The micro-arcsecond scale evolution of quasar PKS 1257-326

H.E. Bignall (JIVE), J-P. Macquart (Kapteyn Inst. Groningen), 
D.L. Jauncey, A.K. Tzioumis, J.E.J. Lovell (ATNF)
and L. Kedziora-Chudczer (University of Sydney)

The quasar PKS 1257-326 shows very rapid ISS, and similarly to
J1819+3845, has continued to scintillate rapidly for at least several
years. We now have ATCA monitoring data over three years which shows a
repeated annual cycle in the characteristic timescale of ISS, as well
as three epochs in which we measured a time delay between the ISS
pattern arrival times at the VLA and the ATCA. All of these data are
used simultaneously to fit for the velocity of the scattering screen as
well as the characteristic length scale, axial ratio and position angle
of anisotropy of the scintillation pattern. A fit to the data requires
a highly anisotropic scintillation pattern, and tightly constrains the
position angle of anisotropy, however there is some degeneracy in
fitting for the ISM velocity and length scale. The ATCA data show a
slow increase in the rms modulations of IDV, strongly correlated with an 
intrinsic "outburst" in the source. Cross-correlation of the
scintillation pattern at different frequencies suggests a changing
displacement between the centroids of different frequency
components. Implications of these results for the microarcsecond-scale
source structure and evolution are discussed.