Marzena Sniegowska from the Nicolas Copernicus Astronomical Centre in Poland

Marzena Sniegowska, is a PhD student from the Nicolas Copernicus Astronomical Centre in Poland.
She is doing her PhD with Bozena Czerny and works mostly on Active Galactic Nuclei and in particular on ‘changing-look’ AGN.

Talk title: The mystery of Changing-Look (CL) AGN.

The variability of AGN is stochastic in its nature (e.g. Vaughan et al. 2003). It is well modelled as local fluctuations in the accretion flow propagating inwards, where  most of the energy is dissipated but the flow contains the memory of all the local time-scales. However, apart from this regular, low-level stochastic variability, some AGN occasionally show exceptionally large changes in the luminosity, spectral shape and/or X-ray absorption. The most notable are the changes of the spectral type, when the source classified as a Seyfert 1 becomes a Seyfert 2 galaxy, or vice versa.Thus a name was coined of 'Changing-Look AGN' (CL AGN).

Only a few cases were recorded in the past (e.g. NGC 5548, Peterson et al. 1987). Recently the number of new CL AGN is growing rapidly (MacLeod et al. 2019), so the phenomenon is apparently not so rare as previously thought. The origin of this phenomenon is still unknown, but for most of the sources there are strong arguments in favor of the intrinsic changes (e.g., Kynoch et al. 2019,  Hutsemekers et al. 2019). Understanding the nature of such rapid changes is a challenge to the models of accretion flow onto the black hole. The time-scales are much shorter than the usual viscous time-scales in the accretion disks (of order of hundreds of years, or longer, depending on the black hole mass and the Eddington ratio). There are no convincing models of this phenomena, although some mechanisms are proposed (e.g. Ross et al. 2018, Sniegowska et al. 2019).