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Title:
The Spectral Energy Distribution of Quiescent Black Hole X-Ray Binaries: New Constraints from Spitzer
Authors:
Gallo, Elena; Migliari, Simone; Markoff, Sera; Tomsick, John A.; Bailyn, Charles D.; Berta, Stefano; Fender, Rob; Miller-Jones, James C. A.
Affiliation:
AA(Physics Department, Broida Hall, University of California, Santa Barbara, CA 93106.; Chandra Fellow.), AB(Center for Astrophysics and Space Sciences, 9500 Gilman Drive, University of California, San Diego, La Jolla, CA 92093.), AC(Astronomical Institute ``Anton Pannekoek,'' University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, Netherlands.), AD(Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720.), AE(Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520.), AF(Center for Astrophysics and Space Sciences, 9500 Gilman Drive, University of California, San Diego, La Jolla, CA 92093.; Dipartimento di Astronomia, Università di Padova, Vicolo dell' Osservatorio 2, 35122 Padova, Italy.), AG(School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK.), AH(Astronomical Institute ``Anton Pannekoek,'' University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, Netherlands.)
Publication:
The Astrophysical Journal, Volume 670, Issue 1, pp. 600-609. (ApJ Homepage)
Publication Date:
11/2007
Origin:
UCP
ApJ Keywords:
Radiation Mechanisms: General, Stars: Individual: Alphanumeric: A0620-00, stars: individual (V404 Cyg), Stars: Individual: Alphanumeric: XTE J1118+480, X-Rays: Binaries
DOI:
10.1086/521524
Bibliographic Code:
2007ApJ...670..600G

Abstract

Among the various issues that remain open in the field of accretion onto black hole X-ray binaries (BHBs) is the question of how gas accretes at very low Eddington ratios, in the so-called quiescent regime. While there is general agreement that X-rays are produced by a population of high-energy electrons near the BH, there is controversy concerning the modeling of the contributions of inflowing versus outflowing particles and their relative energy budget. Recent Spitzer observations of three quiescent BHBs have shown evidence for excess emission with respect to the Rayleigh-Jeans tail of the companion star between 8-24 μm. We suggest that synchrotron emission from a partially self-absorbed outflow might be responsible for the observed mid-IR excess, in place of, or in addition to, thermal emission from circumbinary material. If so, then the jet synchrotron luminosity, integrated from radio to near-IR frequencies, exceeds the measured 2-10 keV luminosity by a factor of a few in these systems. In turn, the mechanical power stored in the jet exceeds the bolometric X-ray luminosity by at least 4 orders of magnitude. We compile the broadband spectral energy distribution (SED) of A0620-00, the lowest Eddington-ratio stellar mass BH with a known radio counterpart, by means of simultaneous radio, optical, and X-ray observations, and the archival Spitzer data. We are able to fit the SED of A0620-00 with a maximally jet-dominated model, in which the radio through the soft X-rays are dominated by synchrotron emission, while the hard X-rays are dominated by inverse Compton at the jet base. The fitted parameters land in a range of values reminiscent of the Galactic center supermassive black hole Sgr A*. Most notably, the inferred ratio of the jet acceleration rate to local cooling rates is 2 orders of magnitude weaker than higher luminosity, hard-state sources.
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