8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE Interstellar Scintillation and IDV Twinkle,

8th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006. Giuseppe Cimò – JIVE

Interstellar Scintillation and IDV

Twinkle, twinkle quasi-starBiggest puzzle from afarHow unlike the other onesBrighter than a billion suns.Twinkle, twinkle quasi-starHow I wonder what you are.

George Gamow, "Quasar" 1964.

Giuseppe CimGiuseppe Cimòò JIVEJIVE University of TasmaniaUniversity of Tasmania


This Talk!

• IntraDay Variability and Interstellar Scintillation.

• Episodic scintillation: 1326-5256

• ATCA observations of a sample of known scintillators

• Conclusions


The IDV phenomenon (Radio Wavelengths)

Source-intrinsic explanation:

TBobs= TB

int • D3-α

Relativistic effects can explain only moderate values of D.

IDV requires D~100-1000IDV requires D~100-1000

Special geometry can be taken into account (Qian et al. 1991) and the required factors are ‘similar’ to those involved from superluminal motions.

Variability of flat spectrum radio sources: 30% total flux density variations

In some extreme cases: variations up to 600% in less the one hour In some extreme cases: variations up to 600% in less the one hour (i.e. J1819+384, PKS 0405-385, PKS 1257-326) (i.e. J1819+384, PKS 0405-385, PKS 1257-326)

•Factor 3 (or more) in the polarized flux density•Common phenomenon: present in ~30% of compact radio galaxies

Time scale 1 day Small sizes (cΔt)

TB 1012K (Violation of the Inverse Compton Limit)


McCulloch et al. 2005

The variability pattern of PMN J1326-5256:

Modulation index ~10%Time scale<12 hours

Example of IDV:Ceduna data

COntinousSingle dishMonitoring ofIntraday variability atCeduna

COSMIC


Interstellar Scintillation

Rickett 2004

The turbulence in the interstellar medium causes a change in the phase of the incoming radio waves the paths of the waves are distorted producing spatial variations in the received flux density. The phenomenon of scintillationscintillation then occurs since the turbulent medium is in motion with respect to the observer.

Scattering regimes:

•strong diffractive s < diff

•strong refractive s < scatt

•weak s < weak

For extragalactic objects, one has to take into account the source size, s.


Annual ModulationThe observer moves through a (frozen) spatial pattern caused by a screen at distance D on line of sight.

The relative velocity between the orbital motion of the Earth and the scattering screen is modulated by the composition of the Earth's velocity vector and the velocity vector of the screen: it results in a seasonal change of the variability time scale, so-called, annual modulationannual modulation

The time scale is defined as:

τ = sspatial/vISM

The spatial scale of the scintillation pattern is

sspatial ~ D • θscat

and vISM is the relative speed through the spatial pattern

Rickett 2002



COSMIC data:

The variability pattern of PMN J1326-5256: m~10%, <12hrs

(McCulloch et al. 2005.)

• PARKES-MIT-NRAO(Griffith et al. 1993, AJ 105, 1666)

• No optical identification/No redshift

•Inverted spectrum

• Monitored with the ATCA at 4.8 and 8.6GHz

(Bignall et al. 2003, Bignall PhD Thesis).

PMN J1326-5256



COSMIC data:

The variability characteristics clearly change after day 104

The modulation index dropped to ~4%

(Carter et al. in prep.)


In the following months and up to now, the source appears to be relatively stable around a mean flux density of 1.5Jy apart of brief episodes of rapid variability. During these sporadic events, the flux density slightly increases and decreases on a time scale of ~1 day before coming back to its quiescent phase.

(Carter et al. in prep.)

m<4%


January 2004: An intriguing pattern…

One can see a well defined variability even though with a longer time scale (weeks) and smaller amplitude that shown before.

Questions:What are these sporadic variability events ?Is PMN J1326-5256 trying to start up again ?Does the source present multiple time scales ?


AT Compact Array Observations:

• Target of Opportunity• ATCA Observations: 22, 23 and 25 January 2004• Observing frequencies: 20, 13, 6, 3 and 1 cm

•Calibrators: 1934-638, 0823-500 (primary) 1421-490 (secondary)

• Data reduction with the software package Miriad

Aim of the experiment:

• Compare simultaneous observations at different frequencies.• Analysis of the variability characteristics in different phases of the interstellar scattering

•Towards higher frequencies: If a structural changes occurred, it is probable that the source is still varying in the short wavelengths regime where its size can be still smaller than the scattering size in the ISM.





Q and U variability during Stokes I quiescent phases has been observed.

Some interpretation/ideas:

A change in the mean properties of the ionized ISM along the line of sight could be responsible for the observed cessation of variability.

One should expect also a slow-down in both Q and U

In the case of PMN J1326-5256, we suggest that the scintillation pattern has changed due to an intrinsic ‘long-term’ change in the source jet.

Only the component responsible for the total intensity variations has become large enough to ‘quench’ the scattering.

We observe the fast scintillating polarized subcomponents in the jet.

compact structures in the jets


ATCA Observations – September 2005

• Two weeks of observing time with the split array (3 antennas)

• Observations at 1.6, 2.4, 4.8 and 8.6 GHz

• Full Stokes

Aim:Study of the Total intensity and Polarization

characteristics of a sample of knows scintillators.

Characteristic time scales and variability modulation indexes.

…and now more sources!!!



1.4 GHz 2.4 GHz 4.8 GHz 8.6 GHz

[d] m [%] [d] m [%] [d] m [%] [d] m [%]

0235+164 1.8 2.6 1.7 3.9 1.9 3.9 2.0 , 3.0 5.5

0405-385 1.4 , 2.3 4.5 3.7 8.4 0.7 7.7 0.4, 0.7, 2.8 10.3

1519-273 1.5, 4.5 2.6 1.2 , 3.2 9.9 1.0 4.7 0.5 6

1622-253 0.5 5.8 7.2 5.4 4.5 4.8 0.8 , 5.4 7.2

1622-297 0.6 2.4 1.4 1.6 0.6 1.2 0.6 4.8

1034-293 1.2 9.8 3.3 5.5

1144-379 6.4 12.3 1.1 8

In the case of 0405-385, we know that the screen distance is about 30pc (or less). Therefore the shorter time scale observed at 8.6 GHz gives a scattering size of 230 µas: Too large!!!

Assuming the value of 20 µas found by Rickett et al 2004, we can evaluate the speed of the scattering screen and an indication of the anisotropy.


Conclusions:• Long term changes in the IDV pattern can be due to source-intrinsic effect: emission of new jet components?

• IDV measurements can provide information about the source size at microarcsecond scale (or the velocity of the scattering screen).

• Continuously monitoring flat spectrum radio sources at Ceduna (COSMIC) and Hobart to evaluate time scales and to look for annual modulation (McCulloch et al 2005, Carter et al. & Cimò et al. in prep).

• ATCA data provided high precision measurements of time scales and modulation indexes, both in total intensity and polarization.

Size of the scattering component in the sources and anisotropy of the screen


Question: Why IDV???

•IDV sources are compact: good candidates for geodesy experiments

Geodetic VLBI: RDV campaign + VCS4 (+CONT05?)

• Micro-arcsecond imaging of AGNs:• via Earth Orbit Synthesis (Macquart and Jauncey 2002)• via polarization studies (Rickett & Kedziora-Chudczer 2002)

• Understanding the phenomenon:source structure and ISM

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE Interstellar Scintillation and IDV Twinkle,

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