We present near-infrared polarimetric observations of the black hole X-ray binaries Swift J1357.2–0933 and A0620–00 .
In both sources , recent studies have demonstrated the presence of variable infrared synchrotron emission in quiescence , most likely from weak compact jets .
For Swift J1357.2–0933 we find that the synchrotron emission is polarized at a level of 8.0 \pm 2.5 per cent ( a 3.2 \sigma detection of intrinsic polarization ) .
The mean magnitude and rms variability of the flux ( fractional rms of 19–24 per cent in K _ { S } -band ) agree with previous observations .
These properties imply a continuously launched ( stable on long timescales ) , highly variable ( on short timescales ) jet in the Swift J1357.2–0933 system in quiescence , which has a moderately tangled magnetic field close to the base of the jet .
We find that for A0620–00 , there are likely to be three components to the optical–infrared polarization ; interstellar dust along the line of sight , scattering within the system , and an additional source that changes the polarization position angle in the reddest ( H and K _ { S } ) wave-bands .
We interpret this as a stronger contribution of synchrotron emission , and by subtracting the line-of-sight polarization , we measure an excess of \sim 1.25 \pm 0.28 per cent polarization and a position angle of the magnetic field vector that is consistent with being parallel with the axis of the resolved radio jet .
These results imply that weak jets in low luminosity accreting systems have magnetic fields which possess similarly tangled fields compared to the more luminous , hard state jets in X-ray binaries .