Multiwavelength observations of the black hole X-ray binary A0620-00 in quiescence

Abstract

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 $\Gamma =2.07\pm 0.13$ and a hydrogen column of $N_H=3.0\pm 0.5\times 10^{21}{\mathrm{cm}}^{-2}$. 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 $\alpha =0.74\pm 0.19$ ($S_{\nu }\propto {\nu }^{\alpha }$). This suggests that the peak of the spectral energy distribution is likely to be between $10^{12}$ and $10^{14}$ 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.

13 pages, 6 figures. ApJ, in press. The authors would like to recognize, with sadness and affection, the scientific leadership of Jeff McClintock, who passed away on November 8. Among Jeff's many other contributions, his pioneering work with Ron Remillard (1986, ApJ 308, 110) forms the basis of all subsequent studies of A0620-00 and related objects

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• A MeerKAT Survey of Nearby Novalike Cataclysmic Variables
• Disk-jet coupling changes as a possible indicator for outbursts from GX 339-4 remaining within the X-ray hard state
• GS 2000+25: The Least Luminous Black Hole X-ray Binary
• ALMA observations of A0620-00: fresh clues on the nature of quiescent black hole X-ray binary jets
• Toward a Larger Sample of Radio Jets from Quiescent Black Hole X-ray Binaries
• Scale-invariance of black hole accretion: modeling emission from a black hole X-ray binary with relativistic accretion flow simulations