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Reconciling the spectrum of Sagittarius A* with a two-temperature plasma model

Nature volume 394, pages 651–653 (1998)Cite this article

Abstract

The radio source Sagittarius A* is thought to be powered by gas accreting onto a supermassive black hole at the centre of our Galaxy1,2. Using the high infrared accretion rates3, however, standard accretion models4 are unable to explain the observed low luminosity and spectral energy distribution5,6,7,8, which has led to the consideration of a new model: advection-dominated accretion flows9,10,11,12. In an advection-dominated flow, most of the accretion energy is stored as thermal energy in the gaswhich is then lost as the gas falls into the black hole. This model requires the protons to have a much higher temperature than the electrons, and the gas therefore has a two-temperature structure10,13,14. Although this model explains the low total luminosity15,16,17,18 and much of the spectral energy distribution (from millimetre wavelengths to hard X-rays), it has been difficult to reconcile with low-frequency radio observations. Here we show that a neglected emission process associated with the protons naturally explains the radio observations without any ‘fine tuning’ of the model parameters. This result simultaneously supports the two-temperature model of the gas and suggests that an advection-dominated accretion flow onto a black hole of 2.5 × 106 solar masses provides an accurate description of Sagittarius A*.

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Figure 1: A comparison of the predicted emission from an ADAF model of the Galactic Centre with the observations.
Figure 2: The energy spectrum, R (E ), of positrons and electrons that are created by colliding power-law protons with energy index s = 2.75.

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References

  1. Mezger, P. G., Duschl, W. J. & Zylka, R. The Galactic Center: a laboratory for AGN? Annu. Rev. Astron. Astrophys. 7, 289–388 (1996).

    Article  Google Scholar 

  2. Genzel, R., Hollenbach, D. & Townes, C. H. The nucleus of our Galaxy. Rep. Prog. Phys. 57, 417–479 (1994).

    Article  ADS  Google Scholar 

  3. Melia, F. An accreting black hole model for Sagittarius A*. Astrophys. J. 387, L25–L28 (1992).

    Article  ADS  Google Scholar 

  4. Frank, J., King, A. & Raine, D. Accretion Power in Astrophysics (Cambridge Univ. Press, (1992).

    Google Scholar 

  5. Rogers, A. E. E. et al. Small-scale structure and position of Sagittarius A* from VLBI at 3 millimeter wavelength. Astrophys. J. 434, L59–L62 (1994).

    Article  ADS  Google Scholar 

  6. Menten, K. M., Reid, M. J., Eckart, A. & Genzel, R. The position of Sagittarius A*: accurate alignment of the radio and infrared reference frames at the Galactic Center. Astrophys. J. 475, L111–L114 (1997).

    Article  ADS  CAS  Google Scholar 

  7. Predehl, P. & Trümper, J. ROSAT observation of the Sgr A region. Astron. Astrophys. 290, L29–L32 (1994).

    ADS  Google Scholar 

  8. Merck, M. et al. Study of the spectral characteristics of unidentified galactic EGRET sources. Are they pulsar-like? Astron. Astrophys. Suppl. 120, 465–469 (1996).

    ADS  Google Scholar 

  9. Ichimaru, S. Bimodal behavior of accretion disks — Theory and application to Cygnus X-1 transitions. Astrophys. J. 214, 840–855 (1977).

    Article  ADS  Google Scholar 

  10. Rees, M. J., Begelman, M. C., Blandford, R. D. & Phinney, E. S. Ion supported tori and the origin of radio jets. Nature 295, 17–21 (1982).

    Article  ADS  CAS  Google Scholar 

  11. Narayan, R. & Yi, I. Advection-dominated accretion: underfed black holes and neutron stars. Astrophys. J. 452, 710–735 (1995).

    Article  ADS  Google Scholar 

  12. Abramowicz, M. A., Chen, X., Kato, S., Lasota, J.-P. & Regev, O. Thermal equilibria of accretion disks. Astrophys. J. 438, L37–L39 (1995).

    Article  ADS  Google Scholar 

  13. Shapiro, S. L., Lightman, A. P. & Eardley, D. M. Atwo-temperature accretion disk model for Cygnus X-1 — Structure and spectrum. Astrophys. J. 204, 187–199 (1976).

    Article  ADS  Google Scholar 

  14. Phinney, E. S. in Plasma Astrophysics (eds Guyenne, T. D. & Levy, G.) 337–341 (SP-161, ESA, Paris, (1981)).

    Google Scholar 

  15. Rees, M. J. in The Galactic Center (eds Riegler, G. R. & Blandford, R. D.) 166–176 (Am. Inst. Phys., New York, (1982)).

    Google Scholar 

  16. Narayan, R., Yi, I. & Mahadevan, R. Explaining the spectrum of Sagittarius A* with a model of an accreting black hole. Nature 374, 623–625 (1995).

    Article  ADS  CAS  Google Scholar 

  17. Manmoto, T., Mineshige, S. & Kusunose, M. Spectrum of optically thin advection-dominated accretion flow around a black hole: application to Sagittarius A*. Astrophys. J. 489, 791–803 (1997).

    Article  ADS  CAS  Google Scholar 

  18. Narayan, R., Mahadevan, R., Grindlay, J. E., Popham, R. G. & Gammie, C. Advection-dominated accretion model of Sagittarius A*: evidence for a black hole at the Galactic Center. Astrophys. J. 492, 554–568 (1998).

    Article  ADS  Google Scholar 

  19. Mahadevan, R., Narayan, R. & Krolik, J. Gamma-ray emission from advection-dominated accretion flows around black holes: application to the Galactic Center. Astrophys. J. 486, 268–275 (1977).

