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

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.