The class of stellar-mass black holes that are persistent X-ray sources (e.g. Cygnus X-1, 1E 1740-2942, GRS 1758-258, etc.) and some super-massive black holes at the centre of galaxies (e.g. Sgr A* and many AGN) do not exhibit luminous outbursts with large-scale sporadic ejections. However, despite the enormous differences in mass, steadily accreting black holes have analogous radio cores with steady, flat (S ; ~ 0) emission at radio wavelengths. The fluxes of the core component in AGN are typically of a few Janskys (e.g. Sgr A* ~ 1Jy) allowing VLBI high resolution studies, but in stellar mass black holes the cores are much fainter, typically of a few mJy, which makes difficult high resolution observations of the core.
From the spectral shape it was proposed that the steady compact radio emission in black hole X-ray binaries are jets (e.g. Rodríguez et al. 1995; Fender et al. 1999, 2000; Corbel et al. 2000). Recently, this has been confirmed by VLBI observations at AU scale resolution of GRS 1915+105 (Dhawan, Mirabel & Rodríguez, 2000), and Cyg X-1 (Stirling et al. 2001) in the low-hard X-ray state. VLBA images of GRS 1915+105 show compact jets with sizes ~ 10cm AU along the same position angle as the superluminal large-scale jets. As in the radio cores of AGN, the brightness temperature of the compact jet in GRS 1915+105 is TB 109 K. The VLBA images of GRS 1915+105 are consistent with the conventional model of a conical expanding jet with synchrotron emission (Hjellming & Johnston, 1988; Falcke & Biermann, 1999) in an optically thick region of solar system size. These compact jets are also found in neutron star X-ray binaries such as LS 5039 (Paredes et al. 2000) and Sco X-1 (Fomalont et al. 2001), and are currently used to track the path of black holes and neutron stars in our Galaxy (see Mirabel & Rodrigues, 2002 for a review).