Watch: June 30 Prof. Martin Ward: The Variable Universe: the Changing Appearance of Active Galactic Nuclei(AGN)

Speaker: Prof. Martin Ward
30 June 2022
4 pm GMT+8
Watch it on Bilibili

The Universe is ever changing, whether by evolution over cosmic time, via powerful outbursts such as novae and supernovae, or by virtue of changes in the physical processes at work within an object. Some active galactic nuclei (AGN) and their luminous examples the quasars, have now been observed to increase/decrease their luminosity dramatically over relatively short timescales ie. months to years. These changes are not consistent with previous models of the properties of the accretion disc that feeds material into the black hole and so provides the source of energy. I will describe progress in understanding the origin of these changes. The conventional term used for this phenomenon is “changing look”, but I will propose that a better and more physically motivated term would be “changing state”. This is based on an interesting analogy with black hole binary systems, whose black hole masses are many orders of magnitude less than the supermassive black holes in AGN/quasars, and the timescale for the observed changes is correspondingly much shorter. But the fundamental physics of an accretion disc feeding material into the compact object is basically the same.

Current repeated surveys of large areas of the sky are finding large numbers of such objects. However, it is clear that dramatic changes in the luminosity of the nucleus of a galaxy can result from a very different physical cause. An important challenge is to identify ways to distinguish between these possibilities.

About the Speaker

Martin Ward is Emeritus Chevallier Professor of Astronomy and Leverhulme Fellow at Durham University England. He has previously held positions at Cambridge, Oxford and Leicester, and was a vice-President of the Royal Astronomical Society. He is an observational astrophysicist whose research interests include black holes and quasars. He served as a consultant for the European Space Agency and he is a UK co-investigator on the James Webb Telescope mid-infrared instrument. He is also interested in science public outreach, and is involved in projects related to the search for life elsewhere in the Universe.


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