The polarisation signature from microlensing of circumstellar envelopes in caustic crossing events

Abstract

The high gradient of magnification across the source during both small impact parameter events and caustic crossings offers a unique opportunity for determining the surface brightness profile of the source. Furthermore, models indicate that these events can also provide an appreciable polarisation signal -- arising from differential magnification across the otherwise symmetric source. In a previous paper, polarimetric variations from point lensing of a circumstellar envelope were considered, as would be suitable for an extended envelope around a red giant. In this work we examine the polarisation in the context of caustic crossing events, the scenario which represents the most easily accessible situation for actually observing a polarisation signal in Galactic microlensing.