Anomalous Radio-Wave Scattering from Interstellar Plasma Structures

This paper considers scattering screens that have arbitrary spatial variations of scattering strength transverse to the line of sight, including screens that are spatially well confined, such as disks and filaments. We calculate the scattered image of a point source and the observed pulse shape of a scattered impulse. The consequences of screen confinement include (1) source image shapes that are determined by the physical extent of the screen rather than by the shapes of much smaller diffracting microirregularities (these include image elongations and orientations that are frequency dependent); (2) variation with frequency of angular broadening that is much weaker than the trademark ν-2 scaling law (for a cold, unmagnetized plasma), including frequency-independent cases; and (3) similar departure of the pulse-broadening time from the usually expected ν-4 scaling law. We briefly discuss applications that include scattering of pulses from the Crab pulsar by filaments in the Crab Nebula; image asymmetries from Galactic scattering of the sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active galactic nuclei by intervening galaxies. We also address the consequences for inferences about the shape of the wavenumber spectrum of electron density irregularities, which depend on scaling laws for the image size and the pulse broadening. Future low-frequency (<100 MHz) array observations will also be strongly affected by the Galactic structure of scattering material. Our formalism is derived in the context of radio scattering by plasma density fluctuations. It is also applicable to optical, UV, and X-ray scattering by grains in the interstellar medium.

[1]  T. Joseph W. Lazio,et al.  The Low-Frequency Array (LOFAR): opening a new window on the universe , 2000, Astronomical Telescopes and Instrumentation.

[2]  D. Backer,et al.  A Plasma Prism Model for an Anomalous Dispersion Event in the Crab Pulsar , 2000, astro-ph/0006220.

[3]  M. Kramer,et al.  IAU Colloquium 177: Pulsar Astronomy: 2000 and Beyond , 2000 .

[4]  K. Johnston,et al.  The Extreme Scattering Event toward PKS 1741–038: VLBI Images , 1999, astro-ph/9910323.

[5]  M. Kramer,et al.  Pulsar astronomy - 2000 and beyond : IAU Colloquium 177 : proceedings of a conference held on the Max-Planck-Institut fur Radioastronomie, Bonn, Germany 30 August - 3 September 1999 , 2000 .

[6]  J. Butcher,et al.  MERCURY TMDLS FOR ARIVACA AND PEÑA BLANCA LAKES, AZ , 2000 .

[7]  M. Kramer,et al.  Pulsar Astronomy - 2000 and Beyond , 2000 .

[8]  Denver,et al.  Simultaneous Dual-Frequency Observations of Giant Pulses from the Crab Pulsar , 1999, astro-ph/9902386.

[9]  W. Goss,et al.  G359.87+0.18, an FR II Radio Galaxy 15' from Sagittarius A*: Implications for the Scattering Region in the Galactic Center , 1998, astro-ph/9811317.

[10]  J. Cordes,et al.  Diffractive Interstellar Scintillation Timescales and Velocities , 1998 .

[11]  B. Rickett,et al.  On the Theory of Pulse Propagation and Two-Frequency Field Statistics in Irregular Interstellar Plasmas , 1998 .

[12]  J. Cordes,et al.  Hyperstrong Radio-Wave Scattering in the Galactic Center. II. A Likelihood Analysis of Free Electrons in the Galactic Center , 1998, astro-ph/9804157.

[13]  M. Wardle,et al.  Extreme Scattering Events and Galactic Dark Matter , 1998, astro-ph/9802111.

[14]  G. Bower,et al.  7 Millimeter VLBA Observations of Sagittarius A* , 1998, astro-ph/9802030.

[15]  J. Moran,et al.  Anisotropic Radio Scattering of NGC 6334B , 1998 .

[16]  T. Lazio,et al.  The Gaussian Plasma Lens in Astrophysics: Refraction , 1997, astro-ph/9709249.

[17]  J. Cordes,et al.  Finding Radio Pulsars in and beyond the Galactic Center , 1996, astro-ph/9608028.

