Observations confront simulations: How X-ray polarisation can put observational limits on large scale ordered magnetic fields in the accretion flow

Black hole accretion flows release the enormous gravitational potential energy of the infalling matter as radiation, winds and the jet, yet our understanding of this structure is quite limited. The two known solutions for the accretion flow, the Shakura-Sunyaev thin discs and advection dominated accretion flows, both collapse for luminosities greater than ~0.1LEdd. Large scale magnetic fields can stabilize these flows, but numerical simulations show that the resulting field configuration depends on the initial conditions and these cannot be tested as the fields themselves are invisible. Here I show how 

new data on X-ray polarisation from IXPE can put stringent observational constraints on the magnetic field strength and structure within a stellar mass black hole's accretion flow.