We study the radio/X-ray correlation in Cyg X-3. It has been known that the
soft and hard X-ray fluxes in the hard spectral state are correlated positively
and negatively, respectively, with the radio flux. We show that this implies
that the observed ∼1--100 keV flux (which is a fair approximation to the
bolometric flux) is completely uncorrelated with the radio flux. We can recover
a positive correlation (seen in other sources and expected theoretically) if
the soft X-rays are strongly absorbed by a local medium. Then, however, the
intrinsic X-ray spectrum of Cyg X-3 in its hard state becomes relatively soft,
similar to that of an intermediate spectral state of black-hole binaries, but
not to their true hard state. We also find the radio spectra in the hard state
of Cyg X-3 are hard on average, and the flux distributions of the radio
emission and soft X-rays can be described by sums of two log-normal functions.
We compare Cyg X-3 with other X-ray binaries using colour-colour,
colour-Eddington ratio and Eddington ratio-radio flux diagrams. We find Cyg X-3
to be spectrally most similar to GRS 1915+105, except that Cyg X-3 is
substantially more radio loud, which appears to be due to its jet emission
enhanced by interaction with the powerful stellar wind from the Wolf-Rayet
donor.Comment: An error in the BAT light curve shown in Fig. A2 corrected. Published
in MNRAS + erratu
