Chandra image of the black hole at the center of spiral galaxy M81. Credit: X-ray: NASA/CXC/Wisconsin/D.Pooley & CfA/A.Zezas; Optical: NASA/ESA/CfA/A.Zezas; UV: NASA/JPL-Caltech/CfA/J.Huchra et al.; IR: NASA/JPL-Caltech/CfA
A high-performance computing researcher has predicted a physical effect that would help physicists and astronomers provide fresh evidence of the correctness of Einstein’s general theory of relativity. Bin Chen, who works at the university’s Research Computing Center, describes the yet-to-be-observed effect in the paper “Probing the Gravitational Faraday Rotation Using Quasar X-ray Microlensing,” in the journal Scientific Reports. “To be able to test general relativity is of crucial importance to physicists and astronomers,” Chen said.
This testing is especially so in regions close to a black hole, because the current evidence for Einstein’s general relativity — light bending by the sun, for example mainly comes from regions where the gravitational field is very weak, or regions far away from a black hole.
Electromagnetism demonstrates that light is composed of oscillating electric and magnetic fields. Linearly polarized light is an electromagnetic wave whose electric and magnetic fields oscillate along fixed directions when the light travels through space.
Gravitational microlensing magnification pattern of a random star field with mean lens mass , external shear γ = (0.2, 0), surface mass density κc = 0.4, κ* = 0.2.
The gravitational Faraday effect, predicted in the 1950s, theorizes that when linearly polarized light travels close to a spinning black hole, orientation of its polarization rotates according to Einstein’s theory of general relativity. Currently, there is no practical way to detect gravitational Faraday rotation. Chen predicts a new effect that can be used to detect the gravitational Faraday effect. His proposed observation requires monitoring X-ray emissions from gravitationally lensed quasars.
“This means that light from a cosmologically distant quasar will be deflected, or gravitationally lensed, by the intervening galaxy along the line of sight before arriving at an observer on the Earth,” said Chen of the phenomenon of gravitational lensing, which was predicted by Einstein in 1936. More than 100 gravitational lenses have been discovered so far.
Microlensing polarization light curve: The black hole spin a = 0.998, and the accretion disk inclination angle θ = 75°. The trajectory of the quasar is shown as the dashed orange line in Fig. 1. The time is measured in pixels in the x direction (one pixel is about 6.8 days assuming the source is moving with a transversal velocity about 500 kms−1). The red solid, magenta long-dashed, and blue dashed curves show respectively the variation of the X-ray flux magnitude (i.e., the traditional microlensing light curve), the angle χ, and the degree δ of the X-ray polarization. The observed X-ray flux, and the degree and angle of the X-ray polarization vary rapidly and concurrently during microlensing caustic crossing.
“Astronomers have recently found strong evidence showing that quasar X-ray emissions originate from regions very close to supermassive black holes, which are believed to reside at the center of many galaxies,” Chen said. “Gravitational Faraday rotation should leave its fingerprints on such compact regions close to a black hole. “Specifically, the observed X-ray polarization of a gravitationally microlensed quasar should vary rapidly with time if the gravitational Faraday effect indeed exists,” he said. “Therefore, monitoring the X-ray polarization of a gravitationally lensed quasar over time could verify the time dependence and the existence of the gravitational Faraday effect.”
If detected, Chen’s effect – a derivative of the gravitational Faraday effect would provide strong evidence of the correctness of Einstein’s general relativity theory in the “strong-field regime,” or environment in close proximity to a black hole.Chen generated a simulation for the paper on the FSU Research Computing Center’s High-Performance Computing cluster — the second-largest computer cluster in Florida. http://www.nature.com/articles/srep16860
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