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Fast interstellar scintillation of quasars, and PKS 1257-326 revisited

Hayley Bignall
ICRAR - Curtin University


3 well-studied "extreme" scintillating quasars
8.6 GHz

4.9 GHz: weak scattering

4.8 GHz

2.3 GHz 1.4 GHz PKS 0405-385 ­ ATCA (Kedziora-Chudczer et al. 1997, ApJL, 490, L9)

1.4 GHz: strong scattering (refractive scintillation)
J1819+3845 - WSRT (Dennett-Thorpe & de Bruyn, 2000, ApJL, 529, L65)

PKS 1257-326 - ATCA (Bignall et al. 2003, ApJ, 585, 653)



Two-station pattern arrival time delays

Depends on baseline r, scintillation velocity v(DOY), position angle and axial ratio R of anisotropy in the scintillation pattern


Annual cycle in scintillation timescale

Fit to annual cycle + time delay (at 3 different times of year) implies highly anisotropic scintillation pattern: R>6 but not well constrained. Bignall et al. (2006) Walker et al. (2009)

Minor axis scale is well constrained and relating this to Fresnel scale, r F = implied screen distance is D~10pc

cD / 2


The annual cycle & scintillation pattern projected on the sky


"Core shifts" from multi-frequency scintillation pattern offsets
2003

Line corresponds to pattern displacement of 22,000 km for preferred annual cycle model = 15 as for screen distance of 10pc Offset is significantly smaller in subsequent years ­ optical depth effect post-outburst?

2006


Evolution of PKS 1257-326 during ATCA monitoring programme


CABB data: 2GHz bandwidth (x2) Obtained by Jeff Hodgson


0.07

mpt~(0/)

17/12

Source has large fraction of flux density in steep-spectrum, nonscintillating components
0.03 4



0.15 -17/

12

Can fit frequency dependence assuming inverted spectrum for scintillating component. Also need to consider source size effects (>F; ~1/?)

0.049 0.6

-17/12


That PKS 1257-326 is still scintillating after >15 years (since 1995 archival data) implies "screen" is at least 100 AU in extent



Resolution "beyond VLBI": some results from other fast scintillators...

Modelling of asscale polarization structure in PKS 0405-385 (Rickett et al. 2002)

Power spectral modelling of screen + source evolution in J1819+3845 (Macquart & de Bruyn 2007)


Fast scintillators from the MASIV Survey
Detection of Six Rapidly Scintillating AGNs and the Diminished Variability of J1819+3845
Koay, Bignall, Macquart, Jauncey, Rickett, Lovell. A&A Letters, accepted 07/09/2011 · MASIV 5 GHz VLA Survey: ~18% of 443 compact quasars had characteristic timescales of ISS < 12 hours (Lovell et al. 2008, ApJ, 689, 108) · J1819+3845 stood out with modulation index of 50% (cf <12% for all other sources in MASIV) · MASIV Survey follow-up (Koay et al. 2011, AJ, 142, 108) [Poster "ISS Surveys as a Cosmological Probe: Prospects and Challenges" by Kevin Koay, J-P Macquart et al.] · 6 out of 128 sources observed over 11 days in Jan 2009 had characteristic timescales < 2 hours (=the sampling interval) · J1819+3845 showed diminished variability with 2% variations on ~6 hour timescale (previously reported by CimР 2008, in Proceedings of the 9th
EVN symposium)


How to find fast variability
Long timescale variability amplitude

Fast scintillators

Short timescale variability amplitude


Example light curve of a fast scintillator from MASIV Survey VLA follow-up


J1819+3845 no longer shows extreme, fast scintillation


Implications of fast scintillation


c

har

< 2 hr attributed to scattering at D<12 pc for a







~200 as source or D<250 pc for a ~10 as source Can constrain compact fraction and brightness temperature ­ cf mas-scale structure from VLBI Correlation between fast scintillation and low line-ofsight H: suggestive of lower effective scattering screen distances (although PKS 1257-326 has high H! - multiple screens possible) Change in J1819+3845 ­ interpreted as nearby "screen" drifting out of the line of sight Similar to episodic behaviour of PKS 0405-385


Implications of fast scintillation








Fast scintillators are not as rare as initially thought, but large-amplitude (>10%) ISS of quasars is rare. Quasar ISS is a combination of source and screen.. screen may dominate behaviour! Need methods for dealing with variability in synthesis imaging ­ e.g. Stewart et al. (A&A, accepted, arXiv.org/abs/1107.4282): generalisation of N-beam CLEAN algorithm Need to consider the effects of ISS on radio transient population studies


ASKAP simulation including (extreme) variable sources

Simulation by Stewart et al. (2011) of variable point source + extended emission using (c) standard clean and (d) 2-beam decomposition