GEMS: The Gravity and Extreme Magnetism SMEX

GEMS will use an X-ray telescope 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 magnetars, 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 is forced to vibrate in only one direction. This happens when light scatters off of a surface, for example.

GEMS will reveal:

• How spinning black holes affect space-time and matter as it is drawn in and compressed by strong gravitational fields.
• What happens in the super strong magnetic fields near pulsars and magnetars.
• How cosmic rays are accelerated by shocks in supernova remnants.

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.

The heart of GEMS is a small chamber filled with gas. When an X-ray is absorbed in the gas, an electron carries off most of the energy, and starts out in a direction related to the polarization direction of the X-ray. This electron loses energy by ionizing the gas; the instrument measures the direction of the ionization track, and thereby the polarization of the the X-ray. The GEMS detector readout employs a time projection chamber to image the track.

Goddard's GEMS proposal is part of NASA's Explorer program. The proposal was submitted in response to NASA's Announcement of Opportunity for Small Explorers (SMEX) and Missions of Opportunity issued on September 28, 2007.

Six proposals, including GEMS, were selected for detailed concept study. In June 2009 GEMS was chosen to be the second of these missions to go forward, starting in 2010 for a launch in 2014.

NASA Goddard will be responsible for the GEMS instrument and the overall program management. Orbital Sciences Corporation, Dulles, Va., will be responsible for building the spacecraft and mission operations. ATK Space, Goleta, Calif., will build a boom to place the X-ray telescopes the proper distance from the detectors.

NASA's Ames Research Center, Moffett Field, Calif., will assist the project by managing the spacecraft development contract and by providing simulations of how the data will appear.

The University of Iowa will provide instrument calibration assistance, and will have students prepare an experiment that could be part of the mission. GEMS includes collaborators from universities including MIT, Johns Hopkins University, Cornell University, Rice University, the University of Oulu (Finland), North Carolina State University and Washington University.

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