In the last 15 years several categories of X-ray binaries and X-ray transients (X-ray ``novae'') have been studied. They demonstrate a strong need for the A+ configuration to allow sensitive imaging with 5-10 milli-arcsec resolution, and for simultaneous observing with the VLBA to cover size scales from 0.5 milli-arcsec up to a few arcsec. VLBA and VLA observations of the newly discovered relativistic jets in the X-ray transient GRO J1655-40 show both the potential for the study of relativistic, X-ray source jets, and the critical information gap between the VLA and the VLBA. The VLA studies of GRS1915+105 show that it is a similar relativistic jet, with an estimated (but not directly measured) jet velocity of 0.92c. It shows tantalizing information on the 20 milli-arcsec scales accessible with the VLA: but without the A+ configuration and the VLBA one cannot reach the level of detail seen in GRO J1655-40. The 0.92c jets in GRO J1655-40 show structure moving between 54 and 45 milli-arcsec/day that varies from superluminal to subluminal, linearly polarized features, and ejection on both sides of the core. Detailed structures are seen which are some combination of kinematic modulation of the ejecta and physical substructures. These ejecta quickly evolve from size scales of 5-10 milli-arcsec to large scales, then appear resolved out with the VLBA while they are unresolved with the current VLA, and are unseen substructures until they move/expand to the 100 milli-arcsec scales where they are resolved features in high resolution VLA images.
Imaging with the A+ configuration would provide excellent minute-to-minute snapshots that would avoid the need to calibrate VLBA data with models that include the motions of complex sources during the times of observation. In addition, spectral index imaging of 0.1- jets requires the A+ configuration; and for the 10-100 milli-arcsec scales, the A+ configuration is essential to get the needed higher frequency images at the right size and brightness sensitivity scale. Without this, astrophysical analysis of detailed structures such as in GRO J1655-40 will be difficult.
GRO J1655-40 and GRS1915+105 are certain candidates for the level of detailed study that is not possible for extragalactic jets. What evidence is there that other X-ray binaries and transients exhibit similar phenomena requiring the A+ configuration or cross-linking the A+ configuration with the VLBA? Both SS433 and Cyg X-3 are known to have continuous or flare-produced jets with speeds of 0.26c and (0.2-0.4)c, respectively. EVN observations of SS433 have directly shown that jet ejecta have a ``brightening'' region which extends from the core to about 30 milli-arcsec (for a distance of 5.5 kpc). Until now this is the only case where in situ particle acceleration near a ``central engine'' has been directly observed. On three occasions, Cyg X-3 has been seen to have 0.2-0.4c North-South expansion, and it was once imaged as a triplet on size scales of . The A+ configuration, or inner VLBA elements with the A+ configuration, are the only instruments with the combination of surface brightness sensitivity and resolution to examine the acceleration regions in these relativistic jets.
Six other X-ray transients have been shown to have the sort of decaying, post-flare, synchrotron emission seen in GRO J1655-40: GS2023+338 (V404 Cyg), GS1124-683 (``Nova'' Muscae 1991), GS2000+35, A0620-00, Cen X-4 and Aql X-1. The time scale for these events ranges from a few days to a few tens of days. In only two cases was it possible to look at these sources with the VLA A configuration, and they were unresolved; however, this covered very little of the decay events. Some of these events may have been image-able with the A+ configuration, or the VLBA. Without knowing that there are resolved ejecta from VLA or A+ configuration snapshots, such objects are unlikely to get VLBA attention during the critical 1-5 day time scales when they are strongest. (The most important VLBA images of GRO J1655-40 began one day after the peak in the radio light curve and covered the next 10-15 days after two interesting flares.)
About 15 other X-ray binaries are known radio sources. One class, the so-called Z-sources (after their Z-shaped X-ray color-color diagrams) show state changes in which changes in the structure of their accretion disks are followed by state changes in radio and UV emission (Sco X-1, GX17+2, Cyg X-2). A change in a large-scale incoherent synchrotron emission region tens of minutes after an alteration in accretion disk structure requires bulk propagation velocities of the order of 0.1c or greater. Since this emission is typically weak (1-30 mJy) the imaging of this ejecta, estimated to be several mas or larger in size, will require the sensitivity of the A+ configuration or the cross-linked VLA+VLBA.
The ``great annihilator'' 1E1740.7-2941 and GRS1758-258 are both associated with -ray sources that are similar to GRO J1655-40 and GRS 1915+105. Both have twin jets on scales from several to tens of arc seconds. Their jets are variable, but neither has had a regular sequence of high resolution imaging to see structure variations in their core and inner jets. They may be similar to the two highly-relativistic jet sources during and after flare events but have only been observed long afterwards, when only surviving large ejecta are present.
It is a mixture of the short time scales of significant motion, expansion, and evolution, and the fact that they quickly achieve angular sizes within the resolution of the VLA/VLBA ``gap'' that makes the A+ configuration, preferably cross-linked with the VLBA, critical for studying the resolved relativistic jets in the X-ray binary and transient systems in our own galaxy. The detailed spatial and spectral index imaging of these phenomena as a function of time, coupled with -ray, X-ray, UV, and optical studies, will provide a level of detailed, evolution-tracing information unattainable for AGN jets.