PRAXyS Science

Principal Investigator:
Keith Jahoda, NASA's Goddard Space Flight Center, Greenbelt, Md.

PRAXyS, the Polarimeter for Relativistic Astrophysical X-ray Sources, 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 magnetic field around neutron stars.

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. PRAXyS 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. This will open new discovery space because PRAXyS 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. This means it is forced to vibrate primarily in only one direction. This happens when light scatters off of a surface, for example, or when it traverses a strong magnetic field.

PRAXyS 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.

PRAXyS will be able to tell the shape of the X-ray-emitting matter 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. The paths of X-rays, and their polarization, are bent by the strong gravity near a spinning black hole. PRAXyS therefore 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, PRAXyS will open new phase space.

The heart of PRAXyS 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 PRAXyS detector readout employs a time projection chamber to image the track.

Goddard's PRAXyS 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 September, 2014.

NASA Goddard will be responsible for the PRAXyS instrument and the overall program management. Orbital ATK, headquartered in Dulles, Va., will be responsible for building the spacecraft, performing mission operations, and building an extendable boom to place the X-ray telescopes the proper distance from the detectors.

The University of Iowa will provide instrument calibration assistance. PRAXyS includes collaborators from universities including George Washington University, Johns Hopkins University, Cornell University, Rice University, the University of Arizona, Stanford University and North Carolina State University.