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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

Using SKA to observe relativistic jets from X-ray binary systems

I briefly outline our current observational understanding of the relativistic jets observed from X-ray binary systems, and how their study may shed light on analogous phenomena in Active Galactic Nuclei and Gamma Ray Bursts. How SKA may impact on this field is sketched, including the routine tracking of relativistic ejections to large distances from the binaries, detecting and monitoring the radio counterparts to 'quiescent' black holes, and detecting the radio counterparts of the brightest X-ray binaries throughout the Local Group of galaxies.Comment: To appear in "Science with the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews (Elsevier: Amsterdam

Spectral differences between the jets in `radio loud' and `radio quiet' hard state black hole binaries

We have compiled from the available literature a large set of radio measurements of black hole binaries in the hard X-ray state for which measurements of the gigahertz frequency radio spectral index are possible. We separate the sample into `radio loud' and `radio quiet' subsets based upon their distribution in the radio -- X-ray plane, and investigate the distribution of radio spectral indices within each subset. The distribution of spectral indices of the `radio loud' subset is well described by a Gaussian distribution with mean spectral index $\alpha = +0.2$ and standard deviation $0.2$ (here spectral index is defined such that a positive spectral index means more flux at higher frequencies). The sparser sample for the `radio quiet' subset can be approximated, less well, by a Gaussian with mean $\alpha = -0.2$ and standard deviation $0.3$; alternatively the simple mean of the distribution of the radio quiet subset is $-0.3$. The two spectral index distributions are different at high statistical significance. Confirming previous work in the literature, we test to see if the differences in observed spectra could result from different distributions of jet viewing angles, but find no evidence for this. We conclude therefore that the jets in the two groups are physically different in some way, and briefly discuss possible origins and further possible diagnostics. Finally we note that extrapolating to lower frequencies the two subsets move closer together in the radio -- X-ray plane, and approximately merge into a single distribution at around 400 MHz.Comment: Accepted for publication in MNRA

Chandra imaging spectroscopy of 1E 1740.7 - 2942

We have observed the black hole candidate 1E 1740.7 - 2942, the brightest persistent hard X-ray source within a few degrees of the Galactic centre, for 10 ksec with Chandra (ACIS-I) on August 2000. Attempting to compensate for pile-up effects we found the spectra were well-fit by an absorbed power law, with photon indices Gamma = 1.54 ^{+0.42}_{-0.37} (readout streak) and Gamma = 1.42^{+0.14}_{-0.14} (annulus), consistent with a black hole low/hard state. We have analysed a public observation performed by Chandra which utilised short frames in order to avoid severe pile-up effects: subtracting the core point spread function from the whole image, we did not find evidence for any elongated feature perpendicular to the radio jet axis, as reported in a recent analysis of the same data. Moreover, comparing the radial profiles with those of an unscattered X-ray point source, we found indication of an extended, previously undetected, X-ray scattering halo. The measured halo fractional intensity at 3 keV is between 30 and 40 percent within 40 arcsec but drops below detectable levels at 5 keV. Finally, by placing a limit on the X-ray flux from the radio emitting lobe which has been identified as the hot spot at the end of the northern jet of 1E 1740.7 - 2942, we are able to constrain the magnetic energy density in that region.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

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