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A review is given of the interplay between studies of compact radio sources and the scattering and scintillations that occur as the signals travel through the irregular refractive index of the interstellar and interplanetary plasmas.
I first review the observables and optics of interstellar seeing associated with radio wave scattering in the interstellar medium. I then describe the Galactic distribution of electron density and its fluctuations, as inferred from a number of observables, including angular and pulse broadening, diffractive scintillations, and dispersion measures. Propects for improving the Galactic model are outlined.
The inertial range of incompressible MHD turbulence is most conveniently described in terms of counter propagating waves. Shear Alfvén waves control the cascade dynamics. Slow waves play a passive role and adopt the spectrum set by the shear Alfvén waves. Cascades composed entirely of shear Alfvén waves do not generate a significant measure of slow waves. MHD turbulence is anisotropic with energy cascading more rapidly along k⊥ than along k‖. Anisotropy increases with k⊥ such that the excited modes are confined inside a cone bounded by . The opening angle of the cone, , defines the scale dependent anisotropy. MHD turbulence is generically strong in the sense that the waves which comprise it are critically damped. Nevertheless, deep inside the inertial range, turbulent fluctuations are small. Their energy density is less than that of the background field by a factor θ2(k⊥) « 1. MHD cascades are best understood geometrically. Wave packets suffer distortions as they move along magnetic field lines perturbed by counter propagating wave packets. Field lines perturbed by unidirectional waves map planes perpendicular to the local field into each other. Shear Alfvén waves are responsible for the mapping’s shear and slow waves for its dilatation. The former exceeds the latter by θ−1 (k⊥) » 1 which accounts for dominance of the shear Alfvén waves in controlling the cascade dynamics.
Radio signals from pulsars are significantly affected by scattering in the interstellar medium. A review of this phenomenon of pulsar scintillation forms the main objective of this paper. The basic concepts are described and some new results related to the following aspects are presented: (i) understanding of refractive scintillation effects and (ii) constraining the spectrum of electron density fluctuations in the interstellar medium.
Chapter Two Pulsars: Their Scattering and Intrinsic Properties
The Parkes multibeam pulsar survey is a major survey for pulsars lying within a 10°-wide strip along the southern Galactic plane, using the multibeam receiver on the Parkes 64-m radiotelescope. It is an international collaboration between groups at Jodrell Bank Observatory, Massachusetts Institute of Technology, Bologna Astronomical Observatory and the ATNF. The survey commenced in 1997 August, and has so far succeeded in finding more than 550 previously unknown pulsars. Many of these are distant, with some beyond the centre of the Galaxy according to current models of the interstellar electron density distribution. Interstellar scattering affects the pulse profile of many of the more distant pulsars even at 1374 MHz, the centre frequency of the survey. Preliminary results from the survey are presented.
Scintillation times and decorrelation bandwidths for the pulsars B0329+54, B1641-15, B1508+55 and B1919+21 are determined. The results are based on observations made with different instruments and at different radio frequencies. All objects but the pulsar B1508+55 were detected to have more than one frequency scale. The obtained values of scattering parameters are not contrary in general to the Kkolmogorov form of density fluctuation spectrum.
Propagation effects are well known to limit the sensitivity of pulsar searches based on periodicity detections. I define several regimes for pulsar searches that are based on whether the search sensitivity is luminosity limited, dispersion limited or scattering limited. Consideration of these regimes allows general statements to be made about pulsar searches in and out of the Galactic plane. Telescope size matters, but only to a point. Once scattering becomes important it is better to search more sky (in a blind survey) than to integrate longer on a given sky position. Example surveys are described.
We report the results of the measurements and analysis of the pulse broadening due to interstellar scattering on 43 pulsars at 102 MHz. This is the largest uniform sample of direct measurements of pulsar scattering τsc, which make it feasible to analyze the dependence of this value on other pulsar parameters. The measured dependence of τsc on dispersion measure τsc(DM) = 40(DM /100)2.1 is close to theoretically expected relation τsc(DM) α DM2. A frequency dependence of the scattering pulse broadening is weaker than commonly accepted τsc α v−4.4.
