[abridged] The radio:X-ray correlation for hard and quiescent state black
hole X-ray binaries is critically investigated in this paper. New observations
of known sources, along with newly discovered ones, have resulted in an
increasingly large number of outliers lying well outside the scatter about the
quoted best-fit relation. Here, we employ and compare state of the art data
clustering techniques in order to identify and characterize different data
groupings within the radio:X-ray luminosity plane for 18 hard and quiescent
state black hole X-ray binaries with nearly simultaneous multi-wavelength
coverage. Linear regression is then carried out on the clustered data to infer
the parameters of a relationship of the form {ell}_{r}=alpha+beta {ell}_x
through a Bayesian approach (where {ell} denotes log lum). We conclude that the
two cluster model, with independent linear fits, is a significant improvement
over fitting all points as a single cluster. While the upper track slope
(0.63\pm0.03) is consistent, within the errors, with the fitted slope for the
2003 relation (0.7\pm0.1), the lower track slope (0.98\pm0.08) is not
consistent with the upper track, nor it is with the widely adopted value of
~1.4 for the neutron stars. The two luminosity tracks do not reflect systematic
differences in black hole spins as estimated either from reflection, or
continuum fitting method. These results are insensitive to the selection of
sub-samples, accuracy in the distances, and to the treatment of upper limits.
Besides introducing a further level of complexity in understanding the
interplay between synchrotron and Comptonised emission from black hole X-ray
binaries, the existence of two tracks in the radio:X-ray domain underscores
that a high level of caution must be exercised when employing black hole
luminosity relations for the purpose of estimating a third parameter, such as
distance or mass.Comment: MNRAS, in press (10 pages, 7 figures
