Retro-lensing light curves near a black hole

TL;DR

This paper models the light-curves from clouds near a black hole, highlighting how gravitational lensing and scattering produce sharp flux spikes and polarization changes, revealing photon orbit signatures.

Contribution

It introduces a detailed model of retro-lensed light curves near black holes, including polarization and time lag effects, extending previous work on radiation-driven clouds.

Findings

Sharp flux spikes due to gravitational lensing are predicted.

Retro-lensed photons cause observable peaks with characteristic time lags.

Polarization varies abruptly on the light-crossing time scale.

Abstract

We model the light-curves from radiation-driven clouds near an accreting black hole. Taking into account the multiple images due to strong gravitational lensing, we find that sharp spikes can significantly enhance the observed flux. Following our previous work (Horak & Karas 2006a,b) we assume that scattering of ambient light takes place in a cloud that is in radial motion under a combined influence of black hole gravity and the radiation field. The retro-lensed photons give rise to peaks in the observed signal that follow with a characteristic time lag after the direct-image photons. Duration of these features is very short and the predicted polarization varies abruptly on the time-scale comparable with the light-crossing time of the system -- a signature of the photon orbit. We also consider the polarization properties of scattered light.