Probing the gravitational Faraday rotation using quasar X-ray microlensing.

Bin Chen

doi:10.1038/srep16860

Abstract

The effect of gravitational Faraday rotation was predicted in the 1950s, but there is currently no practical method for measuring this effect. Measuring this effect is important because it will provide new evidence for correctness of general relativity, in particular, in the strong field limit. We predict that the observed degree and angle of the X-ray polarization of a cosmologically distant quasar microlensed by the random star field in a foreground galaxy or cluster lens vary rapidly and concurrently with flux during caustic-crossing events using the first simulation of quasar X-ray microlensing polarization light curves. Therefore, it is possible to detect gravitational Faraday rotation by monitoring the X-ray polarization of gravitationally microlensed quasars. Detecting this effect will also confirm the strong gravity nature of quasar X-ray emission.

Highlights

Gravitational Faraday rotation, the rotation of the plane of polarization of an electromagnetic wave propagating in a curved spacetime, is the gravitational analogue of the electromagnetic Faraday rotation[1,2,3,4,5,6,7,8,9,10]
The X-ray flux and polarization of a microlensed quasar at cosmological redshift zs will be lensed first by the supermassive black hole powering the active galactic nuclei (AGN), microlensed by the foreground random star field in the lens galaxy at redshift zd
Given the large cosmological distance from the lens galaxy to the background quasar, the strong lensing by the Kerr black hole can be decoupled from the foreground microlensing using the numerical schemes outlined in[25]

Summary

Introduction

Gravitational Faraday rotation, the rotation of the plane of polarization of an electromagnetic wave propagating in a curved spacetime, is the gravitational analogue of the electromagnetic Faraday rotation[1,2,3,4,5,6,7,8,9,10]. While this effect exists in both the strong and weak field regimes[11,12,13,14,15], significant gravitational Faraday rotations are expected only for strong gravitational field, for example, for photons transiting regions very close to a Kerr black hole (a black hole with spin[16]) Measuring this effect is difficult for at least two reasons: first, the regions producing significant rotations (a few gravitational radii from a back hole), are too small to be directly resolved by current telescopes; and the lack of polarimeters with high sensitivity which can measure polarization fraction at a few percent level and resolve polarization angle at a few degrees level. The X-ray polarization of a microlensed quasar fluctuates with time, producing a microlensing polarization light curve strongly correlated with the classical flux light curve

Methods

Results

Conclusion