A thermal bremsstrahlung model for the quiescent X-ray emission from Sagittarius A*

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I consider the thermal bremsstrahlung emission from hot accretion flows (Bondi/ADAFs), taking into account the finite size of the observing telescope's beam (Rbeam) relative to the Bondi accretion radius (RA). For Rbeam ≫ RA, soft X-ray emission from the hot interstellar medium surrounding the black hole dominates the observed emission, while for Rbeam ≪ RA, hard X-ray emission from the accretion flow dominates. I apply these models to Chandra observations of the Galactic center, for which Rbeam ≈ RA. 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 ∼RA ∼ 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.

Original languageEnglish (US)
Pages (from-to)855-859
Number of pages5
JournalAstrophysical Journal
Issue number2 I
StatePublished - Aug 20 2002
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Accretion, accretion disks
  • Galaxy: center


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