Optical/infrared polarised emission in X-ray binaries

TL;DR

This paper reviews optical/infrared polarimetric observations of X-ray binaries, revealing synchrotron emission from jets with magnetic fields that are turbulent and variable, providing insights into the connection between inflow and outflow in these systems.

Contribution

It presents new polarimetric measurements of X-ray binaries' jets, highlighting the magnetic field configurations and their variability, and predicts future X-ray polarisation detections.

Findings

Polarisation levels of 1-10% in optical/infrared jets.

Magnetic fields are turbulent, variable, and generally aligned with jets.

Potential for future X-ray polarisation detection from jets.

Abstract

Recently, evidence for synchrotron emission in both black hole and neutron star X-ray binaries has been mounting, from optical/infrared spectral, polarimetric, and fast timing signatures. The synchrotron emission of jets can be highly linearly polarised, depending on the configuration of the magnetic field. Optical and infrared (OIR) polarimetric observations of X-ray binaries are presented in this brief review. The OIR polarimetric signature of relativistic jets is detected at levels of ~ 1-10 %, similar to AGN cores. This reveals that the magnetic geometry in the compact jets may be similar for supermassive and stellar-mass BHs. The magnetic fields near the jet base in most of these systems appear to be turbulent, variable and on average, aligned with the jet axis, although there are some exceptions. These measurements probe the physical conditions in the accretion (out)flow and demonstrate a new way of connecting inflow and outflow, using both rapid timing and polarisation. Variations in polarisation could be due to rapid changes of the ordering of the magnetic field in the emitting region, or in one case, flares from individual ejections or collisions between ejecta. It is predicted that in some cases, variable levels of X-ray polarisation from synchrotron emission originating in jets will be detected from accreting Galactic black holes with upcoming spaceborne X-ray polarimeters.