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.
Highlights
X-ray binary jets come in two main “flavours”, both producing synchrotron emission—steady, continuously launched ejecta, characterised by a flat/inverted radio spectrum, and discrete ejecta that are optically thin
By studying the time evolution of the jet spectrum and how it relates to changes in the inflow, it is possible to test how the jet properties respond to accretion flow changes, which is more difficult in active galactic nuclei (AGN) because these vary on much longer timescales
The position angle (PA) of polarisation was compared to the axis of the resolved radio jet of GX 339–4 [9] and implied the be highly polarised if the magnetic field (B-field) was approximately parallel to the jet axis
Summary
X-ray binary jets come in two main “flavours”, both producing synchrotron emission—steady, continuously launched ejecta, characterised by a flat/inverted radio spectrum, and discrete ejecta that are optically thin. Compact radio jet is present, there is usually an optical and infrared (OIR) excess above the disc emission, which is generally attributed to the jet [2,16,17] (and references therein) This is likely to be optically thin synchrotron emission from near the jet base in the post-accelerated plasma. We note that at OIR wavelengths, photons do not suffer from Faraday rotation or dispersion because of their strong dependency on wavelength, which can hamper the interpretation of radio results [26] In this concise review, examples are given of polarimetric studies of the optically thin synchrotron emission in steady jets, from optical or near-infrared (NIR) data of both BH and neutron star X-ray binaries (NSXBs)
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