Test of relativistic gravity using microlensing of relativistically broadened lines in gravitationally lensed quasars

TL;DR

This paper proposes using microlensing of relativistically broadened emission lines in gravitationally lensed quasars to test the predictions of general relativity near supermassive black holes by analyzing the evolution of the line edges during caustic crossing.

Contribution

It introduces a method to measure the gravitational redshift and orbital velocities near black holes by observing the time-dependent microlensing effects on emission lines, enabling tests of relativistic gravity models.

Findings

The evolution of line edges is insensitive to many astrophysical parameters.

Measurement precision can reach the innermost stable circular orbit.

Potential to discriminate between GR and alternative gravity theories.

Abstract

We show that observation of the time-dependent effect of microlensing of relativistically broadened emission lines (such as e.g. the Fe Kalpha line in X-rays) in strongly lensed quasars could provide data on celestial mechanics of circular orbits in the direct vicinity of the horizon of supermassive black holes. This information can be extracted from the observation of evolution of red / blue edge of the magnified line just before and just after the period of crossing of the innermost stable circular orbit by the microlensing caustic. The functional form of this evolution is insensitive to numerous astrophysical parameters of the accreting black hole and of the microlensing caustics network system (as opposed to the evolution the full line spectrum). Measurement of the temporal evolution of the red / blue edge could provide a precision measurement of the radial dependence of the gravitational redshift and of velocity of the circular orbits, down to the innermost stable circular orbit. These measurements could be used to discriminate between the General Relativity and alternative models of the relativistic gravity in which the dynamics of photons and massive bodies orbiting the gravitating centre is different from that of the geodesics in the Schwarzschild or Kerr space-times.