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 ∼1/3 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.