A clean sightline to quiescence: multiwavelength observations of the high Galactic latitude black hole X-ray binary Swift J1357.2−0933

We present coordinated multiwavelength observations of the high Galactic latitude (b =+ 50 ◦ ) blackholeX-raybinary(BHXB)SwiftJ1357.2−0933inquiescence.Ourbroad-bandspectrum includes strictly simultaneous radio and X-ray observations, and near-infrared, optical, and ultraviolet data taken 1–2 d later. We detect Swift J1357.2−0933 at all wavebands except for the radio (f5GHz < 3.9µJy beam −1 ;3 σrms). Given current constraints on the distance (2.3– 6.3 kpc), its 0.5–10 keV X-ray flux corresponds to an Eddington ratio LX/LEdd = 4 × 10 −9 – 3 × 10 −8 (assuming a black hole mass of 10M� ). The broad-band spectrum is dominated by synchrotron radiation from a relativistic population of outflowing thermal electrons, which we argue to be a common signature of short-period quiescent BHXBs. Furthermore, we identify the frequency where the synchrotron radiation transitions from optically thick-to-thin (νb ≈ 2–5 × 10 14 Hz), which is the most robust determination of a ‘jet break’ for a quiescent BHXB to date. Our interpretation relies on the presence of steep curvature in the ultraviolet spectrum, a frequency window made observable by the low amount of interstellar absorption along the line of sight. High Galactic latitude systems like Swift J1357.2−0933 with clean ultraviolet sightlines are crucial for understanding black hole accretion at low luminosities.