Evidence for quiescent synchrotron emission in the black hole X-ray transient Swift J1357.2-0933

We present high time resolution ULTRACAM optical and NOTCam infrared observations of the edge-on black hole X-ray transient Swift J1357.2-0933. Our data taken in 2012 and 2013 show the system to be at its pre-outburst magnitude and so the system is in quiescence. In contrast to other X-ray transients, the quiescent light curves of Swift J1357.2-0933 do not show the secondary star's ellipsoidal modulation. The optical light curve is dominated by variability with an optical fractional rms of similar to 35 per cent, a factor of >3 larger than what is observed in other systems at similar time resolution. Optical flare events lasting 2-10 min with amplitudes of up to similar to 1.5 mag are seen as well as numerous rapid similar to 0.8 mag dip events which are similar to the optical dips seen in outburst. Similarly, the infrared J-band light curve is dominated by variability with a fractional rms of similar to 21 per cent, and flare events lasting 10-30 min with amplitudes of up to similar to 1.5 mag are observed. The quiescent optical to mid-infrared spectral energy distribution (SED) in quiescence is dominated by a non-thermal component with a power-law index of -1.4 (the broad-band rms SED has a similar index) which arises from optically thin synchrotron emission most likely originating in a weak jet; the lack of a peak in the SED rules out advection-dominated models. Using the outburst amplitude-period relation for X-ray transients, we estimate the quiescent magnitude of the secondary star to lie in the range V-min = 22.7-25.6, which when combined with the absolute magnitude of the expected M4.5 V secondary star allows us to constrain the distance to lie in the range 0.5-6.3 kpc. The short orbital period argues for a nuclearly evolved star with an initial mass similar to 1.5 M-circle dot, which has evolved to a 0.17 M-circle dot star. The high Galactic latitude of Swift J1357.2-0933 implies a scaleheight in the range 0.4-4.8 kpc above the Galactic plane, possibly placing Swift J1357.2-0933 in a sub-class of high-z short-period black hole X-ray transients in the Galactic halo.