The very flat radio–millimetre spectrum of Cygnus X‐1

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 2×10 31  erg s −1 (2×10 24  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.