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We present coordinated multiwavelength observations of the high Galacticlatitude (b=+50 deg) black hole X-ray binary (XRB) J1357.2-0933 in quiescence.Our broadband spectrum includes strictly simultaneous radio and X-rayobservations, and near-infrared, optical, and ultraviolet data taken 1-2 dayslater. We detect Swift J1357.2-0933 at all wavebands except for the radio(f_5GHz < 3.9 uJy/beam). Given current constraints on the distance (2.3-6.3kpc), its 0.5-10 keV X-ray flux corresponds to an Eddington ratio Lx/Ledd =4e-9 -- 3e-8 (assuming a black hole mass of 10 Msun). The broadband spectrum isdominated by synchrotron radiation from a relativistic population of outflowingthermal electrons, which we argue to be a common signature of short-periodquiescent BHXBs. Furthermore, we identify the frequency where the synchrotronradiation transitions from optically thick-to-thin (approximately 2-5e14 Hz,which is the most robust determination of a 'jet break' for a quiescent BHXB todate. Our interpretation relies on the presence of steep curvature in theultraviolet spectrum, a frequency window made observable by the low amount ofinterstellar absorption along the line of sight. High Galactic latitude systemslike Swift J1357.2-0933 with clean ultraviolet sightlines are crucial forunderstanding black hole accretion at low luminosities.

A Clean Sightline to Quiescence: Multiwavelength Observations of the High Galactic Latitude Black Hole X-ray Binary Swift J1357.2-0933