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Variability at radio and submillimeter wavelengths

The point-like source Sgr A* is always visible in the long-wavelength regimes (i.e. radio to sub-millimeter). Toward higher frequencies (i.e. near-infrared to X-rays) its steady, quiescent emission faints out. Sgr A* also shows short time-scale (of the order minutes to hours) variability across the entire observable spectrum. Time travel arguments imply, that the source regions of these radiation outbursts, also called flares, have to be in the direct environment of the black-hole's event horizon. Therefore the investigation of flares offer a unique possibility to shed light on the physical conditions in the strong-gravity regime.

We have monitored the emission of Sgr A* from 2010 to 2012 in the 100 GHz radio (ATCA) and from 2008 to 2014 in the 350 GHz submm (LABOCA APEX-ESO) regime. We also performed a homogeneous reduction and calibration of the observational data, and a statistical analysis of the full data sets. Our results indicate that both flux-density distributions can be well described by a power-law with the same scaling index. Together with observed flare-peak delays from higher to lower frequencies, this can be interpreted as evidence for adiabatically expanding synchrotron blobs giving rise to the radiation outbursts at these wavelengths. These blobs are knots of hot plasma located in the mid-accretion plane of the material that is being accreted onto the black hole. Our analysis also allows us to derive their physical properties, such as synchrotron turnover frequencies, initial blob sizes and expansion velocities.

350 GHz map of the innermost few parsec of the galactic center taken with LABOCA/APEX in September 2013. The radiation is dominated by thermal emission from molecular clouds. The central point-like structure is associated with Sgr A*. Zooming in to that object and removing the quiescent background emission reveals short time-scale flux density variations. Three snapshots of the center from the 1st Septembre 2013 can be seen at the right hand side of the picture. They are seperated by ~0.5 hours and show a raise of flux density of up to 3 Jy. A bright submm flares has been observed.
Credits: Eckart group, Ph1-UniKoeln.

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