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Radio variability of Active Galactic Nuclei

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					Long-term variability behaviour of
             AGN
                   The X Finnish-Russian
                  Radioastronomy Symposium
                     Orilampi, 1-5 September, 2008



                   Talvikki Hovatta
             Metsähovi Radio Observatory

                        In collaboration with:
             M. Tornikoski, A. Lähteenmäki, E. Nieppola,
                  I. Torniainen, M. Lainela, H.J. Lehto,
                   E. Valtaoja, M.F. Aller, H.D. Aller
                          Outline
   Introduction
   Sample
   Variability timescales
    • Different methods
   Flare characteristics
   Conclusions




      Talvikki Hovatta              The X Finnish-Russian Radio
      Metsähovi Radio Observatory   Astronomy Symposium
                    Introduction
   A sample of ~100 AGN has been
    monitored at Metsähovi Radio Observatory
    for nearly 30 years
   The large dataset enables studies of long-
    term behaviour
   Aim is to better understand the observed
    properties, radiation mechanisms and
    physics of the sources


      Talvikki Hovatta              The X Finnish-Russian Radio
      Metsähovi Radio Observatory   Astronomy Symposium
                   Sample data
   22, 37 and 87 GHz data from Metsähovi
   4.8, 8 and 14.5 GHz from the University of
    Michigan (UMRAO)
   90 and 230 GHz data from the SEST
   90, 150, and 230 GHz data from the
    literature




      Talvikki Hovatta              The X Finnish-Russian Radio
      Metsähovi Radio Observatory   Astronomy Symposium
                Sample sources
   80 sources for timescale analysis
    • At least 10 years of monitoring data at 22 or 37 GHz
    • Bright sources with a flux density of at least 1 Jy in the
      active state
   55 sources for flare analysis
    • At least one well-monitored distinguishable flare at 2 of
      the frequencies (22, 37 or 90 GHz being one of the
      frequencies)
   Altogether 90 sources
   HPQs, LPQs, BLOs and Radio Galaxies

      Talvikki Hovatta                    The X Finnish-Russian Radio
      Metsähovi Radio Observatory         Astronomy Symposium
    Timescale analyses: methods
   Structure function (SF)

   Discrete autocorrelation function (DCF)

   Lomb-Scargle periodogram

   Wavelets
    • Only 22, 37 and 90 GHz
    • Morelet wavelet


      Talvikki Hovatta              The X Finnish-Russian Radio
      Metsähovi Radio Observatory   Astronomy Symposium
     Timescale analyses: results
   Large flares are seen on average every 4
    to 6 years
    • Average at 37 GHz is 4.2 years (DCF, wavelets)


   Redshift-corrected timescales are shorter
    • 2 years for quasars
    • 3-4 years for BL Lacertae objects (BLOs)
    -> shocks could be produced less frequently in BLOs


   Rise and decay times of flares are
    between 1 to 2 years
      Talvikki Hovatta                 The X Finnish-Russian Radio
      Metsähovi Radio Observatory      Astronomy Symposium
     Timescale analyses: results
   No strict periodicities were found
    • Episodes of quasi-periodic behaviour are common


   Multiple timescales are common

   Timescales change, get weaker in power
    or disappear over long time periods



      Talvikki Hovatta                The X Finnish-Russian Radio
      Metsähovi Radio Observatory     Astronomy Symposium
     Timescale analyses: results
   DCF and wavelets give similar results but
    wavelets also give information on the
    continuity of the timescales

   Lomb-Scargle periodogram easily
    produces spurious timescales

   Hovatta et al. 2007 (A&A, 469, 899-912),
    Hovatta et al. 2008b (A&A, in Press)

      Talvikki Hovatta              The X Finnish-Russian Radio
      Metsähovi Radio Observatory   Astronomy Symposium
                    Example: 4C 29.45 at 22GHz
                    6
                           a)
                                                                   ~3.4 years
                    5
                                Flux curve
Flux density (Jy)




                    4

                    3

                    2

                    1

                    0
                                1985             1990          1995             2000            2005

                                                        Time (years)



                    1.2
                                b)
                    1.0
                    0.8              DCF                                               3.49 years
                    0.6
                    0.4
DCF




                    0.2
                    0.0
                    -0.2
                    -0.4
                    -0.6
                    -0.8
                       -4000                 -2000             0                2000                4000
                                                        Time lag (days)
                           Talvikki Hovatta                               The X Finnish-Russian Radio
                           Metsähovi Radio Observatory                    Astronomy Symposium
                                            Example…

                           3.29 years                                             SF
        80                                                                1
                                                                                  d)

                              LS-periodogram                              0
        60
                                                                      -1




                                                               log (SF)
Power




        40                                                            -2

                                                                      -3               1.21 years
        20
                                                                      -4

         0                                                            -5
         0.000    0.001     0.002   0.003      0.004   0.005                  0           1         2          3    4
                          Frequency (1/ day)                                              log (time scale) [days]




                 Talvikki Hovatta                                                       The X Finnish-Russian Radio
                 Metsähovi Radio Observatory                                            Astronomy Symposium
                      Example…



1.7 years

                                                          3.4 years




        Talvikki Hovatta              The X Finnish-Russian Radio
        Metsähovi Radio Observatory   Astronomy Symposium
                Example: results
   Same timescale of ~3.4 years is obtained with all
    the methods

   Only with wavelets it is possible to see that it is
    present only in the latter half of the flux curve

   Comparison of new SF analysis to Lainela &
    Valtaoja (1993) also showed the difference
    • L&V 1993, SF timescale >6.68 years




       Talvikki Hovatta                    The X Finnish-Russian Radio
       Metsähovi Radio Observatory         Astronomy Symposium
           Flare characteristics
   Sample of 55 sources with 159 flares
   4.8 – 230 GHz
   Flare amplitudes (peak, relative)
   Duration of flares
   Variability indices
   Testing of the shock model
   Hovatta et al. 2008 (A&A, 485, 51-61),
    Nieppola et al. in preparation

      Talvikki Hovatta              The X Finnish-Russian Radio
      Metsähovi Radio Observatory   Astronomy Symposium
                           Example
                                                           4 years              0235+164
   Times between flares
    • On average 4 years                         10
                                                                                       1992.8
                                                                                                     1998.2

   Rise / decay times                           8                  1987.1
                                                                              1991.1


    • 1-2 years                                  6       Peak




                                  Flux density
                                                                                           1994.3
                                                         4.4Jy
   Duration                                     4
                                                                                                                2004.6



    • Median 2.5y @ 22 & 37GHz
                                                 2
    • Range between 0.3-13.2y
    Peak flux
                                                 0

                                                           Dur 2.5 y
    • Median 4.5Jy @22 & 37GHz                    1980       1985            1990             1995       2000     2005

    • Range between 0.7-57 Jy                                                          Time




                                                                                       0.95 years

        Talvikki Hovatta                                     The X Finnish-Russian Radio
        Metsähovi Radio Observatory                          Astronomy Symposium
      Testing of the shock model

                                                        α=-0.24
                          α<-0.5
                                     α=0.41




-Otherwise good general
correspondence with the
shock model


       Talvikki Hovatta              The X Finnish-Russian Radio
       Metsähovi Radio Observatory   Astronomy Symposium
                   Conclusions
   Variability behaviour is complex
   Episodes of quasi-periodic behaviour are
    common
   Flares are seen on average every 4 years
   Median duration is 2.5 years

=> Long-term monitoring is essential!


      Talvikki Hovatta              The X Finnish-Russian Radio
      Metsähovi Radio Observatory   Astronomy Symposium

				
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