1. What is the scattering screen in front of quasar PKS B1257-326?
Hayley Bignall
Jean-Pierre Macquart (NRAO/Caltech)
Dave Jauncey, Jim Lovell, Tasso Tzioumis (ATNF)
Lucyna Kedziora-Chudczer (University of Sydney)
VLBI: Roopesh Ojha (ATNF), Cormac Reynolds (JIVE),
Juan Carlos Algaba (University of Valencia)

2. SINS conference, Socorro, NM
Outline
Observations of PKS B
Rapid intra-hour variability (“IHV”) at cm wavelengths
Two-station time delays (VLA-ATCA)
Annual cycle in timescale
Analysis of the scintillation pattern
Implications for the structure responsible for the scintillation
15 February 2019
SINS conference, Socorro, NM

3. SINS conference, Socorro, NM
PKS B
“Typical” flat-spectrum radio quasar at z=1.256 (Perlman et al. 1998), except…
Flux density observed with ATCA at 4.8 GHz over 12 h on 2005 December 29
Flux density (Jy)
Time
15 February 2019
SINS conference, Socorro, NM

4. Intra-hour variability (IHV)
PKS B was third confirmed “intra-hour” variable
after PKS B (Kedziora-Chudczer et al. 1997) and J (Dennett-Thorpe & de Bruyn 2000)
IHV in PKS B has evidently persisted over a decade
1995 archival data, monitored since 2000
MASIV Survey  IHV seen in <<1% of flat spectrum radio sources
15 February 2019
SINS conference, Socorro, NM

5. SINS conference, Socorro, NM
Comparison: PKS B
8.6 GHz
4.8 GHz
2.3 GHz
1.4 GHz
Kedziora-Chudczer et al. (1997)
15 February 2019
SINS conference, Socorro, NM

6. SINS conference, Socorro, NM
Comparison: PKS B
Kedziora-Chudczer (2006, accepted by MNRAS)
“IHV” is episodic
Recent observations show rapid variations again, with very fine structure
15 February 2019
SINS conference, Socorro, NM

7. SINS conference, Socorro, NM
PKS B : time delays
Up to 8 minutes delay between pattern arrival times at VLA and ATCA
15 February 2019
SINS conference, Socorro, NM

8. SINS conference, Socorro, NM
Time delays:
May: 483 +/- 15s
January: 333 +/- 12s
March: 318 +/- 10s
15 February 2019
SINS conference, Socorro, NM

9. SINS conference, Socorro, NM
Time delay geometry
For baseline r, scintillation pattern of axial ratio R elongated along Ŝ, moving with velocity v relative to baseline, gives rise to pattern arrival time delay:
(Coles & Kaufman, 1978)
15 February 2019
SINS conference, Socorro, NM

10. Scintillation timescale
Annual cycle in characteristic timescale (Bignall et al. 2003)
s0 = characteristic scintillation length scale
15 February 2019
SINS conference, Socorro, NM

11. SINS conference, Socorro, NM
Results: scintillation pattern is highly anisotropic. Position angle and length scale along short axis are well-determined
Pattern scale and velocity component parallel to long axis poorly constrained
15 February 2019
SINS conference, Socorro, NM

12. Distance to the scattering screen
Scintillation length scale (1/e) along minor axis = (4.2 +/- 0.1) x 104 km at 5 GHz
Weak scattering theory:
For anisotropic scattering, s0  0.78rF
rF=(lL/2p) Fresnel scale
 L < 10 pc
 Scintillation scale (minor axis) is ~30 microarcseconds
15 February 2019
SINS conference, Socorro, NM

13. Constraints on the anisotropy
Degree of cross-correlation between the observed patterns at each telescope, for large R, and pattern scale large compared with baseline:
15 February 2019
SINS conference, Socorro, NM

14. Constraints on the anisotropy
De-correlation of pattern between the two telescopes
calculate linear Pearson correlation coefficients for time-shifted data
In all cases, r > 0.995
At both frequencies & both days, see somewhat more decorrelation in May than in other epochs
Observing spatial decorrelation as each telescope samples different parts of scintillation pattern
15 February 2019
SINS conference, Socorro, NM

15. Time delays and pattern decorrelation
Decorrelation  major axis length scale of scintillation pattern
Based on geometry, expect largest decorrelation in May, which is observed
Results imply major axis length scale at least 4x105 km at 4.9 GHz
R  10
We effectively measure a scale length ~40x the baseline
15 February 2019
SINS conference, Socorro, NM

16. “Annual cycle” in time delay
Measuring the time delay near DOY 220 should give tighter constraints on the velocity (and anisotropy)
15 February 2019
SINS conference, Socorro, NM

17. SINS conference, Socorro, NM
Summary
Two-station time delays + annual cycles in timescale combined to fit for characteristic scintillation scale, anisotropy & velocity
Scintillation pattern is highly elongated (minimum axial ratio R ~10:1)
This leads to degeneracy in fitting velocity
Evidence for highly anisotropic scattering
Nearby screen: within 10 pc
Scintillation persistent over a decade  scattering structure at least 1015 cm in linear extent
15 February 2019
SINS conference, Socorro, NM

18. SINS conference, Socorro, NM
Questions…
Are the structures causing “normal” IDV the same as those causing IHV but at larger distances?
Alignment of source structure: another possible IHV selection effect?
15 February 2019
SINS conference, Socorro, NM

19. SINS conference, Socorro, NM
x250
15 February 2019
SINS conference, Socorro, NM