We report on deep , coordinated radio and X-ray observations of the black hole X-ray binary XTEÂ J1118+480Â in quiescence .
The source was observed with the Karl G. Jansky Very Large Array for a total of 17.5 hrs at 5.3 GHz , yielding a 4.8 \pm 1.4 \mu Jy radio source at a position consistent with the binary system .
At a distance of 1.7 kpc , this corresponds to an integrated radio luminosity between 4-8 \times 10 ^ { 25 } erg s ^ { -1 } , depending on the spectral index .
This is the lowest radio luminosity measured for any accreting black hole to date .
Simultaneous observations with the Chandra X-ray Telescope detected XTE J1118+480 at 1.2 \times 10 ^ { -14 } erg s ^ { -1 }  cm ^ { -2 }  ( 1-10 keV ) , corresponding to an Eddington ratio of \sim 4 \times 10 ^ { -9 } for a 7.5 M _ { \odot }  black hole .
Combining these new measurements with data from the 2005 and 2000 outbursts available in the literature , we find evidence for a relationship of the form \ell _ { r } = \alpha + \beta \ell _ { X } Â ( where \ell denotes logarithmic luminosities ) , with \beta = 0.72 \pm 0.09 .
XTEÂ J1118+480Â is thus the third system – together with GX339-4Â and V404 Cyg – for which a tight , non-linear radio/X-ray correlation has been reported over more than 5 dex in \ell _ { X } .
Confirming previous results , we find no evidence for a dependence of the correlation normalisation of an individual system on orbital parameters , relativistic boosting , reported black hole spin and/or black hole mass .
We then perform a clustering and linear regression analysis on what is arguably the most up-to-date collection of coordinated radio and X-ray luminosity measurements from quiescent and hard state black hole X-ray binaries , including 24 systems .
At variance with previous results , a two-cluster description is statistically preferred only for random errors \buildrel < \over { \sim } 0.3 dex in both \ell _ { r } Â and \ell _ { X } , a level which we argue can be easily reached when the known spectral shape/distance uncertainties and intrinsic variability are accounted for .
A linear regression analysis performed on the whole data set returns a best-fitting slope \beta = 0.61 \pm 0.03 and intrinsic scatter \sigma _ { 0 } = 0.31 \pm 0.03 dex .