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)?
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