The ATCA c1730 program

There are thought to be about a hundred thousand million galaxies in the Universe, yet less than a thousand of these have been detected to emit gamma-rays. The Fermi gamma-ray satellite, launched in June 2008, is revealing more and more of these powerful galaxies, and we are using the Australia Telescope Compact Array (ATCA) to study them at radio wavelengths in order to understand exactly what powers these fascinating objects. Fermi is completing a full-sky survey every three hours and providing unprecedented views of the gamma-ray universe. This project will observe a selection of 80 active galaxies detected by Fermi which, together with optical, X- ray and Very Long Baseline Interferometry (VLBI) data (from the TANAMI project will enable correlations in the multi-wavelength variability of these objects to be identified and a better understanding gained of the physical processes powering these AGN.

ATCA observations are (from April 2009) made at 5.5, 9, 17, 19, 38 and 40 GHz, using the 2GHz bandwidth provided by the Compact Array Broadband Backend (CABB) at each frequency. The observations are made in "snapshot" mode, with a single scan at each frequency. This observing mode is well-suited to bright compact radio-sources, such as those being studied in this program. Data from c1730 are processed in a data reduction and immediately incorporated in the ATCA calibrator database for use by all astronomers.

Illustrative multi-epoch results from C1730 are shown in Figures below. The contrasting spectral evolutions of PKS0454-234 and PKS1730--130 are apparent: PKS0454-234 is effectively constant in the 7mm band, with the flux densities decaying at lower frequencies. PKS1730-130, however, shows pronounced variation in all bands, with a quiescent flat spectrum in the first epoch, an inverted spectrum at the higher frequencies in the second epoch, and a steeper spectrum in the third epoch as the flare emission evolves from higher to lower frequencies. This behaviour would be expected to be accompanied by the emergence of a new jet-component on VLBI scales, and increased gamma-ray activity (however as the peak is likely to have occurred before the Fermi launch, this particular case cannot by verified).

A summary of observations made to date is provided in the table below.

For further information on the c1730 program, please contact Phil Edwards (philip . edwards [at] csiro . au).

This page last modified 26-aug-2011.