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 | \alpha| \leq 0.15 ( 3 \sigma ) .
There is no evidence for either a low- or high-frequency cut-off , but in the mid-infrared ( \sim 30 \mu m ) thermal emission from the OB-type companion star becomes dominant .
The integrated luminosity of this flat-spectrum emission in quiescence is \geq 2 \times 10 ^ { 31 } erg s ^ { -1 } ( 2 \times 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 reponsible 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 .