Publication Date:
2016-01-02
Description:
We present coordinated multiwavelength observations of the high Galactic latitude ( b = +50°) black hole X-ray binary (BHXB) Swift J1357.2–0933 in quiescence. Our broad-band spectrum 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 ( f 5 GHz 〈 3.9 μJy beam –1 ; 3 rms ). Given current constraints on the distance (2.3–6.3 kpc), its 0.5–10 keV X-ray flux corresponds to an Eddington ratio L X / L Edd = 4 x 10 –9 –3 x 10 –8 (assuming a black hole mass of 10 M ). 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 ( b 2–5 x 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.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics