42 research outputs found

    Pulsar scattering in space and time

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    We report on a recent global VLBI experiment in which we study the scatter broadening of pulsars in the spatial and time domain simultaneously. Depending on the distribution of scattering screen(s), geometry predicts that the less spatially broadened parts of the signal arrive earlier than the more broadened parts. This means that over one pulse period the size of the scattering disk should grow from pointlike to the maximum size. An equivalent description is that the pulse profile shows less temporal broadening on the longer baselines. This contribution presents first results that are consistent with the expected expanding rings. We also briefly discuss how the autocorrelations can be used for amplitude calibration. This requires a thorough investigation of the digitisation and the sampler statistics and is not fully solved yet.Comment: 6 pages. Talk presented at the 11th European VLBI Network Symposium (Bordeaux, France, 9-12 October 2012), PoS(11th EVN Symposium)04

    Degeneracies and scaling relations in general power-law models for gravitational lenses

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    The time delay in gravitational lenses can be used to derive the Hubble constant in a relatively simple way. The results of this method are less dependent on astrophysical assumptions than in many other methods. The most important uncertainty is related to the mass model used. We discuss a family of models with a separable radial power-law and an arbitrary angular dependence for the potential psi = r^beta * F(theta). Isothermal potentials are a special case of these models with beta=1. An additional external shear is used to take into account perturbations from other galaxies. Using a simple linear formalism for quadruple lenses, we can derive H0 as a function of the observables and the shear. If the latter is fixed, the result depends on the assumed power-law exponent according to H0 proportional to (2-beta)/beta. The effect of external shear is quantified by introducing a `critical shear' gamma_c as a measure for the amount of shear that changes the result significantly. The analysis shows, that in the general case H0 and gamma_c do not depend on the position of the lens galaxy. We discuss these results and compare with numerical models for a number of real lens systems.Comment: accepted for publication in MNRAS, 10 pages, 4 figures (eps included), uses mn2e.cls, amsmath.sty, times.st