Research: Accretion properties and extreme variability

The physical mechanism responsible for the X-ray emission, and its relation to the accretion flow, is still largely debated. With my research I plan to shed light on the accretion process, by combining X-ray, γ-ray, mm and optical observations of nearby AGN, and by studying the most extreme cases of AGN variability.

Accretion physics

 

With my team we are studying the characteristics of the X-ray plasma in AGN by looking at the X-ray variability properties of a large number of sources from the BASS sample, and by analyzing XMM-Newton/NuSTAR observations of a sample of nearby super-Eddington AGN. These episodes of super-Eddington accretion could have been extremely important for the growth of massive black holes in the early Universe, and a clear understanding of their broad-band X-ray properties is very important to better understand future observations that will be carried out by the next generation of X-ray facilities, such as Athena.

 Artistic representation of an X-ray corona in a supermassive black hole (Credits: NASA/JPL)

We are also looking at the multi-wavelength properties of all the unobscured AGN in the BASS sample, with the goal of improving our understanding on the relation between the emission from the disk and that from the X-ray corona.

Extreme AGN variability

 

Understanding the physical mechanisms at play during transient episodes of extreme accretion onto SMBHs is a key step to understand how these objects grow. Over the past couple of years I have been working intensively on this subject, following up several of these transient events in the X-ray band (e.g., Trakhtenbrot et al. 2019, Nature Astronomy). In this framework, we recently discovered the first case of an AGN in which the X-ray corona disappeared, possibly due to the destruction of the accretion disk following a tidal disruption event (Ricci et al. 2020). We have been following up this object for two years in the X-ray band, and we have several papers in preparation based on these observations. I am also a member of the SDSS-V and the Vera Rubin Observatory collaborations, which will be complete game changers for the study of rapid and dramatic episodes of accretion onto supermassive black holes.

Long-term light curve of 1ES 1927+654 in the X-rays (top panel) and optical/UV (bottom panel). The middle panels show how the hardness ratio and the contribution of the power-law component changed with time.