Abstract
Sagittarius A*, which is located at the Galactic center, is a puzzling source. It has a mass of M = (2.5 ± 0.4) × 106 M☉, which makes it an excellent black hole candidate. Observations of stellar winds and other gas flows in its vicinity suggest a mass accretion rate of ≳ few × 10−6 M☉ yr-1. However, such an accretion rate would imply a luminosity greater than 1040 ergs-1 if the radiative efficiency is the usual 10%, whereas observations indicate a bolometric luminosity less than 1037 ergs-1. The spectrum of Sgr A* is unusual, with emission extending over many decades of wavelength. We present a model of Sgr A* that is based on a two-temperature optically thin advection-dominated accretion flow. The model is consistent with the estimated M and and fits the observed fluxes in the centimeter/millimeter and X-ray bands, as well as upper limits in the submillimeter and infrared bands; the fit is less good in the radio spectrum below 86 GHz and in γ-rays above 100 MeV. The very low luminosity of Sgr A* is explained naturally in the model by means of advection. Most of the viscously dissipated energy is advected into the central mass by the accreting gas, and therefore the radiative efficiency is extremely low, ~5 × 10-6. A critical element of the model is the presence of an event horizon at the center that swallows the advected energy. The success of the model could thus be viewed as confirmation that Sgr A* is a black hole.
Highlights
The enigmatic radio source, Sagittarius A∗ (Sgr A∗), has for many years been a puzzle (Genzel & Townes 1987; Genzel, Hollenbach & Townes 1994; Mezger, Duschl & Zylka1996)
The main result of this paper is that the two-temperature advection-dominated accretion flow (ADAF) model (Narayan et al 1995, Rees 1982) provides a viable explanation of the spectral properties of Sgr
Rees et al (1982) made the interesting point that the plasma physics involved is so complex that perhaps the best way of testing the two-temperature paradigm is by comparison of astrophysical models with observations
Summary
The enigmatic radio source, Sagittarius A∗ (Sgr A∗), has for many years been a puzzle Over the years a number of models have been proposed for Sgr A∗ Some of these have been phenomenological approaches which aim to explain the radio and infrared spectrum without including any detailed dynamics Most current work on ADAFs has been concerned with a branch of low Msolutions (Ichimaru 1977; Rees et al 1982; Narayan & Yi 1995b; Abramowicz et al 1995) which is present for mass accretion rates below a few percent of the Eddington rate (Narayan 1997; Esin, McClintock & Narayan 1997) These low-Msolutions make use of the standard α viscosity and assume a two-temperature plasma with an equipartition magnetic field.
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