Jet-dominated advective systems of all mass scales

We show that the radio emission of black hole (BH) and neutron star (NS) X-ray binaries (XRBs) follows the analytical prediction of a jet model where the jet carries a constant fraction of the accretion power. The radio emission can therefore be used as a tracer of the accretion rate. This measure is normalised with efficiently radiating objects. As it is independent of the X-ray fluxes, the measure allows us to compare the accretion rate dependency of the bolometric X-ray luminosity of BHs and NSs. For NSs, it scales linearly with accretion rate while the scaling for BHs is quadratic - as expected for inefficient accretion flows. We find the same behaviour in AGN. This new approach uses the jet power to obtain the accretion rate. Thus, we know both the jet power and the radiated power of an accreting BH. This allows us to show that some accretion power is likely to be advected into the black hole, while the jet power dominates over the bolometric luminosity of a hard state BH.Comment: conference proceedings of the Sixth Microquasar workshop: Microquasars and Beyond, 18-22 September 2006 in Como, Italy (eds: T. Belloni et al. 2006

Microquasars: hard X-ray/gamma-ray emission

I review some of the basic observational details of jets from X-ray binaries, or `microquasars'. It is shown that in both (Z and Atoll) NS and BHC systems radio emission, and therefore jet formation, is correlated with the presence of hard (30-500 keV) X-ray emission. At gamma-ray (> 500 keV) energies, the relation is not so clear. Possible physical connections between the presence of a jet, with a population of relativistic electrons, and the emission of hard X-rays, are briefly discussed.Comment: To be published in Proc. International Symposium on High Energy Gamma-Ray Astronomy, Heidelberg, Eds. F. Aharonian & H. Voelk, 2001, AIP, in pres

The closest black holes

Starting from the assumption that there is a large population (> 10^8) of isolated, stellar-mass black holes (IBH) distributed throughout our galaxy, we consider the detectable signatures of accretion from the interstellar medium (ISM) that may be associated with such a population. We simulate the nearby (radius 250 pc) part of this population, corresponding to the closest ~35 000 black holes, using current best estimates of the mass distribution of stellar mass black holes combined with two models for the velocity distribution of stellar-mass IBH which bracket likely possibilities. We distribute this population of objects appropriately within the different phases of the ISM and calculate the Bondi-Hoyle accretion rate, modified by a further dimensionless efficiency parameter \lambda. Assuming a simple prescription for radiatively inefficient accretion at low Eddington ratios, we calculate the X-ray luminosity of these objects, and similarly estimate the radio luminosity from relations found empirically for black holes accreting at low rates. The latter assumption depends crucially on whether or not the IBH accrete from the ISM in a manner which is axisymmetric enough to produce jets. Comparing the predicted X-ray fluxes with limits from hard X-ray surveys, we conclude that either the Bondi-Hoyle efficiency parameter \lambda, is rather small (< 0.01), the velocities of the IBH are rather high, or some combination of both. The predicted radio flux densities correspond to a population of objects which, while below current survey limits, should be detectable with the Square Kilometre Array (SKA). Converting the simulated space velocities into proper motions, we further demonstrate that such IBH could be identified as faint high proper motion radio sources in SKA surveys.Comment: Accepted for publication in MNRA

Circularly polarised radio emission from GRS 1915+105 and other X-ray binaries

I report the detection of circular polarisation, associated with relativistic ejections, from the `microquasar' GRS 1915+105. I further compare detections and limits of circular polarisation and circular-to-linear polarisation ratios in other X-ray binaries. Since in at least two cases the dominance of linear over circular polarisation or vice versa is a function of frequency, this seems to indicate that this is a strong function of depolarisation in the source. Furthermore, I note that circular polarisation has only been detected from sources whose jets lie close to the plane of the sky, whereas we have quite stringent limits on the circular polarisation of jets which lie close to the line of sight.Comment: To appear in the proceedings of "Circular Polarisation from Relativistic Jet Sources", Eds R. P. Fender and J.-P. Macquart, Kluwer / Astrophysics and Space Science, in pres