Scintillation of pulsar radio emission provides information about the interstellar medium along the path to the pulsar and the velocities of pulsars. It also affects the precision of pulse timing observations. Using a pulsar timing system developed at the Urumqi Astronomical Observatory 25 m telescope, we observed diffractive scintillation dynamic spectra for several strong northern pulsars. This paper introduces the observing system and discusses the observational results.
We present some results from our observations of giant pulses from PSR B0950+08 at 103 MHz. These observations, now extending over a year, have shown the highest rate of occurrence of giant pulses seen from any known pulsar. Large fluctuations in the intensity levels of individual giant pulses and in their occurrence rate per unit interval of time are seen during a single day’s observations, as well as from one day to the next. We conclude that these intensity variations are likely to be intrinsic to the pulsar.
Radio-wave scattering in the interstellar plasma provides the means to circumvent the diffraction limit for earth-based instruments, and to image the emission regions of pulsars. For the past 25 years, observers have sought to exploit this fact to learn how pulsars shine. I review the techniques developed, and summarize measurements of size of emission regions of pulsars to date.
We discuss the resolution of pulsar magnetospheres using interstellar scintillation. The two-dimensional spatial structure of pulsar emission zones can be obtained from analysis of diffractive scintillations at low frequencies. Based on refractive and diffractive scintillation of pulsars we can also reconstruct the distribution of turbulent plasma along the line of sight, and using this analysis a new approach to pulsar distance estimation can be made.
We have developed a numerical code for the propagation of different electromagnetic modes in a pulsar magnetosphere filled by a relativistic, streaming electron-positron plasma in a strong, curved magnetic field. We determine the trajectories, limiting polarization and damping of the waves leaving the magnetosphere.
Diffractive and refractive magnetospheric scintillations may allow direct probing of the plasma inside the pulsar light cylinder. The unusual electrodynamics of the strongly magnetized electron-positron plasma allows separation of the magnetospheric and interstellar scattering. The most distinctive feature of the magnetospheric scintillations is their independence of frequency. Diffractive scattering due to small scale inhomogeneities produces a scattering angle that may be as large as 0.1 radians, and a typical decorrelation time of 10−8 seconds. Refractive scattering due to large scale inhomogeneities is also possible, with a typical angle of 10−3 radians and a correlation time of the order of 10−4 seconds. Some of the magnetospheric propagation effects may have already been observed.
Chapter Three Intra-Day Variability, Gravitational Lensing and Polarization
Intra-day variability (IDV) of active galactic nuclei (AGN) has been detected from gamma-ray energies to radio wavelengths. At high energies, such variability appears to be intrinsic to the sources themselves. However, at radio wavelengths, brightness temperatures as high as 1018 to 1021 K are encountered if the IDV is intrinsic to the source. We discuss here the accumulating evidence showing that, at radio wavelengths where the highest brightness temperatures are encountered, interstellar scintillation (ISS) is the principal mechanism causing IDV. While ISS reduces the implied brightness temperatures, they still remain uncomfortably high.
This report presents preliminary results of daily observations, over 60 and 100 days, of a complete, flux-limited sample of radio sources with flat spectra. The existence of flicker up to 21.7 GHz was confirmed, for sources with flat spectra, on a time-scale of 4 days. A model explaining the flux density variations of the unique radio source 0524+034, on long and short time-scales, by an intrinsic mechanism is proposed.
We present three epochs of VSOP observations of the BL Lac object 2007+777 at 5 GHz. Compared with the ground-based VLBA data, the space baselines with HALCA clearly reveal a more detailed and finer source structure. Mainly based on the quite uniform and circular UV-coverages of the VLBA, and using a new cross-selfcalibration method, we have found evidence for weak structural changes on a timescale of two weeks in the core region of this intraday variable source. The physical causes for these variations are discussed.
We present the results of a year-long monitoring campaign on J1819+3845. We interpret the results of this WSRT campaign to infer critical source parameters such as source lifetime and structure on tens of microarcseconds. The long lifetime of the source at such high brightness temperatures requires continuous energy injection or exotic emission processes. We have previously interpreted the extreme scintillation of Jl819+3845 as due to a relatively nearby (~ 20 pc) scattering screen. We show this screen has a velocity w.r.t. the LSR of ~ 25 km s−1, as measured by the changing scintillation properties throughout the year: the ‘velocity parallax’.