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)

SINS conference, Socorro, NM Outline Observations of PKS B1257-326 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

SINS conference, Socorro, NM PKS B1257-326 “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

Intra-hour variability (IHV) PKS B1257-326 was third confirmed “intra-hour” variable after PKS B0405-385 (Kedziora-Chudczer et al. 1997) and J1819+3845 (Dennett-Thorpe & de Bruyn 2000) IHV in PKS B1257-326 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

SINS conference, Socorro, NM Comparison: PKS B0405-385 8.6 GHz 4.8 GHz 2.3 GHz 1.4 GHz Kedziora-Chudczer et al. (1997) 15 February 2019 SINS conference, Socorro, NM

SINS conference, Socorro, NM Comparison: PKS B0405-385 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

SINS conference, Socorro, NM PKS B1257-326: time delays Up to 8 minutes delay between pattern arrival times at VLA and ATCA 15 February 2019 SINS conference, Socorro, NM

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

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

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

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

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

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

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

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

“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

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

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

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