Welcome to the home of the Jet Acceleration and Collimation Probe Of Transient X-Ray Binaries (JACPOT XRB)!
Collimated outflows, or jets, are believed to exist in accreting compact objects, powered by the process of accretion onto the central compact object. In our own Milky Way Galaxy, stellar-mass compact objects may exist in binary systems with less evolved companion stars, and in certain evolutionary phases, material is transferred from those companion stars to the compact objects. Not all of the matter transferred reaches the accretor, with some of the material being ejected in the form of collimated jets, which in some cases may be relativistic.
Such accreting compact objects evolve through well-defined states, defined by characteristic X-ray spectral and timing properties. For accreting stellar mass black holes, the duty cycles are believed to be of order 10-50 years, with the systems spending the majority of their time in a low-luminosity quiescent state, but undergoing occasional outbursts where they brighten by several orders of magnitude in the X-ray and radio bands, and show evidence for discrete relativistic ejecta moving away from the system. Similar trends have also been observed in neutron star and white dwarf systems.
Studies of the X-ray and radio properties of the outbursts of such systems shows that the sources all trace out similar patterns in a plot of X-ray hardness against X-ray intensity, and the radio properties of the systems are believed to correlate with position in such an X-ray hardness-intensity diagram (Fender, Belloni & Gallo, 2004). When the source is in a low-luminosity hard or quiescent X-ray state, the radio emission is thought to arise from a partially self-absorbed, steady, compact jet. As the X-ray intensity rises, the power in this jet increases. As the X-ray spectrum softens during an outburst, the jet velocity is believed to increase, leading to internal shocks in the flow which form the bright, relativistic ejecta seen during outbursts. The radio jet then switches off until the outburst is over and the X-ray spectrum hardens once again. However, this paradigm has never been directly tested with high-resolution radio imaging.
JACPOT XRB aims to make use of the unparalleled angular resolution and sensitivity of the VLBA to monitor outbursts of each of the three classes of system, in order to test the paradigm outlined above. This will provide the most comprehensive high-resolution datasets ever taken in order to determine the similarities and differences between jet formation and evolution in accreting stellar mass compact objects. Together with simultaneous X-ray, optical and infrared monitoring, this will provide unprecedented datasets in which to test the current paradigm for the jet-disc coupling, and, by comparing the three, to probe the importance of the depth of the gravitational potential well, the stellar surface and the stellar magnetic field, in the production, acceleration and collimation of jets.