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H.E.S.S. observations of Sgr A*


Poon,  Helen
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

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Poon, H. (2016). H.E.S.S. observations of Sgr A*. PhD Thesis, Ruprecht-Karls-Universität, Heidelberg.

Cite as: https://hdl.handle.net/11858/00-001M-0000-002A-EDA7-1
The Galactic Centre has been studied with the High Energy Stereoscopic System
(H.E.S.S.) for over 10 years, showing a bright, complex gamma-ray morphology. Other
than a strong point-like very-high-energy gamma-ray source coincident with the supermassive
black hole Sgr A*, previous analyses also revealed a diffuse ridge of gamma-ray
emission, indicative of a powerful cosmic-ray accelerator in this region. It has been widely
studied in the multiwavelength, exhibiting flares from radio to X-ray, but in the VHE
regime it has always been stable. A gas cloud called G2 was found travelling straight
to the Galactic Centre in 2011. Accretion rate was expected to increase which might
possibly lead to flares in different wavebands.
The addition of a fifth telescope with 600 m2 mirror area to the centre of the H.E.S.S.
array has increased the energy range accessible, allowing observations to take place down
to 100 GeV and potentially below. The sensitivity is also increased which means it has
better ability to detect VHE flares from the Galactic Centre. In this work, long-term
study of Sgr A* with H.E.S.S. I and data involving H.E.S.S. II around pericentre passage
time of G2 is presented. Varibility tests are also applied to check for transient events.
The overall spectrum and spectra for individual years have been stable over the years.
The results of variability tests also revealed stability in the flux.