Sagittarius A* Accretion Flow and Black Hole Parameters From General Relativistic Dynamical and Polarized Radiative Modeling

Abstract

We obtain estimates of Sgr A* accretion flow and black hole parameters by fitting polarized sub-mm observations with spectra computed using three-dimensional (3D) general relativistic (GR) magnetohydrodynamical (MHD) (GRMHD) simulations. Observations are compiled from averages over many epochs from reports in 29 papers for estimating the mean fluxes F ν , linear polarization (LP) fractions, circular polarization (CP) fractions, and electric vector position angles (EVPAs). GRMHD simulations are computed with dimensionless spins a * = 0, 0.5, 0.7, 0.9, 0.98 over a 20, 000M time interval. We perform fully self-consistent GR polarized radiative transfer using our new code to explore the effects of spin a * , inclination angle θ, position angle (PA), accretion rateṀ , and electron temperature T e (T e is reported for radius 6M ). By fitting the mean sub-mm fluxes and LP/CP fractions, we obtain estimates for these model parameters and determine the physical effects that could produce polarization signatures. Our best bet model has a * = 0.5, θ = 75 • , PA = 115 • ,Ṁ = 4.6×10 −8 M ⊙ year −1 , and T e = 3.1 × 10 10 K at 6M . The sub-mm CP is mainly produced by Faraday conversion as modified by Faraday rotation, and the emission region size at 230 GHz is consistent with the VLBI size of 37µas. Across all spins, model parameters are in the ranges θ = 42 • − 75 • ,Ṁ = (1.4 − 7.0) × 10 −8 M ⊙ year −1 , and T e = (3 − 4) × 10 10 K. Polarization is found both to help differentiate models and to introduce new observational constraints on the effects of the magnetic field that might not be fit by accretion models so-far considered.

Measuring the Direction and Angular Velocity of a Black Hole Accretion Disk via Lagged Interferometric Covariance

“…However, fractional polarization is baseline-based and provides identical immunity after calibrating slowly changing differential gain and leakage terms (e.g., Roberts et al 1994). Comparisons with simulations that include polarization information (e.g., Shcherbakov et al 2012;Shcherbakov & McKinney 2013;Dexter 2014) will allow us to assess whether lagged correlation of fractional polarization on close baseline pairs can likewise reflect the dynamics of the bulk flow. However, because the polarization direction can change throughout an orbit, from the changing local magnetic field direction or relativistic aberration or from strong-field relativistic effects such as lensing or parallel transport, the application to polarization may require significant modification.…”

“…However, given that the current data are extremely sparse, the inclination and image properties cannot yet be confidently constrained. In addition, current estimates of the inclination are subject to a degeneracy between supplementary inclinations, q p q -, , { } (see Figure 1) because simulated images of accretion flows exhibit a near symmetry orthogonal to the rotation axis (Mościbrodzka et al 2009;Dexter et al 2010;Broderick et al 2011;Shcherbakov et al 2012;Psaltis et al 2015).…”

Coupled jet-disk model for Sagittarius A*: explaining the flat-spectrum radio core with GRMHD simulations of jets

“…In these models relativistic jets are often produced (e.g., Beckwith et al 2008). The synchrotron emission from a GRMHD-RIAF can now be calculated with a high degree of precision using general relativistic radiative-transfer codes (e.g., Dolence et al 2009;Dexter & Agol 2009;Shcherbakov et al 2012). Advances in numerical modeling, together with less scattered, mm-wave VLBI measurements of Sgr A*, provide an opportunity to put tight constraints on jet-plus-disk models for BHs Dexter et al 2012).…”

Jet Signatures in the Spectra of Accreting Black Holes

“…The disadvantage of this approach is that it is only applicable in regimes in which the radiation is dynamically unimportant. These codes have been used by many authors to calculate the observational signatures of low luminosity systems in which the radiation pressure can be neglected (e.g., Chan et al 2009Chan et al , 2015aChan et al , 2015bMościbrodzka et al 2009;Mościbrodzka et al 2014;Shcherbakov et al 2012;Mościbrodzka & Falcke 2013;Shcherbakov & McKinney 2013). These works mainly focussed on reproducing the spectra and variability properties of Sgr A * , and place important constraints on quantities such as the black hole spin, proton-to-electron temperature ratio, and inclination angle.…”