A Thermal Bremsstrahlung Model for the Quiescent X‐Ray Emission from Sagittarius A*

I consider the thermal bremsstrahlung emission from hot accretion flows (Bondi/ADAFs), taking into account the finite size of the observing telescope's beam ( R beam ) relative to the Bondi accretion radius ( R A ). For R beam ≫ R A , soft X-ray emission from the hot interstellar medium surrounding the black hole dominates the observed emission, while for R beam ≪ R A , hard X-ray emission from the accretion flow dominates. I apply these models to Chandra observations of the Galactic center, for which R beam ≈ R A . I argue that bremsstrahlung emission accounts for most of the "quiescent" (nonflaring) flux observed by Chandra from Sgr A*; this emission is spatially extended on scales ~ R A ~ 1'' and has a relatively soft spectrum, as is observed. If accretion onto the central black hole proceeds via a Bondi or ADAF flow, a hard X-ray power law should be present in deeper observations with a flux ~ of the soft X-ray flux; nondetection of this hard X-ray component would argue against ADAF/Bondi models. I briefly discuss the application of these results to other low-luminosity active galactic nuclei.