[en] The Crab Nebula has been observed with the Cambridge 5-km radio telescope at a frequency of 2.695 GHz, giving an angular resolution of 3.7 x 10 arcsec. Maps are presented showing total intensity and linear polarization. The nebula has a filamentary structure at this wavelength, strikingly similar to that seen in optical line emission. An upper limit of 0.1 per cent is placed on any circular polarization. (author)

[en] Radio polarization observations with the VLA of 3C 275.1, a 'bent' quasar at z = 0.555, show strong depolarization associated with the host galaxy and two other galaxies, one of which is thought to be in the foreground and one of which may be interacting strongly with the quasar itself. (author)

[en] Observations at five radio frequencies are used to determine the structure and spectra of a complex region in the direction 0648 + 19. We show that it consists of three separate radio sources; two tailed sources in one cluster, and an unrelated classical double. We have used spectral-aging calculations to derive ages and velocities for the two tailed sources. Combining these results with other data we discuss whether galaxy motion alone can account for the distribution of plasma. We find that the velocities derived from spectral ageing are not consistent with this hypothesis and favour an explanation which includes bulk streaming in the tails at velocities of several thousand km s^-1

[en] The results of three EXOSAT observations of XO142+614 made in 1984 August, 1985 November and December are reported. A 25-min coherent modulation was detected during the first observation, but not during the following two. The modulation peak-to-peak amplitude was 55 per cent between 3 and 8 keV and < 1 per cent between 1 and 3 keV. The 1-10 keV spectrum was fit with a power law of energy index approx. 3 in all three observations, except during the observation when X-ray pulsations were detected when a second power-law component with an energy index of approx. 1 and a high and a high absorption column density was also required. The data are consistent with the modulation being confined to the flatter spectral component. New position measurements from both EXOSAT and the Einstein HRI are presented. (author)

[en] To date, mid-infrared properties of Galactic black hole binaries have barely been investigated in the framework of multi-wavelength campaigns. Yet, studies in this spectral domain are crucial to get complementary information on the presence of dust and/or on the physical processes such as dust heating and thermal bremsstrahlung. Here, we report a long-term multi-wavelength study of the microquasar GRS 1915+105. On the one hand, we aimed at understanding the origins of the mid-infrared emission, and on the other hand, at searching for correlation with the high-energy and/or radio activities. We observed the source at several epochs between 2004 and 2006 with the photometer IRAC and spectrometer IRS, both mounted on the Spitzer Space Telescope. When available, we completed our set of data with quasi-simultaneous RXTE/INTEGRAL high-energy and/or Ryle radio observations from public archives. We then studied the mid-infrared environment and activities of GRS 1915+105 through spectral analysis and broadband fitting of its radio to X-ray spectral energy distributions. We detected polycyclic aromatic hydrocarbon molecules in all but one IRS spectra of GRS 1915+105 which unambiguously proves the presence of a dust component, likely photoionized by the high-energy emission. We also argue that this dust is distributed in a disk-like structure heated by the companion star, as observed in some Herbig Ae/Be and isolated cool giant stars. Moreover, we show that some of the soft X-ray emission emanating from the inner regions of the accretion disk is reprocessed and thermalized in the outer part. This leads to a mid-infrared excess that is very likely correlated to the soft X-ray emission. We exclude thermal bremsstrahlung as contributing significantly in this spectral domain.

[en] We analyzed radio and X-ray observations of GRS 1915+105, between 1995 May and 2006 June, focusing on the times characterized by radio flares and cycles of hard dips-soft spikes in the X-ray light curve. Assuming these flares to be discrete ejections, we applied a plasmon model to the radio data, with good agreement with the light curves. We fitted a total of 687 radio flares with a standard model of a plasmon. We found that the distribution of width is t₀ = 1160 s with an rms deviation of 360 s, while that of the amplitude is S _max = 59 mJy with an rms deviation of 28 mJy. The distribution of width is thus rather peaked, while that of the amplitude is not. Regarding radio and X-ray links, this study confirms previous observations on smaller data sets, namely that X-ray cycles of hard dips-soft spikes are always followed by radio flares. A strong correlation is found between the length of X-ray 'dips' in the X-ray light curves and the amplitude and fluence of the subsequent radio oscillations. A model of an exponential rise of the form L_15GHz(Δt) = L _max(1 - exp(-(Δt - Δt_min)/τ) is in good agreement with the observations, with the maximum fluence L_max on the order of 70 Jy s, and the characteristic time τ on the order of 200-500 s. We discuss possible physical interpretations of this correlation, regarding the nature of the ejected material and the physical process responsible for the ejection.

[en] We report the discovery of a systematic constant time lag between the X-ray and radio flares of the gamma-ray binary LSI +61 303, persistent over a long, multi-year timescale. Using the data from the monitoring of the system by RXTE we show that the orbital phase of X-ray flares from the source varies from φ_X ≅ 0.35 to φ_X ≅ 0.75 on the superorbital 4.6 yr timescale. Simultaneous radio observations show that periodic radio flares always lag the X-ray flare by Δφ_X–R ≅ 0.2. We propose that the constant phase lag corresponds to the time of flight of the high-energy particle-filled plasma blobs from inside the binary to the radio emission region at the distance of ∼10 times the binary separation distance. We put forward a hypothesis that the X-ray bursts correspond to the moments of formation of plasma blobs inside the binary system.

