The Quiescent Spectral Energy Distribution of V404 Cyg

TL;DR

This study provides the most comprehensive multiwavelength spectral energy distribution of the quiescent black hole binary V404 Cyg, revealing an IR excess likely from a jet or cool disc and confirming a stable X-ray spectral shape.

Contribution

It offers the first complete simultaneous multiwavelength SED of V404 Cyg in quiescence, constraining models of accretion and jet emission in this system.

Findings

No significant UV to near-IR accretion light contribution.

IR excess suggests jet or cool disc presence.

X-ray spectrum consistent with Gamma~2 power-law, stable over luminosity changes.

Abstract

We present a multiwavelength study of the black hole X-ray binary V404 Cyg in quiescence, focusing upon the spectral energy distribution (SED). Radio, optical, UV, and X-ray coverage is simultaneous. We supplement the SED with additional non-simultaneous data in the optical through infrared where necessary. The compiled SED is the most complete available for this, the X-ray and radio brightest quiescent black hole system. We find no need for a substantial contribution from accretion light from the near-UV to the near-IR, and in particular the weak UV emission constrains published spectral models for V404 Cyg. We confirm that no plausible companion spectrum and interstellar extinction can fully explain the mid-IR, however, and an IR excess from a jet or cool disc appears to be required. The X-ray spectrum is consistent with a Gamma~2 power-law as found by all other studies to date. There is no evidence for any variation in the hardness over a range of a factor of 10 in luminosity. The radio flux is consistent with a flat spectrum (in f(nu)). The break frequency between a flat and optically thin spectrum most likely occurs in the mid or far-IR, but is not strongly constrained by these data. We find the radio to be substantially variable but with no clear correlation with X-ray variability.