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X-ray Polarimetry to Determine Geometry of Black Holes and Neutron Stars

GEMS is an upcoming NASA space-based observatory whose purpose is to help determine the geometry of black holes and neutron stars. Neutron stars and black holes possess the strongest gravitational fields in the Universe, and because of these enormous gravitational fields, these objects distort the very fabric of space and time in their vicinities. Measuring the geometric distortions of spacetime near these objects provides the ultimate test of Einstein's gravitational theory. The nearest neutron star is over 300 light-years from earth, and the nearest black hole is about 10 times farther away, so it's impossible to directly image these objects. GEMS will measure space-time distortions using a technique called X-ray polarimetry, which measures the orientation, or polarization, of X-rays emitted very close to the surface of the collapsed object. Polarimetry is routinely done in visible light and using radio waves, but is extremely challenging at X-ray energies. The GEMS polarimeters, built in collaboration with USRA/CRESST scientists, use an entirely new design to allow measurement of X-ray polarization at never-before achievable sensitivity. GEMS will, for the first time, routinely measure the polarization of high-energy X-ray radiation being emitted near the surface of many neutron stars and black holes to help define how strong gravity (and strong magnetic fields, in the case of neutron stars) shapes both space and time.

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