Observations of Intra-Hour Variable Quasars

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 Weak scattering 4.8 GHz 2.3 GHz Strong scattering 1.4 GHz Kedziora-Chudczer et al. 1997

4. PKS B0405-385: the first IHV quasar • Kedziora-Chudczer et al. (1997) • ISS model (n0 ~ 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 mas-scale polarized structure (not unique)

7. PKS B0405-385: new data 1.8 Jy • 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! I 1.5 Jy 0.06 Jy Q 0.02 Jy 0.08 Jy U 0.04 Jy

8. J1819+3845 – the 2nd IHV quasar • 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 Dennett-Thorpe & de Bruyn (2000)

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

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

11. PKS 1257-326: first year of ATCA monitoring • 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

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

14. 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

15. 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:

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

17. Annual cycle in scintillation timescale Bignall et al. 2003, ApJ, 585, 653 s0 = characteristic scintillation length scale

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

19. 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

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

21. Annual cycle in ISS timescale

22. Annual cycle in 2-station time delay

23. 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

24. PKS 1257-326 time delay geometry

25. PKS B1257-326: screen distance • Scintillation length scale (1/e) along minor axis: amin = (4.2 +/- 0.1) x 104 km at 5 GHz • Weak scattering theory: • rF=(lL/2p) Fresnel scale • For anisotropic scattering, amin 0.78rF • Screen distance L < 10 pc • Minor axis angular scale is ~30 microarcseconds • If source has flux density of 100mJy distributed within 30x30 mas, brightness temperature Tb ~ 1013 K

26. 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)?