Scintillation in Extragalactic Radio Sources

Marco Bondi

Istituto di Radioastronomia CNR Bologna, Italy

References

Conference Proceedings & Review Papers: AIP #74 “Radio Wave Scattering in the Interstellar Medium” 1988, Eds J.M. Cordes,

B.J. Rickett & D.C. Backer IAU Colloquium #182 “Sources and Scintillations: Refraction and Scattering in Radio

Astronomy” 2001, Eds: R. Strom Rickett 1990, Annu. Rev. Astron. Astrophys.

Papers: Bondi et al. 1994, A&A 287, 390 Blandford, Narayan & Romani 1986, ApJL 301, 53 Dennett-Thorpe & De Bruyn 2000, ApJL 529, 65 Ferrara & Perna 2001, MNRAS 325, 1643 Heeschen & Rickett 1987, AJ 93, 589 Padrielli et al 1987, A&ASS 67, 63 Rickett et al. 1995, A&A 293, 479 Rickett et al. 2000, ApJL 550, 11 Spangler et al. 1993, A&A 267, 213 Walker 1998, MNRAS 294, 307

Outline

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

en

Density and Intensity Fluctuations

Typically it is assumed a power-law spectrum for the spatial power spectrum of the density irregularities:

CN is a strength parameter and q is the wave number of density fluctuations in the plasma.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:

qrCqP Nn )()( 2

dzqz

Vzq

qPqPD

nI

222

0 24sin)()(

Define the point source scintillation index (rms fractional intensity fluctuation):

Define the scattering strength:

A relevant quantity in scintillation is the Fresnel scale (units are cm):

The angular size of the Fresnel scale is given (in arcseconds) by

7.15.06.0 DSM dzzCSM N )(2

Scintillation Jargon

p

rmsp S

Sm

GHzkpcf DkDr /102.1/ 11

GHzkpc

ffD

Dr

6108

/

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.

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

The spatial scale for weak intensity variations is the Fresnel scale rf . 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

For a point source ( ) the following relations hold:

For a source with

Weak Scintillation

GHzkpc

ffweakD

Dr

6108

/

weak

hrs 2 and 01217

065 weakpm

weak

hrs and weak67 weakweakpmm

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

It is necessary to observe with frequency resolution of or better in order to be sensitive to diffractive scintillation.

The angular size on which phase changes of order 1 rad are introduced into the wavefront is:

The corresponding time-scale is For sources with d the modulation index is reduced to d/ and the

time-scale for variations increased by a factor /d . No recorded examples of diffractive scintillation of extragalactic sources.

Strong Scintillation: Diffractive

0

2 and 1

pm

5600

1 ffd

hr 2 560

1 fd

Can be understood in terms of ray-optics and correspond to lens-like phenomena.

It is characterized by slow, broad-band variability. The refractive scale is given by the scattering disk, much larger than the

Fresnel scale, and the time-scale is correspondingly longer.

Again if r the modulation index is reduced by a factor while the time-scale increases with

Strong Scintillation: Refractive

51100 ffr

51102 r

67 rr

30170

31 pm

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

Low Frequency Variability - I

zk

cSTburst

1

2 2

2max

212

zD

c

L

burstburst

3

1

12min 10

burstT

Refractive scintillation was proposed as the mechanism responsible for low frequency variability: dependence of variability on galactic latitude.

Results from analysis of a 15 years monitoring at 408 MHz coupled with VLBI observations at 610 MHz to derive the source sizes: 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

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).

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.

Sources along the line of sight of the apex show longer time-scales.

Low Frequency Variability - III

LSRISMpatt VVVV

There is no measurable evidence for a finite propagation speed of the turbulent irregularities responsible for the refractive scintillation:

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;

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

km/s 5AV

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.

In some cases the variations can have substantial amplitude (10-15 %) over few hours (e.g. 0917+624).

If intrinsic these variations would imply Lorentz factors of the order of 100.

Variations are observed also in polarized flux and position angle.

Flickering & Intra-Day Variability - I

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.

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

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.

Annual modulation detected in IDV sources (0917+624, J1819+3845).

2

2

2ln2 bBTkS

Scintillation as a Probe of the ICM - I

Most of the baryons reside in a warm/hot component which is difficult to detect with standard absorption/emission line techniques.

Refractive scintillation of a compact quasar behind a cluster can be used to probe the intracluster medium.

The cluster will act as a foreground screen: relevant parameters are: radial profile of the cluster mass density (isothermal model) mass fraction of the gas (0.04 - 0.2) distance of the cluster (0.02) and of the quasar (1.0) velocity of the inhomogeneities (1000 km/s) the size of the quasar 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