Galactic black hole binaries produce powerful outflows which emit over almost the entire electromagnetic spectrum. Here, we report the first detection with the Herschel observatory of a variable far-infrared source associated with the compact jets of the black hole transient GX 339−4 during the decay of its recent 2010–2011 outburst, after the transition to the hard state. We also outline the results of very sensitive radio observations conducted with the Australia Telescope Compact Array, along with a series of near-infrared, optical (OIR) and X-ray observations, allowing for the first time the re-ignition of the compact jets to be observed over a wide range of wavelengths. The compact jets first turn on at radio frequencies with an optically thin spectrum that later evolves to an optically thick synchrotron emission. An OIR reflare is observed about 10 d after the onset of radio and hard X-ray emission, likely reflecting the necessary time to build up enough density, as well as to have acceleration (e.g. through shocks) along an extended region in the jets. The Herschel measurements are consistent with an extrapolation of the radio inverted power-law spectrum, but they highlight a more complex radio to OIR spectral energy distribution for the jets.

We present the first robust evidence of an anticorrelation between the X‐ray photon index, Γ, and the X‐ray luminosity in a single low‐luminosity active galactic nucleus (LLAGN), NGC 7213. Until today, such anticorrelation trends have been seen only in large samples of LLAGN that span a wide range of X‐ray fluxes, although the opposite behaviour (i.e. a positive correlation between Γ and X‐ray luminosity) has been extensively studied for individual X‐ray bright active galactic nuclei. For NGC 7213, we use the long‐term X‐ray monitoring data of the Rossi X‐ray Timing Explorer (RXTE), regularly obtained on average every two days from 2006 March to 2009 December. Based on our X‐ray data, we derive the Γ versus flux and the hardness ratio versus flux relations, indicating clearly that NGC 7213 follows a ‘harder when brighter’ spectral behaviour. Additionally, by analysing radio and optical data, and combining data from the literature, we form the most complete spectral energy distribution (SED) of the source across the electromagnetic spectrum yielding a bolometric luminosity of 1.7 × 1043 erg s−1. Phenomenologically, the SED of NGC 7213 is similar to that of a low‐ionization nuclear emission‐line region. The robust anticorrelation trend that we find between Γ and X‐ray luminosity together with the low accretion rate of the source, 0.14 per cent that of the Eddington limit, makes NGC 7213 the first LLAGN exhibiting a similar spectral behaviour with that of black hole X‐ray binaries in the ‘hard state’.

We have conducted a deep radio survey of a sample of black hole X-ray binaries in the hard and quiescent states, to determine whether any systems were sufficiently bright for astrometric follow-up with high-sensitivity very long baseline interferometric (VLBI) arrays. The one hard-state system, Swift J1753.5-0127, was detected at a level of 0.5 mJy beam −1 . All eleven quiescent systems were not detected. In the three cases with the highest predicted quiescent radio brightnesses (GRO J0422+32, XTE J1118+480, and GRO J1655-40), the new capabilities of the Expanded Very Large Array were used to reach noise levels as low as 2.6 µJy beam −1 . None of the three sources were detected, to 3σ upper limits of 8.3, 7.8, and 14.2 µJy beam −1 , respectively. These observations represent the most stringent constraints to date on quiescent radio emission from black hole X-ray binaries. The uncertainties in the source distances, quiescent X-ray luminosities at the times of the observations, and in the power-law index of the empirical correlation between radio and X-ray luminosities, make it impossible to determine whether these three sources are significantly less luminous in the radio band than expected. Thus it is not clear whether that correlation holds all the way down to quiescence for all black hole X-ray binaries.

We present the results of radio observations of the black hole binaries GRO J1655−40 and XTE J1550−564 in quiescence, with the upgraded Australia Telescope Compact Array. Neither system was detected. Radio flux density upper limits (3σ) of 26 μJy (at 5.5 GHz), 47 μJy (at 9 GHz) for GRO J1655−40 and 1.4 mJy (at 1.75 GHz), 27 μJy (at 5.5 GHz), 47 μJy (at 9 GHz) for XTE J1550−564 were measured. In conjunction with quasi‐simultaneous Chandra X‐ray observations (in the case of GRO J1655−40) and Faulkes Telescope optical observations (XTE J1550−564) we find that these systems provide the first evidence of relatively ‘radio‐quiet’ black hole binaries at low luminosities, indicating that the scatter observed in the hard state X‐ray–radio correlation at higher luminosities may also extend towards quiescent levels.

We present the results of a ∼3 yr campaign to monitor the low-luminosity active galactic nucleus (LLAGN) NGC 7213 in the radio (4.8 and 8.4 GHz) and X-ray bands (2-10 keV). With a reported X-ray Eddington ratio of 7 × 10 −4 L Edd , NGC 7213 can be considered to be comparable to a hard-state black hole X-ray binary. We show that a weak correlation exists between the X-ray and radio light curves. We use the cross-correlation function to calculate a global time-lag between events in the X-ray and radio bands to be 24 ± 12 d lag (8.4 GHz radio lagging X-ray) and 40 ± 13 d lag (4.8 GHz radio lagging X-ray), respectively. The radioradio light curves are extremely well correlated with a lag of 20.5 ± 12.9 d (4.8 GHz lagging 8.4 GHz). We explore the previously established scaling relationship between core radio and X-ray luminosities and black hole mass L R ∝ M 0.6−0.8 L 0.6 X , known as the 'Fundamental Plane of black hole activity', and show that NGC 7213 lies very close to the best-fitting 'global' correlation for the plane as one of the most-luminous LLAGNs. With a large number of quasisimultaneous radio and X-ray observations, we explore for the first time the variations of a single AGN with respect to the Fundamental Plane. Although the average radio and X-ray luminosities for NGC 7213 are in good agreement with the Plane, we show that there is intrinsic scatter with respect to the Plane for the individual data points.