    Article  ADS  Google Scholar 

  20. Mahadevan, R. Scaling laws for advection-dominated flows: applications to low-luminosity galactic nuclei. Astrophys. J. 477, 585–601 (1997).

    Article  ADS  Google Scholar 

  21. Falcke, H. et al. The simultaneous spectrum of Sgr A* from λ20 cm to λ1 mm and the nature of the mm-excess. Astrophys. J.(in the press).

  22. Backer, D. C. et al. Upper limit of 3.3 astronomical units to the diameter of the Galactic Center radio source Sgr A*. Science 262, 1414–1416 (1993).

    Article  ADS  CAS  Google Scholar 

  23. Marcaide, J. M. et al. Position and morphology of the compact non-thermal radio source at the galactic center. Astron. Astrophys. 258, 295–301 (1992).

    ADS  Google Scholar 

  24. Alberdi, A. et al. VLBA image of Sgr A* at λ = 1.35 cm. Astron. Astrophys. 277, L1–L4 (1993).

    ADS  Google Scholar 

  25. Ginzburg, V. L. & Syrovatskii, S. I. The Origin of Cosmic Rays (Macmillan, New York, (1964)).

    Book  Google Scholar 

  26. Dermer, C. D. Binary collision rates of relativistic thermal plasmas. II. Spectra. Astrophys. J. 307, 47–59 (1986).

    Article  ADS  CAS  Google Scholar 

  27. Rybicki, G. & Lightman, A. Radiative Processes in Astrophysics (Wiley, New York, (1979)).

    Google Scholar 

  28. Begelman, M. C. & Chiueh, T. Thermal coupling of ions and electrons by collective effects in two-temperature accretion flows. Astrophys. J. 332, 872–890 (1988).

    Article  ADS  Google Scholar 

  29. Quataert, E. Particle heating by Alfvenic turbulence in hot accretion flows. Astrophys. J.(in the press); also as preprint astro-ph/9710127 (1998).

  30. Gruzinov, A. Radiative efficiency of collisionless accretion. Astrophys. J.(in the press); also as preprint astro-ph/9710132 (1998).

  31. Blackman, E. Fermi energization in magnetized astrophysical flows. Phys. Rev. Lett.(in the press); also as preprint astro-ph/9710137 (1998).

  32. Bisnovatyi-Kogan, G. S. & Lovelace, R. V. E. Influence of ohmic heating on advection-dominated accretion flows. Astrophys. J. 486, L43–L46 (1997).

    Article  ADS  Google Scholar 

  33. Quataert, E. & Gruzinov, A. Turbulence and particle heating in advection-dominated accretion flows. Astrophys. J.(submitted); also as preprint astro-ph/9803112 (1998).

  34. Narayan, R., Mahadevan, R. & Quataert, E. Advection dominated accretion around black holesin The Theory of Black Hole Accretion Discs(eds Abramovicz, M. A., Bjornsson, G. & Pringle, J. E.) (Cambridge Univ. Press, in the press).

  35. Duschl, W. J. & Lesch, H. The spectrum of Sgr A* and its variability. Astron. Astrophys. 286, 431–436 (1994).

    ADS  Google Scholar 

  36. Falcke, H. in Unsolved Problems in the Milky Way (eds Blitz, L. & Teuben, P. J.) 163–170 (IAU Symp. No. 169, Kluwer, Dordrecht, (1996)).

    Google Scholar 

  37. Beckert, T. & Duschl, W. J. Synchrotron radiation from quasi-monoenergetic electrons. Modeling the spectrum of Sgr A*. Astron. Astrophys. 328, 95–106 (1997).

    ADS  CAS  Google Scholar 

  38. Mastichiadis, A. & Ozernoy, L. M. X-ray and gamma-ray emission of Sagittarius A* as a wind-accreting black hole. Astron. Astrophys. 426, 599–603 (1994).

    ADS  Google Scholar 

  39. Melia, F. An accreting black hole model for Sagittarius A*. 2: A detailed study. Astrophys. J. 426, 577–585 (1994).

    Article  ADS  Google Scholar 

  40. Mahadevan, R., Narayan, R. & Yi, I. Harmony in electrons: cyclotron and synchrotron emission by thermal electrons in a magnetic field. Astrophys. J. 465, 327–337 (1996).

    Article  ADS  Google Scholar 

  41. Shakura, N. I. & Sunyaev, R. A. Black holes in binary systems. Observational appearance. Astron. Astrophys. 24, 337–355 (1973).

    ADS  Google Scholar 

  42. Haller, J. et al. Stellar kinematics and the black hole in the Galactic Center. Astrophys. J. 456, 194–205 (1996); Erratum. Astrophys. J. 468, 955 (1996).

    Article  ADS  CAS  Google Scholar 

  43. Eckart, A. & Genzel, R. Stellar proper motions in the central 0.1 pc of the Galaxy. Mon. Not. R. Astron. Soc. 284, 576–598 (1997).

    Article  ADS  Google Scholar 

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Acknowledgements

I thank R. Narayan, E. Blackman, A. Fabian, C. Gammie, Z. Haiman, J. Herrnstein, J.Krolik, A. Mody, M. Rees and E. Quataert for discussions and comments.

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  1. Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA, Cambridge, UK

    Rohan Mahadevan

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Correspondence to Rohan Mahadevan.

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Mahadevan, R. Reconciling the spectrum of Sagittarius A* with a two-temperature plasma model. Nature 394, 651–653 (1998). https://doi.org/10.1038/29241

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