The relation between radio and X-ray luminosity of black hole binaries: affected by inner cool disks?
Abstract
Context. Observations of the black hole X-ray binaries GX 339-4 and V404 Cygni have brought evidence of a strong correlation between radio and X-ray emission during the hard spectral state; however, now more and more sources, the so-called "outliers", are found with a radio emission noticeably below the established "standard" relation. Several explanations have already been considered, but the existence of dual tracks is not yet fully understood.
Aims: We suggest that in the hard spectral state re-condensation of gas from the corona into a cool, weak inner disk can provide additional soft photons for Comptonization, leading to a higher X-ray luminosity in combination with rather unchanged radio emission, which presumably traces the mass accretion rate.
Methods: As an example, we determined how much additional luminosity due to photons from an underlying disk would be needed to explain the data from the representative outlier source H1743-322.
Results: From the comparison with calculations of Compton spectra with and without the photons from an underlying disk, we find that the required additional X-ray luminosity lies well in the range obtained from theoretical models of the accretion flow.
Conclusions: The radio/X-ray luminosity relation resulting from Comptonization of additional photons from a weak, cool inner disk during the hard spectral state can explain the observations of the outlier sources, especially the data for H1743-322, the source with the most detailed observations. The existence or non-existence of weak inner disks on the two tracks might point to a difference in the magnetic fields of the companion stars. These could affect the effective viscosity and the thermal conductivity, hence also the re-condensation process.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- February 2014
- DOI:
- 10.1051/0004-6361/201322423
- arXiv:
- arXiv:1401.7525
- Bibcode:
- 2014A&A...562A.142M
- Keywords:
-
- accretion;
- accretion disks;
- black hole physics;
- X-rays: individuals: GX 339-4;
- X-rays: individuals: V404 Cyg;
- X-rays: individuals: H1743-322;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 7 pages, 2 figures. Accepted for publication in A &