2018ApJ...852....4D

We present results from simultaneous multiwavelength X-ray, radio, and optical/near-infrared observations of the quiescent black hole X-ray binary A0620-00 performed in 2013 December. We find that the Chandra flux has brightened by a factor of 2 since 2005, and by a factor of 7 since 2000. The spectrum has not changed significantly over this time, being consistent with a power law of Γ=2.07±0.13 and a hydrogen column of N_H=(3.0±0.5)×10²¹ cm^–2. Very Large Array observations of A0620-00 at three frequencies, over the interval of 5.25-22.0 GHz, have provided us with the first broadband radio spectrum of a quiescent stellar mass black hole system at X-ray luminosities as low as 10^–8 times the Eddington luminosity. Compared to previous observations, the source has moved to lower radio and higher X-ray luminosity, shifting it perpendicular to the standard track of the radio/X-ray correlation for X-ray binaries. The radio spectrum is inverted with a spectral index α=0.74±0.19 (S_ν∝ν^α^). This suggests that the peak of the spectral energy distribution is likely to be between 10¹² and 10¹⁴ Hz, and that the near-IR and optical flux contain significant contributions from the star, the accretion flow, and from the outflow. Decomposing these components may be difficult, but holds the promise of revealing the interplay between accretion and jet in low luminosity systems.