Sagittarius A* Variability Studies

One of the most important limiting factors in imaging event horizon scale structures of Sagittarius A* is its short timescale variability in the range of minutes to hours. 

This variability leads to a violation of the stationarity assumption that is at the basis of VLBI aperture synthesis. 

At the same time, this short timescale variability in its own right is another source of information from spatial scales close to the event horizon.

Sagittarius A* is strongly variable at submm wavelengths, and entirely dominated by bright outbursts in the near-infrared and X-ray domains. 

It is an unanswered question how the fast variability at high frequencies is linked to the radio and mm/submm variability, and what its physical mechanisms are. 

We have published two papers addressing an attempt to charac​​terize the fast variability in the NIR and submm in 2019 (Unprecedented Near-infrared Brightness and Variability of Sgr A*, Second-scale Submillimeter Variability of Sagittarius A* during Flaring Activity of 2019: On the Origin of Bright Near-infrared Flares) with the question in mind whether the periapsis passages of the putative gas cloud G2 in 2014 and of the star S-2 in 2018 have changed the accretion state of Sagittarius A* recognizably. 

These studies resulted in the detection of the brightest NIR state observed so far and fairly bright submm variability observed with unprecedented fidelity thanks to the capabilities of ALMA. 

While any relation to G2 and S2 cannot be conclusively elucidated, there is indication that 2019 indeed was an extraordinary year. 

Complementarily, we studied the real time SED of a bright flaring episode in 2019 with data from GRAVITY, Chandra, NuSTAR, and Spitzer (Constraining particle acceleration in Sgr A⋆ with simultaneous GRAVITY, Spitzer, NuSTAR, and Chandra observations), which underlined the extreme state of Sagittarius A* at the time in either terms of electron density or electron energy. 

This conclusion could be reached thanks to comparison with the findings of a forth study, Rapid Variability of Sgr A* across the Electromagnetic Spectrum, in which we could demonstrate that the typical, i.e. statistically most common, variability can well be explained by a one zone Synchrotron model with secondary Synchrotron  self-Compton scattering und changes of its optical depth. 

With this radiative model we were able to reproduce the auto-correlation and cross-correlation properties of the most comprehensive dataset of light curves up to now composed of data from Spitzer, the VLT, Keck, ALMA, Apex, SMA, and Chandra.