Abstract
We calculate spectral models of advection-dominated accretion flows, taking into account the possibility that significant mass may be lost to a wind. We apply the models to the soft X-ray transient V404 Cyg in quiescence and the Galactic center source Sgr A*. We show that there are qualitative degeneracies between the mass loss rate in the wind and parameters characterizing the microphysics of the accretion flow; of particular importance is $\delta$, the fraction of the turbulent energy which heats the electrons. For small $\delta$, current observations of soft X-ray transients and Sgr A* suggest that at least $\sim 10 %$ of the mass originating at large radii must reach the central object. For large $\delta \sim 0.3$, however, models with significantly more mass loss are in agreement with the observations. We also discuss constraints on advection-dominated accretion flow models imposed by recent radio observations of NGC 4649 and other nearby elliptical galaxies. We conclude by highlighting future observations which may clarify the importance of mass loss in sub-Eddington accretion flows.
Highlights
A number of authors have argued that, at sub-Eddington accretion rates, the gravitational potential energy released by turbulent stresses in an accretion flow may be stored as thermal energy, rather than being radiated (Ichimaru 1977; Rees et al 1982; Narayan & Yi 1994, 1995; Abramowicz et al 1995; Chen et al 1995; see Narayan, Mahadevan, & Quataert 1998b, and Kato, Fukue, & Mineshige 1998 for reviews)
As we will show in this paper, mass loss from the accretion flow has a dramatic effect on theoretically predicted spectra
We have investigated under what conditions advection-dominated accretion flows (ADAFs) models with mass loss might account for the observations, and the extent to which observations can distinguish between the various proposals for the physics of the accretion flow
Summary
A number of authors have argued that, at sub-Eddington accretion rates, the gravitational potential energy released by turbulent stresses in an accretion flow may be stored as thermal energy, rather than being radiated (Ichimaru 1977; Rees et al 1982; Narayan & Yi 1994, 1995; Abramowicz et al 1995; Chen et al 1995; see Narayan, Mahadevan, & Quataert 1998b, and Kato, Fukue, & Mineshige 1998 for reviews). If the wind carries away roughly the specific angular momentum and energy appropriate to the radius from which it is launched, they show that the remaining (accreting) gas has a negative Bernoulli parameter only for large values of p ∼ 1 They propose that the majority of the mass originating at large radii is lost to a wind. In a separate study, Di Matteo et al (1998; hereafter D98) measured the flux of radio and submillimeter emission from the nuclei of nearby elliptical galaxies and found fluxes significantly below the values predicted by the ADAF model Their observations are difficult to reconcile with Fabian & Rees’s (1995) proposal that these galactic nuclei contain ADAFs. D98 discuss a number of explanations for the “missing” flux; one of their suggestions is that a significant wind may carry off much of the accreting mass in the ADAF.
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