[18]  John W. Armstrong,et al.  Electron Density Power Spectrum in the Local Interstellar Medium , 1995 .

[19]  K. Johnston,et al.  Interstellar scattering toward Cygnus X-3: Measurements of anisotropy and of the inner scale , 1995 .

[20]  W. Cotton,et al.  Anisotropy in the angular broadening of Sagittarius A* at the Galactic center , 1994 .

[21]  S. Sridhar,et al.  Toward a theory of interstellar turbulence. 2. Strong Alfvenic turbulence , 1994 .

[22]  K. Johnston,et al.  A SUMMARY OF EXTREME SCATTERING EVENTS AND A DESCRIPTIVE MODEL , 1994 .

[23]  R. Narayan,et al.  The scatter-broadened image of Cygnus-X-3 , 1994 .

[24]  I. Cognard,et al.  An extreme scattering event in the direction of the millisecond pulsar 1937 + 21 , 1993, Nature.

[25]  James M. Cordes,et al.  Pulsar distances and the galactic distribution of free electrons , 1993 .

[26]  D. Frail,et al.  A Transient Radio Source near the Center of the Milky Way Galaxy , 1992, Science.

[27]  J. Cordes,et al.  Interstellar scattering effects on the detection of narrow-band signals , 1991 .

[28]  J. Cordes,et al.  VLA and VLBI angular broadening measurements - The distribution of interstellar scattering at low Galactic latitudes , 1991 .

[29]  R. Ekers,et al.  Radio studies of the Galactic Centre – II. The arc, threads, and related features at 90 CM (330 MHz) , 1991 .

[30]  D. Breitschwerdt,et al.  Stellar winds in H II regions – II. Turbulent mixing between an H II layer and a stellar wind bubble in the presence of a magnetic field , 1990 .

[31]  B. J. Rickett,et al.  Radio propagation through the turbulent interstellar plasma. , 1990 .

[32]  J. Moran,et al.  The bipolar H II region NGC 6334A , 1988 .

[33]  Mark R. Morris,et al.  Structural details of the Sagittarius A complex - evidence for a large-scale poloidal magnetic field in the Galactic center region , 1987 .

[34]  R. Blandford,et al.  Radio caustics from localized interstellar medium plasma structures , 1987, Nature.

[35]  K. Johnston,et al.  Extreme scattering events caused by compact structures in the interstellar medium , 1987, Nature.

[36]  J. Cordes,et al.  Refractive and diffractive scattering in the interstellar medium , 1986 .

[37]  J. Higdon Density fluctuations in the interstellar medium: Evidence for anisotropic magnetogasdynamic turbulence. II - Stationary structures , 1986 .

[38]  J. Cordes,et al.  Small-scale electron density turbulence in the interstellar medium. , 1985 .

[39]  J. Higdon Density fluctuations in the interstellar medium: evidence for anisotropic magnetogasdynamic turbulen , 1984 .

[40]  R. Isaacman,et al.  The Crab Nebula pulsar - Variability of dispersion and scattering , 1977 .

[41]  N. Vandenberg Meter-wavelength VLBI. IV - Temporal and spatial scattering of the Crab Nebula pulsar's radiation , 1976 .

[42]  Strong scintillations in astrophysics. II - A theory of temporal broadening of pulses , 1975 .

[43]  D. J. Thorne,et al.  Anomalous Scattering in the Crab Nebula , 1975 .

[44]  I. P. Williamson The broadening of pulses due to multipath propagation of radiation , 1975, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.

[45]  I. P. Williamson Pulse Broadening due to Multiple Scattering in the Interstellar Medium , 1972 .

[46]  P. Scheuer Amplitude Variations in Pulsed Radio Sources , 1968, Nature.

[47]  J. Findlay Radio Telescopes , 1964, IEEE Transactions on Military Electronics.

[48]  A. Hewish Large radio telescopes , 1961 .