Understanding of the spectral and polarimetric characteristics of rapidly scintillating blazars is fundamental in order to describe both the innermost (sub-pc) regions of these compact objects and the interstellar medium responsible for the scintillation. A multi frequency analysis of the intraday variability in PMN J1326-5256, based on the combination of Australia Telescope Compact Array observations with the data from the monitoring projects at the University of Tasmania, will be described. Some implications concerning the structure of compact radio cores and the properties of the interstellar medium will be discussed

Bignall, Hayley

Cimo, G.

Ellingsen, S.

Kedziora-Chudczer, L.

McCulloch, P.

Senkbeil, C.

English

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PoS(8thEVN)009Intraday variability of AGNs in the southernhemisphere: the strange case of PMN J1326 −5256Giuseppe Cimò ∗†Joint Institute for VLBI in EuropeE-mail: cimo@jive.nlSimon EllingsenUniversity of TasmaniaE-mail: Simon.Ellingsen@utas.edu.auCliff SenkbailUniversity of TasmaniaE-mail: cliffs@utas.edu.auPeter McCullochUniversity of TasmaniaE-mail: Peter.McCulloch@utas.edu.auHayley BignallJoint Institute for VLBI in EuropeE-mail: bignall@jive.nlLucyna Kedziora-ChudczerSydney UniversityE-mail: Lucyna.Kedziora-Chudczer@atnf.csiro.auUnderstanding of the spectral and polarimetric characteristics of rapidly scintillating blazars isfundamental in order to describe both the innermost (sub-pc) regions of these compact objects andthe interstellar medium responsible for the scintillation. A multifrequency analysis of the intradayvariability in PMN J1326−5256, based on the combination of Australia Telescope Compact Arrayobservations with the data from the monitoring projects at the University of Tasmania, will bedescribed. Some implications concerning the structure of compact radio cores and the propertiesof the interstellar medium will be discussed.The 8th European VLBI Network SymposiumSeptember 26-29, 2006Toruń, Poland∗Speaker.†This research has been supported by the ARC grant number DP0342500 at the University of Tasmania.c© Copyright owned by the author(s) under the terms of the CreativCommons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(8thEVN)009IDV in the southern hemisphere Giuseppe Cimò1. IntroductionFast flux density variations on intraday time scale, i.e. intraday variability (IDV, Witzel et al,1986 [9], Heeschen et al, 1987 [3]), are observed in some compact sources at centimetre wave-lengths. If we consider these variations as intrinsic, the observed rapid fluctuations imply, via thelight travel time argument, very small source sizes and very high apparentbrigh ness temperatures(up to 1021 K). On the other hand, such compact objects must show interstellar scintillation(ISS)due to the propagation of radio waves in the turbulent interstellar medium (ISM) of our Galaxy.Nowadays, interstellar scattering is believed to be the primary mechanics responsible for rapidvariations observed at centimetre wavelengths (Jauncey et al, 2003 [5]).The main characteristic of IDV is the variability on time scales of a day to less than onehour of both total (S) and linearly polarized (P) flux densities. The amplitudes of the fluctuationsdiffer from source to source but are typically in the rangemS = 5− 20%, wheremS is the totalintensity flux density modulation index (mS = σS/S). Often, the variability is more pronouncedin the polarized than in the total flux (mP > 100%). Usually, the variations inS and P appearcorrelated but in some cases are anti-correlated.An ISS origin for the IDV requires the time scales of the variations to exhibit seasonal modu-lations when the projected velocity of the Earth and the interstellar matter have similar speed anddirection (Rickett et al, 2001 [8], Jauncey & Macquart 2001 [4]). Therefore, a proper understand-ing of the IDV phenomenon needs long-term continuous monitoring of IDV sources. Using theCeduna 30-m radio telescope of the University of Tasmania, we are continuusly monitoring thevariability of a number of compact sources at 6.7 GHz in order to obtain information about thevariability pattern during the course of the year (McCulloch et al, 2005 [7]). The Ceduna contin-uous monitoring programme is essential to collect information about the long-term characteristicsof the observed flux density fluctuations.A peculiar quiescent period has been observed in the otherwise stronglyvariable source PMNJ1326−5256 during our Ceduna intraday variability monitoring project. We have alsodetected fastand pronounced polarization changes during this low-level variability phase of the total flux densityusing the Australia Telescope Compact Array (ATCA).2. ObservationsObservations of PMN J1326−5256 over the last 3 years have shown a complex variabilitypattern in this intraday variable source. This peculiar object has changedits scintillation behavioursince the early observations going through the following stages: fast variations – quiescence –long-term variability – fast variations.Here, we present the observations of J1326−5256 carried out with the 30-m Ceduna radiotelescope at 6.7 GHz (see fig.1). The total intensities of the target source and the calibrators (i.e.B1934−638) have been measured by means of cross-scans in both RA and Dec.Furthermore, we present data from the observations with the ATCA carried out simultaneouslyat 1.6, 2.2, 4.8, and 8.4 GHz (see fig.2, total intensity on the top panels, Stokes Q and U on thebottom panels). The data for both total intensity and polarisation have been reduced using theMIRIAD package.2PoS(8thEVN)009IDV in the southern hemisphere Giuseppe Cimò52736 52738 52740 52742 5274411.21.41.61.82PMN J1326-525652764 52766 52768 52770 52772 5277411.21.41.61.8252798 52800 52802 52804 52806 52808MJD [days]11.21.41.61.82Figure 1: Intraday variations of the total flux density of PMN J1326−5256 at 6.7 GHz. Note the slow-downafter MJD 52744.3PoS(8thEVN)009IDV in the southern hemisphere Giuseppe Cimò3. ResultsTransient IDV has been observed in a number of sources. However,the peculiar behaviourof PMN J1326−5256 differs from that seen in other episodic IDV sources. For example, inB0917+624, the amplitude of the variations changed from∼ 10% to became “stable” at a levelof 1–2% (Fuhrmann et al, 2002 [2]), whereas in PKS 0405−385, the extreme variability lastsseveral months and is followed by periods of the order of the years whenonly slow changes areobserved (Kedziora-Chudczer 2006 [6]).In the course of the Ceduna monitoring project, PMN J1326−5256 has shown pronouncedIDV with a complex pattern. Our time series analysis has revealed multiple characteristi timescales:τ ∼ 0.4days andτ ∼ 1.4days up to long-term (>few days) variability. PMN J1326−5256was also observed with the ATCA during monitoring of compact radio sources at 4.8 and 8.6 GHz(Bignall et al, 2002 [1]). In this set of observations, the variations in polarized and total fluxdensities were strongly correlated in early 2001, but later became uncorrelated. This implies acomplex structure of the radio core.Suddenly, in April 2003, the flux density fluctuations of this strongly variable object ceasedon a time scale of less than two weeks (see fig.1). In the following months up to December 2003,the source appeared to be stable around a mean flux density of 1.5 Jy apart from brief episodes ofvariability. During these sporadic events, the flux density slightly increasednd decreased withtime scale of∼ 1day before reverting to its quiescent phase. Measurements made in January 2004at Ceduna show an intriguing pattern: we noticed a well-defined variability although with a longertime scale (weeks) and smaller amplitude than seen before. Furthermore, the ATCA data showsmulti-frequency variability of the total flux density (in agreement with the Ceduna results), butinterestingly, very fast (∼ 2hour) polarization changes have also been detected (see bottom panelson fig.2).A plausible explanation of the slow-down in the variations is that we have observed an annualmodulation of the scintillation. However, such a seasonal effect can be ruled o t since we knowfrom previous observations that the source showed very strong variability in the first part of theyear (Bignall et al, 2002 [1]). One could interpret this sudden changein the framework of the ISStheory as due to quenched scintillation caused