The Galactic Center Seen in Very High Energy Gamma Ray Light

Over the past two decades, very high energy gamma ray emission has been detected coming from the direction of the Milky Way's Galactic Center. The Galactic Center hosts many potential sites of particle acceleration including the supermassive black hole Sagittarius A* and remnants of supernova activity spread throughout the region. Electrons, protons, or other particles that are accelerated to speeds close to the speed of light will emit very high energy light - gamma rays - via a range of physical processes that depend on the particle types and the environmental conditions. Each gamma ray packs approximately one hundred million times the energy of the highest energy X-ray photon seen by the Chandra X-ray Observatory. The gamma-ray maps of the region show emission coming from one bright spot near the central supermassive black hole, from several other locations scattered within the central five hundred light-years, as well as a diffuse glow between them. However the limited spatial resolution of the detection facilities made it difficult to pinpoint the precise source(s) of the gamma rays. Astronomers have been working to refine the observations in order to identify the objects and constrain the mechanisms responsible for generating the gamma rays.

VERITAS, the Very Energetic Radiation Imaging Telescope Array System, is a CfA observatory consisting of four 12-m telescopes designed to study gamma rays and located at the Fred L. Whipple Observatory at Mt. Hopkins, Arizona. CfA astronomers Wystan Benbow, Michael Daniel, Gareth Hughes, and Emmet Roache with a large team of their colleagues reanalyzed eight years of VERITAS observations of the Galactic Center region using advanced data reduction algorithms. The team finds that the bright central gamma-ray source is centered on the supermassive black hole (in agreement with some earlier studies) but also cannot exclude some neighboring structures as possible origins. Spectral information, however, implies that electrons are the dominant particles, in turn suggesting that supernova remnant activity with high magnetic fields could be powering the emission, not SgrA*. The absence of detected variability in the source is consistent with this model. The astronomers' analysis of the diffuse radiation within the whole region finds that all the energy in that radiation could have been supplied by a single large supernova event in the past, but it is also consistent with the presence of a population of 10-100 thousand pulsars (themselves remnants of supernovae). CfA scientists are leading the development of a next-generation gamma-ray facility that will be able to make stronger source correspondences and also extend the range of detectable gamma-rays.

Reference: "VERITAS Observations of the Galactic Center Region at Multi-TeV Gamma-Ray Energies," C. B. Adams et al., The Astrophysical Journal 913, 115, 2021.