Polarized X-rays may reveal black hole signatures

17 Jun 2002

How can astronomers be sure that black holes exist? The motions of stars and gas near presumed black holes provide provisional evidence, but additional assurances are desirable. Paul Wiita and his colleagues at Georgia State suggest that the polarization (the preferential orientation) of x rays coming from some celestial objects such as x-ray binaries and active galactic nuclei can be used to demonstrate the presence of a black hole.

These x rays are thought to arise when material pulled from nearby stars toward black holes piles up on (and heats up) the accretion disk hovering closely about the hole. According to Wiita, light coming from the inner part of the disk will not only be more energetic than light from further out on the disk, but will show greater changes in polarization as well. Furthermore, the degree of the polarization should be enhanced in a characteristic way by the lensing action of the black hole's huge gravitational field. It will, however, be difficult to test this hypothesis in the near future since the apparatus for measuring polarization was recently dropped from plans for the orbiting AXAF x-ray telescope, to be launched in 1998.