Thermal X-Ray Iron Line Emission from the Galactic Center Black Hole Sagittarius A*

摘要

We model thermal X-ray emission from the accreting supermassive black hole Sagittarius A* at the Galactic center. For the region inside 15 of the center, we use a generalized radiatively inefficient accretion flow (RIAF) model, and for the region out to 10'' we use published models of the ambient hot gas. We calculate the equivalent widths of hydrogen-like and helium-like emission lines of various elements, notably iron. We predict that a strong helium-like iron line with an equivalent width ~1 keV should be emitted by both the external medium and the RIAF. The equivalent width in the external medium is sensitive to the metallicity Z of the gas as well as the mean temperature. For reasonable choices of these parameters, the calculated results agree with Chandra's detection of an iron line with an equivalent width of 1.3 keV within 10''. The emission from within 15 is not sensitive to the external temperature, but is sensitive to the density and, especially, temperature profile inside the Bondi radius. For the range of profiles we consider, we calculate the equivalent width of the iron line to be ~0.6-1.5(Z/Z☉) keV, where Z☉ is the solar metallicity. We present a new Chandra spectrum of the quiescent emission within 15 of Sgr A*. The measured equivalent width of the iron line is 0.7 keV. Although this measurement has a large uncertainty, it is consistent with our predictions, provided the metallicity of the gas is approximately solar.