1 / 26

Observations of Intra-Hour Variable Quasars

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). Introduction.

gay
Download Presentation

Observations of Intra-Hour Variable Quasars

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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)?

More Related