  [Context] The broadband spectral energy distribution of microquasars has
proven to be a valuable tool for assessing the roles of jets and accretion
flows in microquasars, as well as the coupling between the two. The coupling
might manifest itself observationally in the correlated radio and X-ray
variabilities of a source. Such a radio/X-ray correlation has indeed been seen
in several microquasars during the low-hard state and subsequently been claimed
to be universal for all. If proven, the universal correlation would have
profound implications on theoretical models. However, there is already
observational evidence that suggests otherwise. [Aims] In this paper, we
critically examine the radio/X-ray correlation in a sample of microquasars, in
the low-hard state as well as during state transitions, with a goal of testing
the claimed universality of the correlation on observational grounds. [Methods]
We have assembled a comprehensive data set from the
simultaneous/contemporaneous radio and X-ray observations of representative
microquasars. The data have allowed us to quantify the radio/X-ray correlation
on a source by source basis. [Results] We find that the radio/X-ray correlation
of microquasars exhibits diverse behaviors, both in the low-hard and
transitional states, ranging from being very week (or none at all) to very
strong. There is even a hint of spectral dependence of the correlation in some
cases. [Conclusions] Our results rule out the claimed universality of the
radio/X-ray correlation of microquasars, even for the low-hard state. On the
other hand, we do find that the radio and X-ray variabilities are, to varying
degrees, correlated in most cases.