by changes either in the scattering size in the ISMor in the structure of the innermost region of the source.Variations of the electron density or change of the distance of the scatteringscreen couldalso be responsible for the cessation of the rapid variability. Alternatively,one could imagine aclumpy ISM with moving clouds at different distances. On the other hand, theemission of a newjet component can lead to an apparent source size larger than the scattering size in our Galaxy,thereby canceling out the variability. Finally, the observed complex pattern could be a mixture ofscintillation (annual modulation) and source-intrinsic (i.e. pc-scale structures) effects. We haveseen that the source was still scintillating in the polarized components. This suggests that thesestructures are smaller than the scintillating total flux density component.4PoS(8thEVN)009IDV in the southern hemisphere Giuseppe CimòFigure 2: Intraday variations of 1326−5256 at the ATCA in January 2004. Top two panels correspond tototal intensity variations. Bottom four panels show Q and U variation.5PoS(8thEVN)009IDV in the southern hemisphere Giuseppe Cimò4. ConclusionUsing the Ceduna 30-m radio telescope of the University of Tasmania, we monitored the vari-ability at 6.7 GHz of several compact sources (McCulloch et al, 2005 [7]) in order to establish theirannual cycles and variability characteristics.When explaining the observed sudden change in the variability pattern of J1326−5256 takinginto account only propagation effects, changes in the mean properties ofthe ionized ISM alongthe line of sight should be fully responsible for the observed cessation ofvariability. However,one should expect also a slow-down in both Stokes Q and U, which is not thecase. Therefore,a scenario, which also takes into account the source structure at micro-arcsecond scale, seemsto be more plausible in the case of PMN J1326-5256. We suggest that the source scintillationpattern has changed due to an intrinsic “long-term” change in the compact core. An observableeffect of a multi-component scattered core at micro-arcsecond scale is thcomplex polarizationscintillation pattern. Polarization variability is usually faster and stronger and cbe detected alsoduring quiescent phases of the total flux density fluctuations. This is due tothe smaller sizes ofthe polarised components. Accordingly, we have detected fast (∼ 2hour) polarization changesduring our ATCA experiment in late January 2004. Only the component responsible for the totalintensity variations has become large enough to “quench” the scattering whilewe still observe thefast scintillating polarized subcomponents in the jet.References[1] Bignall H. E., Jauncey D. L., Kedziora-Chudczer L. L. et al. 2002,New Results from an ATCA Studyof Intraday Variable Radio Sources, PASA 19, 29[2] Fuhrmann L., Krichbaum T. P., Cimò G. et al. 2002,Annual Modulation in the Variability Propertiesof the IDV Source 0917+624?, PASA 19, 64[3] Heeschen D. S. Krichbaum T. P, Schalinski C. J. & Witzel A,1987,Rapid variability of extragalacticradio sources, AJ 94, 1493[4] Jauncey D. L. & Macquart J.-P.,Intra-day variability and the interstellar medium towards0917+624,2001, A&A 370, L9[5] Jauncey D. L., Bignall H. E., Lovell J. E. J. et al. 2003,Interstellar Scintillation and Radio Intra-DayVariability in "Radio Astronomy at the Fringe", Eds Zensus, Cohen & Ros, PASP Vol 300, p 199[6] Kedziora-Chudczer, L. 2006,Long-term monitoring of the intra-day variable quasar PKS 0405−385,MNRAS 323, 1411[7] McCulloch P. M., Ellingsen S. P., Jauncey D. L. et al. 2005, COSMIC: Microarcsecond Resolutionwith a 30 Meter Radio Telescope, AJ 129, 2034[8] Rickett B. J., Witzel A., Kraus A. et al. 2001,Annual Modulation in the Intraday Variability ofQuasar 0917+624 due to Interstellar Scintillation, ApJ 550L, 11[9] Witzel A., Heeschen D. S., Schalinski C. J & Krichbaum T. P, 1986,Kurzzeit-Variabilitätextragalaktischer Radioquellen, Mitt. Astron. Ges 65, 2396

Intraday variability of AGNs in the southern hemisphere

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Intraday variability of AGNs in the southern hemisphere

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