Two types of relativistic jets are suggested to form near accreting black holes : a potentially ultrarelativistic Poynting-dominated jet and a Poynting-baryon jet .
One source of jet matter is electron-positron pair production , which is driven by neutrino annihilation in GRBs and photon annihilation in AGN and x-ray binaries .
GRB Poynting-dominated jets are also loaded by electron-proton pairs by the collisional cascade of Fick-diffused free neutrons .
We show that , for the collapsar model , the neutrino-driven enthalpy flux ( classic fireball model ) is probably dominated by the Blandford-Znajek energy flux , which predicts a jet Lorentz factor of \Gamma \sim 100 - 1000 .
We show that radiatively inefficient AGN , such as M87 , are synchrotron-cooling limited to \Gamma \sim 2 - 10 .
Radiatively efficient x-ray binaries , such as GRS1915+105 , are Compton-drag limited to \Gamma \lesssim 2 , but the jet may be destroyed by Compton drag .
However , the Poynting-baryon jet is a collimated outflow with \Gamma \sim 1 - 3 .
The jet from radiatively efficient systems , such as microquasar GRS1915+105 , may instead be a Poynting-baryon jet that is only relativistic when the disk is geometrically thick .
In a companion paper , general relativistic hydromagnetic simulations of black hole accretion with pair creation are used to simulate jet formation in GRBs , AGN , and x-ray binaries .