New radio observations of Circinus X-1

New radio observations of the radio-jet X-ray binary Circinus X-1 over nearly an entire 16.6-day orbit are presented. The source continues to undergo radio flaring in the phase interval 0.0 - 0.2 and appears to be brightening since observations in the early 1990s. The radio flux density is well correlated with simultaneous soft X-ray monitoring from the XTE ASM, including a secondary flare event around phases 0.6 --0.8 observed at both energies.Comment: 5 pages, 2 figures. To appear in Proc. 4th Compton Symposium, AIP pres

The very flat radio - millimetre spectrum of Cygnus X-1

We present almost-simultaneous detections of Cygnus X-1 in the radio and mm regimes, obtained during the low/hard X-ray state. The source displays a flat spectrum between 2 and 220 GHz, with a spectral index flatter than 0.15 (3sigma). There is no evidence for either a low- or high-frequency cut-off, but in the mid-infrared (~30 microns) thermal emission from the OB-type companion star becomes dominant. The integrated luminosity of this flat-spectrum emission in quiescence is > 2 x 10^{31} erg/s (2 x 10^{24} W). Assuming the emission originates in a jet for which non-radiative (e.g. adiabatic expansion) losses dominate, this is a very conservative lower limit on the power required to maintain the jet. A comparison with Cyg X-3 and GRS 1915+105, the other X-ray binaries for which a flat spectrum at shorter than cm wavelengths has been observed, shows that the jet in Cyg X-1 is significantly less luminous and less variable, and is probably our best example to date of a continuous, steady, outflow from an X-ray binary. The emissive mechanism reponsible for such a flat spectral component remains uncertain. Specifically, we note that the radio-mm spectra observed from these X-ray binaries are much flatter than those of the `flat-spectrum' AGN, and that existing models of synchrotron emission from partially self-absorbed radio cores, which predict a high-frequency cut-off in the mm regime, are not directly applicable.Comment: Accepted for publication in MNRA

Jets in neutron star X-ray binaries: a comparison with black holes

(Abridged) We present a comprehensive study of the relation between radio and X-ray emission in neutron star X-ray binaries, use this to infer the general properties of the disc-jet coupling in such systems, and compare the results quantitatively with those already established for black hole systems. There are clear qualitative similarities between the two classes of object: hard states below about 1% of the Eddington luminosity produce steady jets, while transient jets are associated with outbursting and variable sources at the highest luminosities. However, there are important quantitative differences: the neutron stars are less radio-loud for a given X-ray luminosity (regardless of mass corrections), and they do not appear to show the strong suppression of radio emission in steady soft states which we observe in black hole systems. Furthermore, in the hard states the correlation between radio and X-ray luminosities of the neutron star systems is steeper than the relation observed in black holes by about a factor of two. This result strongly suggests that the X-ray emission in the black hole systems is radiatively inefficient, with an approximate relation of the form L_X \propto \dot{m}^2, consistent with both advection-dominated models and jet-dominated scenario. On the contrary the jet power in both classes of object scales linearly with accretion rate. This constitutes some of the first observational evidence for the radiatively inefficient scaling of X-ray luminosity with accretion rate in accreting black hole systems.Comment: Accepted for publication in MNRA

Powerful jets from accreting black holes: evidence from the optical and infrared

A common consequence of accretion onto black holes is the formation of powerful, relativistic jets that escape the system. In the case of supermassive black holes at the centres of galaxies this has been known for decades, but for stellar-mass black holes residing within galaxies like our own, it has taken recent advances to arrive at this conclusion. Here, a review is given of the evidence that supports the existence of jets from accreting stellar-mass black holes, from observations made at optical and infrared wavelengths. In particular it is found that on occasion, jets can dominate the emission of these systems at these wavelengths. In addition, the interactions between the jets and the surrounding matter produce optical and infrared emission on large scales via thermal and non-thermal processes. The evidence, implications and applications in the context of jet physics are discussed. It is shown that many properties of the jets can be constrained from these studies, including the total kinetic power they contain. The main conclusion is that like the supermassive black holes, the jet kinetic power of accreting stellar-mass black holes is sometimes comparable to their bolometric radiative luminosity. Future studies can test ubiquities in jet properties between objects, and attempt to unify the properties of jets from all observable accreting black holes, i.e. of all masses.Comment: 26 pages, 4 figures, 1 table. Invited chapter for the edited book "Black Holes and Galaxy Formation", Nova Science Publishers, Inc., at pres

GRS 1915+105 : Flares, QPOs and other events at 15 GHz

Monitoring with the Ryle Telescope at 15 GHz of the Galactic X-ray transient source GRS 1915+105 has revealed a remarkable range of rapid and extended flares which appear to be related to the X-ray emission as recorded by the RXTE all-sky monitor. Quasi-periodic oscillations in the range 20 - 40 min have been found and are probably related to oscillations in the soft X-ray flux.Comment: 2 pages. To be published in Proc. IAU164 : Radio emission from Galactic and Extragalactic Compact Sources (Socorro, 1997