8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE Interstellar Scintillation and IDV Twinkle, twinkle quasi-star Biggest puzzle from afar How unlike the other ones Brighter than a billion suns. Twinkle, twinkle quasi-star How I wonder what you are. George Gamow, "Quasar" Giuseppe Cimò JIVE JIVE University of Tasmania University of Tasmania

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE This Talk! IntraDay Variability and Interstellar Scintillation. Episodic scintillation: ATCA observations of a sample of known scintillators Conclusions

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE The IDV phenomenon (Radio Wavelengths) Source-intrinsic explanation: T B obs = T B int D 3-α Relativistic effects can explain only moderate values of D. IDV requires D~ 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 (i.e. J , PKS , PKS ) 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) T B  K (Violation of the Inverse Compton Limit)

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE McCulloch et al The variability pattern of PMN J : Modulation index ~10% Time scale<12 hours Example of IDV: Ceduna data COntinous Single dish Monitoring of Intraday variability at Ceduna COSMIC

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE Interstellar Scintillation Rickett 2004 scintillation 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 scintillation 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.

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE Annual Modulation The observer moves through a (frozen) spatial pattern caused by a screen at distance D on line of sight. annual modulation 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 modulation The time scale is defined as: τ = s spatial /v ISM The spatial scale of the scintillation pattern is s spatial ~ D θ scat and v ISM is the relative speed through the spatial pattern Rickett 2002

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE COntinous Single dish Monitoring of Intraday variability at Ceduna COSMIC data: The variability pattern of PMN J : m~10%,  <12hrs (McCulloch et al ) 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 J

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE COntinous Single dish Monitoring of Intraday variability at Ceduna COSMIC data: The variability characteristics clearly change after day 104 The modulation index dropped to ~4% (Carter et al. in prep.)

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

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE 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 J trying to start up again ? Does the source present multiple time scales ?

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

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

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

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

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

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

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

8 th EVN Symposium: Exploring the universe with the real-time VLBI. 26 – 29 September 2006.Giuseppe Cimò – JIVE 1.4 GHz2.4 GHz4.8 GHz8.6 GHz  [d] m [%]  [d] m [%]  [d] m [%]  [d] m [%] , , , 0.7, , , , In the case of , 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.

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

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