Wednesday Colloquium


"The Size, Shape, and Scattering of the Black Hole Sagittarius A*"

Sara Issaoun, (Radboud University, Nijmegen)

The Galactic Center supermassive black hole, Sagittarius A* (Sgr A*), is the most promising target to study the dynamics of black hole accretion and outflow via direct imaging. In April 2017, the Atacama Large Millimeter/submillimeter Array (ALMA) operated as a phased array in its first science run with very long baseline interferometry (VLBI). As part of the extensive multi-wavelength campaign for the Event Horizon Telescope, we observed Sgr A* at 3.5-mm with the Global Millimeter VLBI Array (GMVA) and ALMA, reaching an angular resolution of 87 micro-arcseconds, double that of previous experiments at the same wavelength. In this talk, I will present the observations that enabled the first imaging of the intrinsic structure in Sgr A* at 3.5-mm, revealing a nearly isotropic source. I will also discuss the constraints on source size, shape and interstellar scattering that these observations have provided and how these inform horizon-scale expectations for the Event Horizon Telescope.


"Manifestations of MADness (in Accretion Disks)"

Mitch Begelman (JILA, University of Colorado, Boulder, CO)

Magnetically arrested accretion disks (MAD) have been implicated in the formation of powerful relativistic jets by accreting black holes. The basic idea is that the ram pressure of the accretion flow confines a maximum amount of magnetic flux around the black hole, optimizing the extraction of black-hole spin energy. But the situation outside the black hole may be more complicated, with the toroidal magnetic field - not the poloidal flux - dominating the transition to MADness and the resulting disk structure. I will discuss various factors that may come into play when an disk goes MAD, and how recent observations can help with the diagnosis.

Due to the time difference, the lecture will be held at 5:15 pm Warsaw time.


"Blazar Flare and Theoretical Modeling of Multiwavelength Emission"

Raj Prince (Center for Theoretical Physics, PAN, Warsaw)

Blazars are a class of an AGN with jet pointing towards the observer within a few degrees, and due to relativistic effects, all the emission is boosted along the jet axis. They show the spectacular flares across all the wavebands ranging from radio to very high energy gamma-ray. They also offer very strong variability on the time scale of minutes to years during the flaring activity. The reason behind the fast variability and high flaring states are still unknown. However, the broadband study can answer these questions to some extent. The broadband SED of blazar shows a two hump kind of structure one peaks in low energy band and the other in high energy band. I will discuss a few proposed models to explain the fast variability, and further, will talk about the possible physical mechanisms responsible for the multi-wavelength emission.


"The Planck cosmological legacy"

François R. Bouchet (Institut d'Astrophysique de Paris)