Active Galactic Nuclei variability workshop

In the last week of June, the ATNF headquarters in Marsfield hosted a prestigious, Elizabeth & Frederick White, international workshop on the "Variability of Active Galactic Nuclei across the Electro-Magnetic Spectrum". The meeting was sponsored by the Australian Academy of Science, the Anglo-Australian Observatory, the Research School of Astronomy and Astrophysics at the Australian National University, and the Australia Telescope National Facility. More than sixty participants from 14 countries attended the meeting and the majority presented talks and posters. Thanks to the strong Australian sponsorship, the list of invited presentations reflected a large variety of issues researched within this topic.

The workshop participants.

It is recognised that variability of Active Galactic Nuclei (AGN) is one of the most important tools for studying their detailed structure and emission mechanisms. AGN exhibit variability across the entire electromagnetic spectrum, on time-scales ranging from minutes to years.

Long term variability in AGN at radio frequencies has been recognised for almost 40 years and has been interpreted as due to intrinsic outbursts in the source, mostly related to shocks in jets extending from these objects. Relatively recent results from high energy instruments (HEGRA and EGRET in gamma-ray, ROSAT and XTE in X-ray) reveal strong outbursts in a particular class of AGN - blazars.

New developments in instrumentation, including improved sensitivity and stability, have enabled better temporal sampling and accuracy of flux density measurements. This has resulted in the discovery of extremely short time-scale variability over the entire electromagnetic spectrum. Such variability, if intrinsic to the source, suggests extremely small sizes for the emitting source components. This often leads to unacceptable physical conditions such as excessive brightness temperatures and opacity problems.

The interpretation of the short-term fluctuations at radio frequencies is still under debate. The evidence thus far suggests extrinsic mechanisms, such as interstellar scintillation, are dominant.

The intention of the workshop was to focus on the multi-wavelength variability of AGN, in order to improve our understanding of the mechanisms and the sequence of events that lead to the variability in different parts of the electromagnetic spectrum. To achieve this purpose, time for an extensive discussion of the latest observational results and theoretical interpretations was allocated in the workshop program.

Participants discussed monitoring strategies necessary to acquire simultaneous observations of the flux density variations over the entire electromagnetic spectrum. Such monitoring was recognised as a crucial step for a full understanding of activity in the innermost regions of AGN. A proposal to form an International Astronomical Union Commission dedicated to multi-wavelength studies of AGN was an outcome of this discussion.

The international astronomical community was made aware of the strength and activity in observational and theoretical studies of the AGN intraday variability in Australia. In particular the workshop conveyed strongly the importance and implications of the extrinsic interpretation of intraday variability at radio frequencies.

Participants of the workshop were offered a number of accompanying activities by the Local Organizing Committee. One such event was a special presentation by the master of astronomical photography, Dr. David Malin, titled "A Universe of Colour". A display of the Luneberg Lens prototype with Peter Hall's commentary was received with interest. Similarly, the evening screening of the recently released movie "The Dish" brought a sizable audience.

The relaxing atmosphere of the Harbour Cruise Workshop Dinner was enhanced by an opportunity for star-gazing, an unique experience for people who came to Australia from the northern hemisphere.

The scientific benefit of this meeting is already visible in the large number of submitted conference proceeding papers, which are due to be published in Australia's own astronomical journal, the Publications of the Astronomical Society of Australia (PASA), in the January 2002 issue.

The organisers of the Workshop, Lucyna Kedziora-Chudczer and David Jauncey, would like to offer warm thanks to the workshop sponsors for their generous financial support, and also to all the ATNF staff (especially Sue Little, Anne Barends and Phil Sharp) who helped to make this meeting a very enjoyable and memorable experience.

Lucyna Kedziora-Chudczer
(Lucyna.Kedziora-Chudczer@csiro.au)