Scintillation in Extragalactic Radio Sources Marco Bondi Istituto di Radioastronomia CNR Bologna, Italy

References n Conference Proceedings & Review Papers: u AIP #74 “Radio Wave Scattering in the Interstellar Medium” 1988, Eds J.M. Cordes, B.J. Rickett & D.C. Backer u IAU Colloquium #182 “Sources and Scintillations: Refraction and Scattering in Radio Astronomy” 2001, Eds: R. Strom u Rickett 1990, Annu. Rev. Astron. Astrophys. n Papers: u Bondi et al. 1994, A&A 287, 390 u Blandford, Narayan & Romani 1986, ApJL 301, 53 u Dennett-Thorpe & De Bruyn 2000, ApJL 529, 65 u Ferrara & Perna 2001, MNRAS 325, 1643 u Heeschen & Rickett 1987, AJ 93, 589 u Padrielli et al 1987, A&ASS 67, 63 u Rickett et al. 1995, A&A 293, 479 u Rickett et al. 2000, ApJL 550, 11 u Spangler et al. 1993, A&A 267, 213 u Walker 1998, MNRAS 294, 307

Outline n Introduction n Density and intensity fluctuations n Scintillation Jargon n Scintillation regimes: weak, diffractive, refractive n Low frequency variability n Flickering and Intra-Day variability n Intergalactic scintillation

Introduction Electromagnetic waves from an extragalactic radio source pass through several ionized media: the intergalactic gas, the interstellar medium, the interplanetary medium and the ionosphere. In all these cases, the turbulent plasma produces a phase modulation of the wavefront and scattering. This produces a wide variety of observed phenomena such as intensity scintillation, angular broadening and pulse smearing. The study of these phenomena provides information on the angular size of the scattered sources and a unique method for the remote analysis of astrophysical plasmas.

Density and Intensity Fluctuations n Typically it is assumed a power-law spectrum for the spatial power spectrum of the density irregularities: C N is a strength parameter and q is the wave number of density fluctuations in the plasma. n This quantity is related to the power spectrum of intensity fluctuations through the source size (actually the source visibility in interferometer observations). In the case of refractive scintillation we have:

n Define the point source scintillation index (rms fractional intensity fluctuation): n Define the scattering strength: n A relevant quantity in scintillation is the Fresnel scale (units are cm): n The angular size of the Fresnel scale is given (in arcseconds) by Scintillation Jargon

n Scintillation is divided into weak and strong according to whether  is much smaller or greater than unity. In the strong regime the wavefront is highly corrugated on scales smaller than the Fresnel scale, in the weak regime the phase changes over the Fresnel scale are small. n Assuming a model for the distribution of the scattering material it is possible to map the transition frequency 0 (the frequency at which  =1). Scintillation Regimes

n The spatial scale for weak intensity variations is the Fresnel scale r f. For sources with angular extent greater than  f the scintillation patterns from different parts of the source overlap and smear each other out, eliminating a detectable variation n For a point source ( ) the following relations hold: n For a source with Weak Scintillation

n It is an interference effect characterized by fast, narrow-band variations. The modulation index is unity for a point source and the interference fringes have a characteristic frequency scale n It is necessary to observe with frequency resolution of  or better in order to be sensitive to diffractive scintillation. n The angular size on which phase changes of order 1 rad are introduced into the wavefront is: n The corresponding time-scale is n For sources with    d the modulation index is reduced to  d /  and the time-scale for variations increased by a factor  /  d. n No recorded examples of diffractive scintillation of extragalactic sources. Strong Scintillation: Diffractive

n Can be understood in terms of ray-optics and correspond to lens-like phenomena. n It is characterized by slow, broad-band variability. n The refractive scale is given by the scattering disk, much larger than the Fresnel scale, and the time-scale is correspondingly longer. n Again if  r the modulation index is reduced by a factor while the time-scale increases with Strong Scintillation: Refractive

n Low frequency (< 1 GHz) variability has been a puzzling phenomenon in the ‘70s and ‘80s. n Variations of the order of 10% on time-scales of months to years. n Variations could not be explained in terms of expansion of a synchrotron emitting cloud of plasma. n Low frequency bursts would imply  far higher than those derived from proper motion measurements: Low Frequency Variability - I

n Refractive scintillation was proposed as the mechanism responsible for low frequency variability: dependence of variability on galactic latitude. n Results from analysis of a 15 years monitoring at 408 MHz coupled with VLBI observations at 610 MHz to derive the source sizes: u Qualitatively and roughly quantitative agreement between the observed scintillation indices an time-scales and those derived from a “standard model” for interstellar plasma turbulence. Low Frequency Variability - II

n The time-scale of variability is determined by the distance of the effective screen and the pattern-observer velocity in the plane of the sky (pattern speed). u Annual modulation in a sample of low frequency variable. This is interpreted as produced by the Earth orbital motion around the Sun on the pattern produced by refractive scintillation. u Sources along the line of sight of the apex show longer time-scales. Low Frequency Variability - III

n There is no measurable evidence for a finite propagation speed of the turbulent irregularities responsible for the refractive scintillation: u the scattering medium is extended along the line of sight. In this case the random velocities of the density irregularities will not produce any net motion; u the scattering medium is not uniformly distributed along the line of sight, but it is localized in a thin screen at a certain distance. In this case the velocity of the density irregularities should be low suggesting that they could be associated with the HII region envelopes, characterized by a Alfven speed Low Frequency Variability - IV

n Low amplitude (1% --5% rms) short time scale (few hours to days) variability observed in the range 2-20 cm in flat spectrum radio sources. n In some cases the variations can have substantial amplitude ( %) over few hours (e.g ). n If intrinsic these variations would imply Lorentz factors of the order of 100. n Variations are observed also in polarized flux and position angle. Flickering & Intra-Day Variability - I

n Refractive interstellar scintillation has been claimed to be the cause of this phenomena because of a significant trend of increasing flicker amplitude with decreasing galactic latitude. n The combination of a steady and variable component with nearly orthogonal polarisation angles can produce the observed anticorrelation of total flux density and polarized flux Flickering & Intra-Day Variability - II

Flickering & Intra-Day Variability - III n Assuming the source diameter is linearly dependent on wavelength it is possible to reproduce the amplitude and time scale trends with wavelength with a reasonable model of RISS. n Annual modulation detected in IDV sources ( , J ).

Scintillation as a Probe of the ICM - I n Most of the baryons reside in a warm/hot component which is difficult to detect with standard absorption/emission line techniques. n Refractive scintillation of a compact quasar behind a cluster can be used to probe the intracluster medium. n The cluster will act as a foreground screen: relevant parameters are: u radial profile of the cluster mass density (isothermal  model) u mass fraction of the gas ( ) u distance of the cluster (0.02) and of the quasar (1.0) u velocity of the inhomogeneities (1000 km/s) u the size of the quasar u the impact parameter (depending on its value the propagation through the cluster can be in the weak or strong scattering regimes)

Scintillation as a Probe of the ICM - II