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'sbeam (R_beam) relative to the Bondi accretion radius (R_A). For R_beam >> R_Asoft X-ray emission from the hot interstellar medium surrounding the black holedominates the observed emission while for R_beam << R_A hard X-ray emissionfrom the accretion flow dominates. I apply these models to Chandra observationsof the Galactic Center, for which R_beam ~ R_A. I argue that bremsstrahlungemission accounts for most of the ``quiescent'' (non-flaring) flux observed byChandra 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 thecentral black hole proceeds via a Bondi or ADAF flow, a hard X-ray power lawshould be present in deeper observations with a flux ~ 1/3 of the soft X-rayflux; nondetection of this hard X-ray component would argue against ADAF/Bondimodels. I briefly discuss the application of these results to otherlow-luminosity AGN.Comment: final version accepted by ApJ; some rewriting but conclusions unchange