Kinematics show consistency between stellar mass and supermassive black hole parent population jet speeds

TL;DR

This study demonstrates that the apparent differences in jet speeds between stellar-mass and supermassive black holes are mainly due to selection effects, showing their jet Lorentz factors are statistically similar and consistent with relativistic speeds.

Contribution

It provides the first statistical analysis of BHXRB jet Lorentz factors, revealing they follow a power-law distribution similar to AGN jets, challenging previous assumptions about their relative speeds.

Findings

BHXRB jet Lorentz factors follow a power-law distribution with exponent -2.64.

Selection effects largely explain the observed speed differences.

BHXRB jets are consistent with being as relativistic as AGN jets.

Abstract

Jets from stellar-mass and supermassive black holes provide the unique opportunity to study similar processes in two very different mass regimes. Historically, the apparent speeds of black hole x-ray binary (BHXRBs) jets have been observed to be lower than jet speeds from active galactic nuclei (AGN) and specifically blazars. In this work, we show that selection effects could be the primary cause of the observed population differences. For the first time, it is possible to perform a statistical analysis of the underlying BHXRB jet Lorentz factor distribution. We use both the Anderson-Darling test and apply nested sampling to this problem. With Bayes factors, we confirm that the Lorentz factor distribution of BHXRBs is best described with a power law, the same model that has been applied to AGN jets. For a Lorentz factor distribution following $\rm N(\Gamma) \propto \Gamma^b$ we find a value for the exponent of $b=-2.64_{-0.55}^{+0.46}$. This exponent is consistent with values found in AGN population studies, within $1\sigma$ for \textit{Swift}-BAT and \textit{Fermi}-LAT selected AGN. The best-fit exponent for the radio selected MOJAVE sample is just above our $2 \sigma$ limit. This is a remarkable agreement given the different scales at which the jets are observed. The observed slower apparent speeds in BHXRBs are largely due to the much larger inclinations in this sample. Furthermore, nested sampling confirms that $\Gamma_{\rm max}$ is completely unconstrained using this method. Therefore, based on kinematics alone, BHXRB jets are broadly consistent with being just as relativistic as those from supermassive black holes.