The accretion–ejection coupling in the black hole candidate X-ray binary MAXI J1836−194

Abstract

We present the results of our quasi-simultaneous radio, sub-mm, infrared,\noptical and X-ray study of the Galactic black hole candidate X-ray binary MAXI\nJ1836-194 during its 2011 outburst. We consider the full multi-wavelength\nspectral evolution of the outburst, investigating whether the evolution of the\njet spectral break (the transition between optically-thick and optically-thin\nsynchrotron emission) is caused by any specific properties of the accretion\nflow. Our observations show that the break does not scale with the X-ray\nluminosity or with the inner radius of the accretion disk, and is instead\nlikely to be set by much more complex processes. We find that the radius of the\nacceleration zone at the base of the jet decreases from ~10$^6$ gravitational\nradii during the hard intermediate state to ~10$^3$ gravitational radii as the\noutburst fades (assuming a black hole mass of 8 M$_{\\odot}$), demonstrating\nthat the electrons are accelerated on much larger scales than the radius of the\ninner accretion disk and that the jet properties change significantly during\noutburst. From our broadband modelling and high-resolution optical spectra, we\nargue that early in the outburst, the high-energy synchrotron cooling break was\nlocated in the optical band, between $\\approx 3.2 \\times 10^{14}$ Hz and $4.5\n\\times 10^{14}$ Hz. We calculate that the jet has a total radiative power of\n$\\approx 3.1 \\times 10^{36}$ ergs s$^{-1}$, which is ~6% of the bolometric\nradiative luminosity at this time. We discuss how this cooling break may evolve\nduring the outburst, and how that evolution dictates the total jet radiative\npower. Assuming the source is a stellar-mass black hole with canonical state\ntransitions, from the measured flux and peak temperature of the disk component\nwe constrain the source distance to be 4-10 kpc.\n