Abstract.Observations have revealed strong evidence for powerful jets in the Low/Hard states of black hole candidate X-ray binaries. Correlations, both temporal and spectral, between the radio -infrared and X-ray bands suggest that jet synchrotron as well as inverse Compton emission could also be significantly contributing at higher frequencies. We show here that, for reasonable assumptions about the jet physical parameters, the broadband spectrum from radio through X-rays can be almost entirely fit by synchrotron emission. We explore a relatively simple model for a relativistic, adiabatically expanding jet combined with a truncated thermal disk conjoined by an ADAF, in the context of the recently discovered black hole binary XTE J1118+480. In particular, the X-ray power-law emission can be explained as optically thin synchrotron emission from a shock acceleration region in the innermost part of the jet, with a cutoff determined by cooling losses. For synchrotron cooling-limited particle acceleration, the spectral cutoff is a function only of dimensionless plasma parameters and thus should be around a "canonical" value for sources with similar plasma properties. It is therefore possible that non-thermal jet emission is important for XTE J1118+480 and possibly other X-ray binaries in the Low/Hard state.

Abstract. The Galactic black hole candidate X-ray binary GX 339−4 spends most of its time in the low/hard state, making it an ideal candidate for modeling the assumedly low accretion phase. The radio emission correlates very tightly with the X-rays over more than two orders of magnitude in X-ray flux density, suggesting that the jet plasma also plays a role at the higher frequencies. We compare the predictions of our jet model, with and without acceleration, to thirteen broadband simultaneous or quasi-simultaneous spectra over this changing flux history. In addition, we consider a simple standard thin disk which transitions to an optically thin accretion flow, in order to account for the assumedly thermal optical data seen in some observations. A solution without acceleration cannot describe the data without unrealistic energy requirements, nor explain the non-thermal radio spectrum seen during recent radio outbursts. But because of the low disk luminosity, and possibly the assumed disk geometry, acceleration in the jet is limited only by synchrotron cooling and can extend easily into the X-rays. We present a model which can account for all the broadband spectra included here, by changing only two parameters in the jet model: the input power and the location of the first acceleration zone. However, the model is most sensitive to changes in the jet power, the varying of which can also account for the slope of the observed radio/X-ray correlation analytically. At the highest low/hard state luminosities, the synchrotron self-Compton emission from the jet could be detectable with missions such as GLAST, providing a way to test the extent of the synchrotron contribution. We conclude that jet synchrotron is a possible way to explain the broadband features and this correlation, and discuss ways of incorporating this component into the "standard" corona picture.

We have analyzed Rossi X-ray timing explorer (RXTE) pointed observations of the outbursts of black hole and neutron star soft X-ray transients in which an initial low/hard state or 'island' state, followed by a transition to a softer state, was observed. In three sources, the black hole transient XTE J1550-564, the neutron star transient Aquila X-1 and a quasi-persistent neutron star low mass X-ray binary (LMXB) 4U 1705-44, two such outbursts were found. We find that the flux of the soft X-ray peak, which lags the hard X-ray peak by a few days to several weeks, scales with the flux of the hard X-ray peak. We conclude that we are able to predict the soft X-ray outburst peak flux based on the 'preceding' hard X-ray peak flux, implying an early set up of the outbursts. We also find that the X-ray luminosity corresponding to the peak of the hard X-ray flux, which corresponds to the X-ray luminosity of the start of the hard-tosoft state transition, varies by a factor of about 2. If the accretion geometry early in the outburst rise is composed of two flows (e.g. a hot sub-Keplerian halo flow and a Keplerian disk flow, or an outflow and a Keplerian disk flow), the correlation indicates that the two flows are initially related, probably due to processes in the outer part of the accretion disk. We discuss constraints on a single flow model and a disk-jet model from these observations.

In 2000 August, we 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 ks with Chandra (ACIS‐I). Attempting to compensate for pile‐up effects, we found that the spectra were well‐fitted by an absorbed power law, with photon indices Γ= 1.54+0.42−0.37 (readout streak) and Γ= 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 utilized 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 an indication of an extended, previously undetected, X‐ray scattering halo. The measured halo fractional intensity at 3 keV is between 30 and 40 per cent 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 hotspot at the end of the northern jet of 1E 1740.7−2942, we are able to constrain the magnetic energy density in that region.

We have performed simultaneous X‐ray and radio observations of 13 Galactic Centre low‐mass X‐ray binaries in 1998 April using the Wide Field Cameras on board BeppoSAX and the Australia Telescope Compact Array, the latter simultaneously at 4.8 and 8.64 GHz. We detect two Z sources, GX 17+2 and GX 5−1, and the unusual ‘hybrid’ source GX 13+1. Upper limits, which are significantly deeper than previous non‐detections, are placed on the radio emission from two more Z sources and seven atoll sources. Hardness–intensity diagrams constructed from the Wide Field Camera data reveal GX 17+2 and GX 5−1 to have been on the lower part of the horizontal branch and/or the upper part of the normal branch at the time of the observations, and the two non‐detected Z sources, GX 340+0 and GX 349+2, to have been on the lower part of the normal branch. This is consistent with the previous empirically determined relation between radio and X‐ray emission from Z sources, in which radio emission is strongest on the horizontal branch and weakest on the flaring branch. For the first time we have information on the X‐ray state of atoll sources, which are clearly radio‐quiet relative to the Z sources, during periods of observed radio upper limits. We place limits on the linear polarization from the three detected sources, and use accurate radio astrometry of GX 17+2 to confirm that it is probably not associated with the optical star NP Ser. Additionally we place strong upper limits on the radio emission from the X‐ray binary 2S 0921−630, disagreeing with suggestions that it is a Z‐source viewed edge‐on.

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