THE SUBMILLIMETER BUMP IN Sgr A* FROM RELATIVISTIC MHD SIMULATIONS

Abstract

Recent high resolution observations of the Galactic center black hole allow for direct comparison with accretion disk simulations. We compare two-temperature synchrotron emission models from three dimensional, general relativistic magnetohydrodynamic simulations to millimeter observations of Sgr A*. Fits to very long baseline interferometry and spectral index measurements disfavor the monochromatic face-on black hole shadow models from our previous work. Inclination angles ≤ 20 • are ruled out to 3σ. We estimate the inclination and position angles of the black hole, as well as the electron temperature of the accretion flow and the accretion rate, to be i = 50−2 × 10 −9 M ⊙ yr −1 respectively, with 90% confidence. The black hole shadow is unobscured in all best fit models, and may be detected by observations on baselines between Chile and California, Arizona or Mexico at 1.3mm or .87mm either through direct sampling of the visibility amplitude or using closure phase information. Millimeter flaring behavior consistent with the observations is present in all viable models, and is caused by magnetic turbulence in the inner radii of the accretion flow. The variability at optically thin frequencies is strongly correlated with that in the accretion rate. The simulations provide a universal picture of the 1.3mm emission region as a small region near the midplane in the inner radii of the accretion flow, which is roughly isothermal and has ν/ν c ∼ 1 − 20, where ν c is the critical frequency for thermal synchrotron emission.

Millimeter to X-ray flares from Sagittarius A*

“…The timeaveraged data presented by Marrone (2006a,b) result in an average 230 GHz total flux density of 3.75 ± 0.48 Jy and a spectral index of α 230/690 GHz = +0.18 ± 0.34 (1σ uncertainties; Dexter et al 2010). Dexter et al (2010) note that the 230 GHz flux measurements during these SMA observations were 50% higher than during the VLBI measurements from Doeleman et al (2008Doeleman et al ( , 2009). Thus, it is possible that the spectrum between 230 GHz and 690 GHz may actually be flatter than the time-averaged value obtained by Marrone (2006a,b) or even inverted.…”

“…Observations and simulations consistently indicate that at frequencies above 200 GHz the variability amplitudes are of the order of 40%−50% (Garcia-Marin et al 2011a,b;Dexter et al 2009Dexter et al , 2010Li et al 2009;Marrone et al 2006aMarrone et al ,b, 2007. At lower frequencies (corresponding to wavelengths of 0.7 cm to 3.0 cm), the observed variability amplitudes are of the order of 10% (Macquart & Bower 2006;Bower et al 2002;Falcke et al 1998) or lower.…”

“…Falcke & Markoff (2000) find a field strength of ∼25 G for the foot point of a hypothetical jet. Estimates from relativistic MHD simulations result in values in the range of 30−50 G (Moscibrodzka et al 2009;Dexter et al 2010;Shcherbakov & Baganoff 2010).…”

“…The submm spectrum of SgrA* itself is not very well-known owing to resolution effects and the contaminating flux density contributions from the surrounding "mini-spiral" HII region and the circumnuclear disk (CND). However, the so-called sub-millimeter bump is often attributed to SgrA* and is thought to be due to A&A 537, A52 (2012) relativistic, thermal electrons in the innermost section of a hot, thick, advection-dominated accretion flow (Dexter et al 2010;Narayan et al 1995;Yuan et al 2003). Here we refer to the submillimeter bump as the major portion of the low flux density state of SgrA* at sub-millimeter wavelengths.…”

“…However, a general model that provides a quantitative explanation of the frequency-dependent variability of SgrA* in the millimeter and sub-millimeter wavelengths, including the contribution of thermal electrons and relativistic electrons that result in NIR/X-ray flares, is not yet available (see comments in Dexter et al 2010).…”