Changing Modality Behaviour In The Radio Light Curve Of Blazar Pksb1144-379

The highly variable BL Lac object PKSB1144 - 379 was monitored at 6.7 GHz using the Ceduna Radio Telescope with high cadence from 2003 to 2011. Intraday variations due to interstellar scintillation (ISS) were observed throughout the period. To complement our earlier analysis of the ISS and variability of this object, we have investigated the physical origin of changes in the modality of flux density distributions, calculated for similar to 14 d observing blocks. Our analysis shows that the flux density distribution is primarily bimodal, but it changes to unimodal during the core brightening and jet expansion phases of the source. The presence of unimodal flux density distributions during these two phases is most likely due to the compactness of the scintillating component and the intrinsic evolution of the source. The existence of unimodality in the flux density distributions associated with specific phases of the source evolution also suggests that changes in the modality are unlikely due to multiple scattering screens. We propose that the physical origin of changes in the modality of the flux density distribution for PKSB1144 - 379 is most likely due to the combination of multiple bright jet features with interstellar scintillation along the line of sight between observer and source. This new approach complements our previous investigations of the temporal evolution of PKSB1144 - 379 that used interstellar scintillation and very long baseline interferometry, and the combination of these techniques provides a crucial starting point for understanding the system.