The very flat radio–millimetre spectrum of Cygnus X-1 |
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| Authors: | R P Fender G G Pooley P Durouchoux R P J Tilanus C Brocksopp |
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| Institution: | Astronomical Institute 'Anton Pannekoek' and Center for High Energy Astrophysics, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, the Netherlands; Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE; C.E. Saclay, DSM, DAPNIA, Service d'Astrophysique, 91191 Gif-sur-Yvette Cedex, France; Joint Astronomy Centre, 660 N. A'ohōkūPl., Hawaii 96720, USA; Netherlands Foundation for Research in Astronomy, P.O. Box 2, 7990 AA Dwingeloo, the Netherlands; Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH |
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| Abstract: | We present almost-simultaneous detections of Cygnus X-1 in the radio and mm regimes, obtained during the low/hard X-ray state. The source displays a flat spectrum between 2 and 220 GHz, with a spectral index | α | 0.15 (3 σ ). There is no evidence for either a low- or high-frequency cut-off, but in the mid-infrared (∼30 μm) thermal emission from the OB-type companion star becomes dominant. The integrated luminosity of this flat-spectrum emission in quiescence is  2×1031 erg s−1 (2×1024 W). Assuming the emission originates in a jet for which non-radiative (e.g. adiabatic expansion) losses dominate, this is a very conservative lower limit on the power required to maintain the jet. A comparison with Cyg X-3 and GRS 1915+105, the other X-ray binaries for which a flat spectrum at shorter than cm wavelengths has been observed, shows that the jet in Cyg X-1 is significantly less luminous and less variable, and is probably our best example to date of a continuous, steady, outflow from an X-ray binary. The emissive mechanism responsible for such a flat spectral component remains uncertain. Specifically, we note that the radio–mm spectra observed from these X-ray binaries are much flatter than those of the 'flat-spectrum' AGN, and that existing models of synchrotron emission from partially self-absorbed radio cores, which predict a high-frequency cut-off in the mm regime, are not directly applicable. |
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| Keywords: | binaries: close stars: individual: Cygnus X-1 ISM: jets and outflows radio continuum: stars |
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