1. Observations of Intra-Hour Variable Quasars Hayley Bignall (JIVE) Dave Jauncey, Jim Lovell, Tasso Tzioumis (ATNF) Jean-Pierre Macquart (NRAO/Caltech) Lucyna Kedziora-Chudczer (University of Sydney)

2. Introduction MASIV Survey  Intra/inter-day variability very common (56%) in compact flat-spectrum radio sources at cm wavelengths, but more rapid intra-hour variability is extremely rare (<<1%) ! IHV makes it easy to sample ISS pattern in reasonable observing time, so characteristics readily measured Timescale of weak ISS  Fresnel scale at scattering screen –IHV seems to be due to very nearby, localized “screens” (~10pc) 3 best studied IHV quasars –PKS B0405-385 (z=1.285) –J1819+3845 (z=0.54) –PKS B1257-326 (z=1.256) What can they tell us about the sources and the ISM?

3. PKS B0405-385: the first IHV quasar 8.6 GHz 4.8 GHz 2.3 GHz 1.4 GHz Weak scattering Strong scattering Kedziora-Chudczer et al. 1997

4. PKS B0405-385: the first IHV quasar Kedziora-Chudczer et al. (1997) ISS model ( 0 ~ 5 GHz) fit frequency dependence of modulation index (and timescale) IHV in this source is episodic – turns on and off on timescale of months to years

5. PKS B0405-385: long-term variability Kedziora- Chudczer (2006, MNRAS)

6. PKS B0405-385: the first IHV quasar During second “episode” of IHV, pattern arrival time delay of ~2 minutes observed between VLA and ATCA (Jauncey et al. 2000) – Direct proof of ISS origin Rickett et al. (2002) analysed Stokes I,Q and U variability from June 1996: Model of  as-scale polarized structure (not unique)

7. PKS B0405-385: new data Kedziora-Chudczer: ATCA data at 4.9 GHz over 4 hour time range on 8 May 2006 Latest episode of IHV seen since 2004 after 4 year quiescent period (Cimó et al., IAUC 8403) New ATCA monitoring data show very short timescale fluctuations! 1.8 Jy 1.5 Jy 0.06 Jy 0.02 Jy 0.08 Jy 0.04 Jy U I Q

8. J1819+3845 – the 2 nd IHV quasar Dennett-Thorpe & de Bruyn (2000) Monitored over 7 years with WSRT (de Bruyn et al.) “Continuous” IHV Repeated annual cycle with extreme slow-down in November (Dennett- Thorpe & de Bruyn 2003) Pattern arrival time delay between WSRT and VLA (Dennett-Thorpe & de Bruyn, 2002) 21cm frequency-dependent variations – DISS? (Macquart & de Bruyn 2005) Polarized structure & evolution

9. PKS B1257-326: the 3 rd IHV quasar IHV discovered with ATCA in 2000 (actually first in archival data from 1995) Continuous scintillator (like J1819+3845)

10. 4.8 GHz 8.6 GHz PKS 1257-326: first year of ATCA monitoring

11. Peak of cross-correlation between 4.8 and 8.6 GHz data (Bignall et al. 2003, ApJ, 585, 653) Opacity effect in inner jet? Offset has changed with time, possibly due to evolution of intrinsic outburst

12. PKS 1257-326 – long term evolution

14. PKS 1257-326: polarization Stokes parameter cross-correlations show small displacement between I and p component centroids Simple polarized structure compared with other IHVs?

15. ISS as a probe of source structure In order to relate ISS analysis to source structure, need to determine some properties of the scattering –Distance to screen –Velocity –anisotropy

16. Pattern arrival time delay VLA-ATCA Time delay of 8 minutes observed in 2002 May Almost no detectable pattern decorrelation  “frozen-in” pattern, single velocity, characteristic scale >> baseline Coles & Kaufman (1978): for baseline r, pattern axial ratio R elongated along Ŝ, moving with velocity v relative to baseline, time delay is given by:

17. Time delays: –May: 483 +/- 15s –January: 333 +/- 12s –March: 318 +/- 10s

18. Annual cycle in scintillation timescale s 0 = characteristic scintillation length scale Bignall et al. 2003, ApJ, 585, 653

19. Simultaneous fit to time delays and annual cycle ISOTROPIC NO CONSTRAINTS R < 12 LSR VELOCITY

20. The problem of large anisotropy When R is large, can no longer uniquely determine velocity Pattern scale along short axis is well constrained, but length scale and component of v along long axis are not

21. Dennett- Thorpe & de Bruyn (2003) Fit requires highly anisotropic scintillation pattern - also degenerate velocity solutions J1819+3845: annual cycle

22. Annual cycle in ISS timescale

23. Annual cycle in 2-station time delay

24. Time delays and correlation coefficients Largest decorrelation observed in May: large component of velocity parallel to long axis of pattern Scale ~ 500,000 km at 5GHz

25. PKS 1257-326 time delay geometry

26. PKS B1257-326: screen distance Scintillation length scale (1/e) along minor axis: a min = (4.2 +/- 0.1) x 10 4 km at 5 GHz Weak scattering theory: –r F =  ( L/2  ) Fresnel scale –For anisotropic scattering, a min  0.78r F Screen distance L < 10 pc Minor axis angular scale is ~30 microarcseconds If source has flux density of 100mJy distributed within 30x30  as, brightness temperature T b ~ 10 13 K

27. Final remarks ISS of extragalactic sources can be used to probe structure of the sources and the local ISM. –Microarcsecond scales: multi-frequency, polarized substructure through cross-correlation analysis (structure functions, power spectra) –See also Shishov, Smirnova & Tyul’bashev (2005): analysis of asymmetry coefficient to estimate fraction of flux density in scintillating component IHV picks out nearby scattering screens For more distant screens, –ISS occurs on longer timescales –tends to be “quenched” by angular size of AGN Some problems: –Large anisotropy  degenerate solutions for screen velocity –Changes: due to source or screen (or both)?

28. x250

29. PKS B1257-326 at 18.5 GHz

30. J1819+3845: WSRT-VLA time delay Variable time delay (Dennett- Thorpe & de Bruyn, 2002, Nature)

32. PKS 1519-273