Composite image of the Galactic Centre region, combining Hubble images in the near-infrared, Spitzer Space Telescope images in the infrared, and Chandra X-ray images. The bright white region on the bottom right size is Sagittarius A, the Galactic Centre. The image spans about 0.3 degrees across the sky, corresponding to about 170 light years at the distance of the Galactic Centre. See APOD for details, and for an annotated image. Image credit: NASA, ESA, SSC, CXC, and STScI.

The centre of our Galaxy will be one of the prime targets for CTA. The Galactic Centre hosts our nearest supermassive black hole, as well as a variety of other objects likely to generate high-energy radiation, including hypothetical dark-matter particles which may annihilate and produce gamma-rays. Indeed, the Galactic Centre has already been detected as a source of high-energy gamma-rays, and there are indications of high-energy particles diffusing away from the central source and interacting with the dense gas clouds in the central region.

Observations of the Galactic Centre with improved sensitivity and resolution could yield a variety of interesting results on particle acceleration and gamma-ray production in the vicinity of black holes and on particle propagation in the central molecular clouds. We may even detect dark matter annihilation or decay.

Diffuse Emission near the Galactic Centre

The VHE gamma-ray view of the GC region is dominated by two point sources, one
coincident with a pulsar wind nebula (PWN) inside SNR G0.9+0.1, and one coincident with both the supermassive black hole SgrA* and another likely PWN (G359.95-0.04). After subtraction of these sources, diffuse emission along the GC ridge is visible, which shows two important features: it appears correlated with molecular clouds in the region, and it exceeds the gamma-ray emission that would be produced if the same target material was exposed to the cosmic-ray environment in our local neighbourhood by a factor of 3-9.

Top: H.E.S.S. observations of the Galactic Centre region show two prominent point sources, one located right at the Galactic Centre and one coincident with the supernova remnant G0.9+01. Bottom: after subtraction of the emission from the two point sources and an adjustment of the color scale, a band of gamma ray emission emerges, tracing the Galactic Plane. See http://arxiv.org/abs/astro-ph/0603021

The striking correlation of diffuse gamma-ray emission with the density of molecular clouds within less than 150 pc of the GC favours a scenario in which cosmic rays – primarily protons -  interact with the cloud material and produce gamma-rays via decay of neutral pions. The gamma-ray flux is both stronger and harder than expected from just “passive” exposure of the clouds to the average galactic cosmic ray flux, suggesting one or more particle accelerators are present nearby. As the cosmic rays have not yet diffused far, calculations suggest an accelerator age of no more than 10,000 years. The improved sensitivity and angular resolution of CTA would permit the study of the diffusion process in great detail, including possible energy dependence.

An alternative explanation for the diffuse emission, which is to be addressed by CTA,  is the possible existence of a number of sources of electrons rather than protons (e.g. PWNe) along the GC ridge, which just happen to be correlated with the density of molecular clouds. Given the complexity and density of the source population in the GC region, CTA’s improved sensitivity and angular resolution is needed to map the morphology of the diffuse emission, and to investigate its origin.

Further Reading

Goldwurm, An Overview of the High Energy Emission from the Galactic Center; http://arxiv.org/abs/1007.4174