GRO J1655-40 was the second apparently superluminal radio source to be discovered in our Galaxy, after GRS 1915+105 [Mirabel & Rodriguez 1994], but the first to have very high resolution images reveal the detailed morphology of the source. The high and low resolution radio imaging [Tingay et al. 1995; Hjellming and Rupen 1995], the X-ray monitoring [Harmon et al. 1995], and optical observations [Bailyn et al. 1995a; Bailyn et al. 1995b] make GRO J1655-40 one of the best studied X-ray transients yet. The data, when taken together, make a strong empirical case for a direct connection between the accretion of material onto a compact, stellar mass object (possibly a black hole) and the production of relativistic jets. The relationship between evolution in the compact radio source and emissions at other wavelengths has been well demonstrated for this object.
The radio source associated with GRO J1655-40 has been shown to be produced at the time of the initial strong outburst in radio emission, following the first outburst in X-rays. Harmon et al. [1995] show a similar correspondence between X-ray outburst and relativistic jet production for two later outbursts from GRO J1655-40, in 1994 September and 1994 November. On each of these occasions a strong X-ray outburst was followed closely by a radio outburst and the ejection of a new component of radio emission from the core. However, interestingly, while the X-ray outbursts continued at the same strength, each subsequent radio outburst was approximately an order of magnitude weaker than its predecessor [Harmon et al. 1995]. GRO J1655-40 thus made the transition from an accreting radio loud source to an accreting radio quiet source.
Bailyn et al. [1995a; 1995b] reported optical observations of GRO J1655-40 during its quiescent state. They found variations in the total output at optical wavelengths typical of an eclipsing binary system. From the eclipses and optical spectroscopy they calculate the orbital period to be approximately 2.6 days. They also determined the mass function of the system to be 3.35 
0.14 
and suggested that the primary mass of the system is in a black hole with mass 
5.3 .
These conclusions make objects like GRO J1655-40 very interesting. All of the activity associated with an accretion driven relativistic jet can be observed on very small spatial scales and over a short period in time. Since these systems may represent scaled-down versions of galaxy-scale galactic nuclei and radio jets, they may yield important clues to the process of accretion and jet production in galaxies and quasars. Since the discovery of GRS 1915+105 and GRO J1655-40 several investigators have made an attempt to understand the Galactic superluminals and their connection with AGN.
Levinson and Blandford [1996] investigated the physical conditions within the Galactic superluminal sources, their possible formation and the interactions with their environments. They contrast the properties of e - p and e 
- e jets. From their calculations they derive for GRO J1655-40 an equipartition minimum pressure of 
dyn cm 
, magnetic field of 
G, a jet luminosity of 
ergs s 
, and an annihilation radius for electron positron pairs of greater than 
cm, for an e - e jet. The jet luminosity for an e - p jet is higher, 
ergs s .
Levinson and Blandford [1996] also predict the wavelength of blue and red shifted H lines which would support the case for an e - p jet, 1.4 and 1.8 
m respectively. No reports of red and blue shifted H lines have thus far been reported for GRO J1655-40 or GRS 1915+105.
Meier [1996] concentrates on a model of jet production for the Galactic superluminal sources and an explanation of the observed delay between X-ray outburst, signifying accretion, and the production of a relativistic jet. Meier [1996] suggests that the delay between the onset of accretion and jet production occurs when the accretion rate exceeds approximately one third of the Eddington limit, citing the Papaloizou-Pringle compressible shear instability as the agent which disrupts the jet producing region of the source during this high accretion phase. Once the accretion slows and stabilises the relativistic jet forms via the Blandford-Payne MHD acceleration process seeded by an e - e wind. Meier [1996] gives a detailed application of this model to GRO J1655-40 in particular and to other objects such as GRS 1915+105, 1E 1740.7-2942, and Cygnus X-1.
Falke and Beirmann [1996] describe their hypothesis that jets and disks around compact accreting objects are symbiotic features and apply it to the new class of Galactic superluminal sources. They do this by comparing their predictions of disk to radio core luminosities to the observed values. From their model they find that the disk/jet symbiosis can explain the Galactic superluminals and other Galactic jet sources in a radio loud - radio quiet paradigm similar to what they find for radio loud and radio quiet AGN. Thus, they find that their model predictions are reasonable over approximately 15 orders of magnitude in radio power and 10 orders of magnitude in disk luminosity.
Observations of the Galactic superluminals have sparked a significant theoretical effort in attempting to explain their nature and possible relationship to the more powerful extragalactic jet sources. At present it appears that it is plausible that an underlying process controls jet production over a wide range in jet power and spatial scale, and that the process is strongly related to accretion. Strong results and observational predictions regarding the details of the production mechanism are still elusive however.