Motivated by the large body of literature around the phenomenological properties of accreting black hole ( BH ) and neutron star ( NS ) X-ray binaries in the radio : X-ray luminosity plane , we carry out a comparative regression analysis on 36 BHs and 41 NSs in hard X-ray states , with data over 7 dex in X-ray luminosity for both .
The BHs follow a radio to X-ray ( logarithmic ) luminosity relation with slope \beta = 0.59 \pm 0.02 , consistent with the NSs ’ slope ( \beta = 0.44 ^ { +0.05 } _ { -0.04 } ) within 2.5 \sigma .
The best-fitting intercept for the BHs significantly exceeds that for the NSs , cementing BHs as more radio loud , by a factor \sim 22 .
This discrepancy can not be fully accounted for by the mass or bolometric correction gap , nor by the NS boundary layer contribution to the X-rays , and is likely to reflect physical differences in the accretion flow efficiency , or the jet powering mechanism .
Once importance sampling is implemented to account for the different luminosity distributions , the slopes of the non-pulsating and pulsating NS subsamples are formally inconsistent ( > 3 \sigma ) , unless the transitional millisecond pulsars ( whose incoherent radio emission mechanism is not firmly established ) are excluded from the analysis .
We confirm the lack of a robust partitioning of the BH data set into separate luminosity tracks .