this body shows how RX J1131-1231 behaved six billion years ago.
