On the Nature of the X-ray Emission from the Galactic Center Region

Abstract

The origin of the X-ray emission from the central region of the Galaxy has remained a mystery, despite extensive study over the past two decades. A fundamental question is the relative contribution of the point-source and diffuse components of this emission, which is critical to understanding the high-energy phenomena and processes unique to this Galactic nuclear environment. Here, we report on results from a large-scale imaging survey of the Galactic center with sufficient spatial resolution to allow a clean separation of the two components. The highly-ionized Fe emission, previously attributed to the diffuse emission, is found largely due to the discrete X-ray source population. The number and spectrum of such sources indicate the presence of numerous accreting white dwarfs, neutron stars, and/or black holes in the region. The diffuse X-ray emission dominates over the contribution from the faint discrete sources and is shown to be associated with distinct interstellar structures observed at radio and mid-infrared wavelengths, suggesting that it arises from the recent formation of massive stars. We have also mapped out the 6.4-keV fluorescence line emission from neutral to moderately ionized irons. The ubiquitous association of the emission with molecular gas indicates that either the X-ray radiation from the Galactic center was substantially more intense in the past than at present or non-relativistic cosmic-rays are important in producing Fe K shell vacancies.