[en] We present continued radio observations of the tidal disruption event Swift J164449.3+573451 extending to δt ≈ 216 days after discovery. The data were obtained with the EVLA, AMI Large Array, CARMA, the SMA, and the VLBA+Effelsberg as part of a long-term program to monitor the expansion and energy scale of the relativistic outflow, and to trace the parsec-scale environment around a previously dormant supermassive black hole (SMBH). The new observations reveal a significant change in the radio evolution starting at δt ≈ 1 month, with a brightening at all frequencies that requires an increase in the energy by about an order of magnitude, and an overall density profile around the SMBH of ρ∝r^–3/2 (0.1-1.2 pc) with a significant flattening at r ≈ 0.4-0.6 pc. The increase in energy cannot be explained with continuous injection from an L∝t^–5/3 tail, which is observed in the X-rays. Instead, we conclude that the relativistic jet was launched with a wide range of Lorentz factors, obeying E(> Γ_j)∝Γ^–2.5_j. The similar ratios of duration to dynamical timescale for Sw 1644+57 and gamma-ray bursts (GRBs) suggest that this result may be applicable to GRB jets as well. The radial density profile may be indicative of Bondi accretion, with the inferred flattening at r ∼ 0.5 pc in good agreement with the Bondi radius for a ∼few × 10⁶ M_☉ black hole. The density at ∼0.5 pc is about a factor of 30 times lower than inferred for the Milky Way Galactic Center, potentially due to a smaller number of mass-shedding massive stars. From our latest observations (δt ≈ 216 days) we find that the jet energy is E_j,iso ≈ 5 × 10⁵³ erg (E_j ≈ 2.4 × 10⁵¹ erg for θ_j = 0.1), the radius is r ≈ 1.2 pc, the Lorentz factor is Γ_j ≈ 2.2, the ambient density is n ≈ 0.2 cm^–3, and the projected angular size is r_proj ≈ 25 μas, below the resolution of the VLBA+Effelsberg. Assuming no future changes in the observed evolution and a final integrated total energy of E_j ≈ 10⁵² erg, we predict that the radio emission from Sw 1644+57 should be detectable with the EVLA for several decades and will be resolvable with very long baseline interferometry in a few years.

[en] We present continued multi-frequency radio observations of the relativistic tidal disruption event Swift J164449.3+573451 (Sw 1644+57) extending to t ≈ 600 days. The data were obtained with the JVLA and AMI Large Array as part of our on-going study of the jet energetics and the density structure of the parsec-scale environment around the disrupting supermassive black hole. We combine these data with public Swift/XRT and Chandra X-ray observations over the same time-frame to show that the jet has undergone a dramatic transition starting at ≈500 days, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of δt/t ∼< 0.2 (and by a factor of 15 in δt/t ≈ 0.05). The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at ∼< 500 days, and instead points to internal dissipation in the inner jet. On the other hand, our radio data uniquely demonstrate that the low X-ray flux measured by Chandra at ≈610 days is consistent with emission from the forward shock. Furthermore, the Chandra data are inconsistent with thermal emission from the accretion disk itself since the expected temperature of ∼30-60 eV and inner radius of ∼2-10 R_s cannot accommodate the observed flux level or the detected emission at ∼> 1 keV. We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below ∼ M-dot_Edd∼0.006 M_☉ yr^–1 (for a 3 × 10⁶ M_☉ black hole and order unity efficiency) indicating that the peak accretion rate was about 330 M-dot_Edd, and the total accreted mass by t ≈ 500 days is about 0.15 M_☉. From the radio data we further find significant flattening in the integrated energy of the forward shock at t ∼> 250 days with E_j,iso ≈ 2 × 10⁵⁴ erg (E_j ≈ 10⁵² erg for a jet opening angle, θ_j = 0.1) following a rise by about a factor of 15 at ≈30-250 days. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates.

[en] Models of jet production in black hole systems suggest that the properties of the accretion disk—such as its mass accretion rate, inner radius, and emergent magnetic field—should drive and modulate the production of relativistic jets. Stellar-mass black holes in the 'low/hard' state are an excellent laboratory in which to study disk-jet connections, but few coordinated observations are made using spectrometers that can incisively probe the inner disk. We report on a series of 20 Suzaku observations of Cygnus X-1 made in the jet-producing low/hard state. Contemporaneous radio monitoring was done using the Arcminute MicroKelvin Array radio telescope. Two important and simple results are obtained: (1) the jet (as traced by radio flux) does not appear to be modulated by changes in the inner radius of the accretion disk and (2) the jet is sensitive to disk properties, including its flux, temperature, and ionization. Some more complex results may reveal aspects of a coupled disk-corona-jet system. A positive correlation between the reflected X-ray flux and radio flux may represent specific support for a plasma ejection model of the corona, wherein the base of a jet produces hard X-ray emission. Within the framework of the plasma ejection model, the spectra suggest a jet base with v/c ≅ 0.3 or the escape velocity for a vertical height of z ≅ 20 GM/c ² above the black hole. The detailed results of X-ray disk continuum and reflection modeling also suggest a height of z ≅ 20 GM/c ² for hard X-ray production above a black hole, with a spin in the range 0.6 ≤ a ≤ 0.99. This height agrees with X-ray time lags recently found in Cygnus X-1. The overall picture that emerges from this study is broadly consistent with some jet-focused models for black hole spectral energy distributions in which a relativistic plasma is accelerated at z = 10-100 GM/c ². We discuss these results in the context of disk-jet connections across the black hole mass scale.