Today's Colloquium: The evolving polarised jets of stellar mass black holes

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Radio emission from low-mass X-ray binaries (LMXBs) is dominated by synchrotron emission from relativistic jets that are powered by accretion onto the black hole. Polarization measurements allow us to study the orientation of unresolved LMXB jets, the structure of the associated magnetic fields, as well as the jets' interaction with the surrounding medium. In the presence of ordered magnetic fields, linear polarizations of up to ~70% are expected.

However, polarization has been observed in very few LMXBs and at relatively low levels, though it is unclear if this is due to a lack of intrinsic polarization, depolarization, or a lack of observations -- a question complicated by the tendency for null results to remain unpublished.

I will discuss a number of recent (and not so recent) LMXB outbursts where we obtained, among other broadband data, radio polarization measurements. These will include Swift J1745-26, which had linear polarizations of up to 50%, and the spatially resolved polarimetry of XTE J1748-288.

I will also present our recent VLA and VLBA observations of XTE J1908+094, showing expanding polarized ejecta.