Inflow-Outflow Solution with Stellar Winds and Conduction near Sgr A*

arXiv:0912.3255v1 [astro-ph.HE] 16 Dec 2009 Draft version November 9, 2018 Preprint typeset using LATEX style emulateapj v. 08/13/06 INFLOW-OUTFLOW SOLUTION WITH STELLAR WINDS AND CONDUCTION NEAR SGR A* Roman V. Shcherbakov1, Frederick K. Baganoff2 Draft version November 9, 2018 ABSTRACT We propose a 2-temperature radial dynamical model of plasma flow near Sgr A* and fit the bremsstrahlung emission to extensive quiescent X-Ray Chandra data. The model extends from sev- eral arcseconds to black hole (BH) gravitational radius, describing the outer accretion flow together with the infalling region. The model incorporates electron heat conduction, relativistic heat capacity of particles and feeding by stellar winds. Stellar winds from each star are considered separately as sources of mass, momentum and energy. Self-consistent search for the stagnation and sonic points is performed. Most of gas is found to outflow from the region. The accretion rate is limited to below 1% of Bondi rate due to the effect of thermal conduction enhanced by entropy production in a turbulent flow. The X-Ray brightness profile proves too steep near the BH, thus a synchrotron self-Compton point source is inferred with luminosity L ∼3 ·1032 erg/s. We fit the sub-mm emission from the inner flow, thus aiming at a single model of Sgr A* accretion suitable at any radius. Subject headings: accretion, accretion disks, Galaxy: center 1. INTRODUCTION Our Galaxy is known to host a supermassive black hole (BH) with mass M ≈4.5 · 106M⊙at a distance R ≈8.4 kpc (Ghez et al. 2008). The BH exhibits very low luminosity state probably due to inefficient feeding and cooling. The BH is fed by stellar winds within several arcseconds from the compact object roughly around the radius of BH gravitational influence (Cuadra et al. 2008). The stellar winds are expelled at large speeds. They collide, heat up to ∼107 K and emit bremsstrahlung X-Rays, observed by Chandra (Baganoffet al. 2003). A small fraction of mass accretes onto the black hole is thus producing the emission in sub-mm and other wavebands. However, the inferred accretion rate within several Schwarzschild radii is 2 orders of magnitude lower (Marrone 2007) than the inferred Bondi accretion rate (Bondi 1952) at several arcseconds. This disparity is resolved in a present work with a point source revealed coincident with Sgr A*. A brief account of observations is made in § 2. The dynamical model is outlined in § 3. The results are discussed in § 4. 2. OBSERVATIONS We analyze ∼1 Ms of Chandra exposure of Sgr A* and central arcseconds (Muno et al. 2008) significantly improving over the previously analyzed 41 ks exposure (Baganoffet al. 2003). As we are interested in the quiescent emission, we bin the observations in 628 s bins and exclude the flaring states with counts triple the mean. We also subtract the point sources and extended bright emission zones, thus extracting the quiescent count surface brightness profile within 5′′. Having the extensive data we are able to perform the subpixel spatial binning in rings 0.125′′ thickness owing to dithering of spacecraft. The counts from four 90 deg ring segments centered at Sgr A* are compared in order to test the viability of the radial model. It appears that within 2′′ the counts do not differ significantly between ring segments, but the variation was found at > 2′′. The point spread function (PSF) is extracted by observing the nearby binary J174540.9-290014. 3. STELLAR WINDS AND DYNAMICAL MODEL Feeding of the black hole should be a starting point of any accretion model. This approach helps to eliminate a number of arbitrary boundary conditions. A set of ∼30 wind emitters is believed to supply almost all the matter into the feeding region of Sgr A*. Following Cuadra et al. (2008), we identify the important wind emitters, find the wind speeds and ejection rates. We obtain the orbital data from Paumard et al. (2006); Martins et al. (2007); Lu et al. (2009), assuming the stars either belong to the disk or taken to have the minimum eccentricities. As we are constructing the radial model, the radial feeding function q(r) is produced by smoothing wind inputs over radius between the apocenter and the pericenter for each star (see Fig. 1). The averaged wind velocity is found as a root- mean-square average over stars weighed with the ejection rate. We do not account for orbital velocities of stars in energy input as feeding is dominated by only a few stars close to the BH. S2 star is included into the calculation as it may eject more matter (Martins et al. 2008), than falls onto the BH. The dynamical model has sources of mass, radial momentum and energy due to winds (Lamers & Cassinelli 1999). The main feature of the model is the electron thermal conduction proportional to the temperature gradient with conductivity κ = 0.1 p kBTe/mener (Johnson & Quataert 2007). Here we have included the factor of 5 inhibition of Electronic address: rshcherbakov@cfa.harvard.edu 1 Harvard-Smithsonian C

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