Time Variability of Low Angular Momentum Flows Accreting onto Black Holes A Natural Mechani.pdf

a r X i v : a s t r o - p h / 0 4 0 9 4 7 5 v 1 2 0 S e p 2 0 0 4 Time Variability of Low Angular Momentum Flows Accreting onto Black Holes: A Natural Mechanism For Radiation Flaring Daniel Proga1,2 1 JILA, University of Colorado, Boulder CO 80309, USA 2 Present address: Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton NJ 08544, USA Abstract. We present results from our magnetohydrodynamical simulations of ac- cretion flows onto black holes. Our main focus is the interplay between inflows and related outflows. We consider applications of such flows to the Galatic center and low luminosity active galactic nuclei. 1 Introduction Dynamical evidence suggests that the nonluminous matter within 0.015 pc of the Galactic center has a mass of ≈ 2.6 × 106 M⊙ (e.g., [8], [9]). This matter is associated with Sgr A?, a bright, compact radio source [3], and provides very compelling evidence for the existence of a supermassive black hole (SMBH). Observations of Sgr A? in X-ray and radio bands reveal a luminosity substantially below the Eddington limit, LEdd = 3 × 10 44 erg s?1. For example, Chandra observations show a luminosity in 2-10 keV X-rays of ≈ 2 × 1033 erg s?1 (ten orders of magnitude below LEdd) [1]. Chandra observations also revealed an X- ray flare rapidly rising to a level about 45 times as large, lasting for only ～ 104 s, indicating that the flare must originate near the black hole [1]. It is thought that the Sgr A? radiation is due to gas accretion onto the SMBH. Estimates for the accretion luminosity, L, rely on assumptions about the mass accretion rate, M?a, and the efficiency of transforming the gas energy into radiation, η (i.e., L = ηc2M?a). Both M?a and η are uncertain and there is no generally accepted model which could explain the low luminosity of Sgr A? by predicting low enough M?a or η, or both. For example, Coker Melia [5] estimated a rate of 10?4 M⊙ yr ?1 from Bondi-Hoyle accretion of winds from nearby stars, whil