Observations of IntraHour Variable Quasars

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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)

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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 (ltlt1) !
• 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 (z1.285)
• J18193845 (z0.54)
• PKS B1257-326 (z1.256)
• What can they tell us about the sources and the
  ISM?

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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
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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

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PKS B0405-385 long-term variability
Kedziora-Chudczer (2006, MNRAS)
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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)
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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
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J18193845 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)
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PKS B1257-326 the 3rd IHV quasar

• IHV discovered with ATCA in 2000 (actually first
  in archival data from 1995)
• Continuous scintillator (like J18193845)

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

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PKS 1257-326 long term evolution
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PKS 1257-326 polarization

• Stokes parameter cross-correlations show small
  displacement between I and p component centroids
• Simple polarized structure compared with other
  IHVs?

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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

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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 gtgt baseline

Coles Kaufman (1978) for baseline r, pattern
axial ratio R elongated along S, moving with
velocity v relative to baseline, time delay is
given by
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• Time delays
• May 483 /- 15s
• January 333 /- 12s
• March 318 /- 10s

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Annual cycle in scintillation timescale
Bignall et al. 2003, ApJ, 585, 653
s0 characteristic scintillation length scale
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Simultaneous fit to time delays and annual cycle
NO CONSTRAINTS
LSR VELOCITY
ISOTROPIC
R lt 12
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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

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J18193845 annual cycle
Dennett-Thorpe de Bruyn (2003) Fit requires
highly anisotropic scintillation pattern - also
degenerate velocity solutions
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Annual cycle in ISS timescale
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Annual cycle in 2-station time delay
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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

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PKS 1257-326 time delay geometry
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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 lt 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

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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 Tyulbashev (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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x250
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PKS B1257-326 at 18.5 GHz
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J18193845 WSRT-VLA time delay
Variable time delay (Dennett-Thorpe de Bruyn,
2002, Nature)
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PKS 1519-273