Precise calculations of core shift and pressure gradient of AGNs
While the examination of the bright and compact cores of Active Galactic Nuclei (AGNs) by Very Long Baseline Interferometry (VLBI) is limited, due to frequency dependent variations in density of the core and emitted jets, a new technique now offers a frequency-independent approach and unprecedented precision. CAASTRO Associate Investigator Jean-Pierre Macquart and his colleagues used observations with the Australia Telescope Compact Array to test their new approach and calculated a core shift for source ‘PKS 1257-326’ an order of magnitude smaller than respective VLBI measurements.
Their interstellar scintillation analysis was based on frequency-dependent flux density light curves and delays in the arrival times of annually modulated variations obtained from the source. Cross-correlating and fitting the light curves yielded delays of up to 600 seconds that could be related to the structure of its jets. At the lowest observing frequency (4540 MHz), the physical size of the core shift was calculated to be 0.16 parsecs, and 0.10 parsecs at the highest frequency of 9960 MHz.
The team further analysed the morphology of the jets and found that their opening angle increased with distance from the core (‘flaring’) and that their remarkable stability might be due to the high pressure gradient, contained by hydrostatic equilibrium with the surrounding medium.