GEMS will use X-ray telescopes to explore the shape of space that has been distorted by a spinning black hole's gravity, and probe the structure and effects of the formidable magnetic field around neutron stars, dead stars with magnetic fields trillions of times stronger than Earth's.
Current missions cannot do this because the required angular resolution is far beyond what is technically feasible and, in the case of magnetic field imaging, can't do this because magnetic fields are invisible. GEMS will use a new technique to accomplish what has been impossible until now. It will build up a picture indirectly by measuring the polarization of X-rays emitted from these violent regions. This will open new discovery space because GEMS is orders of magnitude more sensitive than previous X-ray polarization experiments.
X-rays are just a powerful kind of light. Like all light, X-rays have a vibrating electric field. When light travels freely through space, it can vibrate in any direction. However, under certain conditions, it becomes polarized, meaning it vibrates in only one direction. This happens when light scatters off of a surface, for example.
GEMS will be able to tell the shapes of the X-ray-emitting matter trapped near black holes better than existing missions can -- in particular, whether matter around a black hole is confined to a flat disk or puffed into a sphere or squirting out in a jet. Since X-rays are polarized by the space swirling around a spinning black hole, GEMS also provides a method of determining black hole spin independent of other techniques.
Attempts to study X-ray polarization date to the beginning of X-ray astronomy, but so far there has been only one detection of polarized X-rays from outside the solar system. Owing to its much greater sensitivity, GEMS will open new phase space.
Photo right : Hubble Space Telescope records the beauty surrounding the black hole at the heart of the Circinus Galaxy.