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• 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?

PKS B0405-385: the first IHV quasar8.6 GHz

4.8 GHz

2.3 GHz

1.4 GHz

Weak scattering

Strong scattering

Kedziora-Chudczer et al. 1997

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

PKS B0405-385: long-term variability

Kedziora-Chudczer (2006, MNRAS)

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)

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 JyU

I

Q

J1819+3845 – the 2nd 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

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)

4.8 GHz8.6 GHz

PKS 1257-326: first year of ATCA monitoring

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

PKS 1257-326 – long term evolution

PKS 1257-326: polarization• Stokes parameter cross-correlations show small

displacement between I and p component centroids

• Simple polarized structure compared with other IHVs?

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

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:

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

Annual cycle in scintillation timescale

s0 = characteristic scintillation length scale

Bignall et al. 2003, ApJ, 585, 653

Simultaneous fit to time delays and annual cycle

ISOTROPICNO

CONSTRAINTS R < 12LSR

VELOCITY

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

Dennett-Thorpe & de Bruyn (2003)Fit requires highly anisotropic scintillation pattern - also degenerate velocity solutions

J1819+3845: annual cycle

Annual cycle in ISS timescale

Annual cycle in 2-station time delay

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

PKS 1257-326 time delay geometry

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=(L/2) 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 as, brightness temperature Tb ~ 1013 K

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)?