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

• Published in: arXiv.org 2015-12
• Main Authors: Plotkin, Richard M, Gallo, Elena, Jonker, Peter G, Miller-Jones, James C A, Homan, Jeroen, Munoz-Darias, Teo, Markoff, Sera, Montserrat Armas Padilla, Fender, Rob, Rushton, Anthony P, Russell, David M, Torres, Manuel A P
• Format: Article
• Language: English
• Subjects: Black holes; Broadband; Curvature; Deposition; Interstellar; Latitude; Spectrum allocation; Synchrotron radiation; Ultraviolet spectra; X ray binaries; X ray stars
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1512.01941

We present coordinated multiwavelength observations of the high Galactic latitude (b=+50 deg) black hole X-ray binary (XRB) J1357.2-0933 in quiescence. Our broadband spectrum includes strictly simultaneous radio and X-ray observations, and near-infrared, optical, and ultraviolet data taken 1-2 days later. 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 (2.3-6.3 kpc), 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 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 (approximately 2-5e14 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.